REVIEW Chapter 6 - McGraw-Hill Ryerson

CHAPTER 

6 

Specifi c Expectations 

In this chapter, you will learn how to . . . 

• D1.1 analyze, on the basis of research, 

some of the social and ethical 

implications of research in genetics and 

genomics (6.3) 

• D1.2 evaluate, on the basis of research, 

the importance of some recent 

contributions to the knowledge, 

techniques, and technologies related to 

genetic processes (6.3) 

• D 2.1 use appropriate terminology 

related to genetic processes (6.1, 6.2, 6.3) 

• D2.3 use the Punnett square method to 

solve basic genetics problems involving 

monohybrid crosses, incomplete 

dominance, codominance, dihybrid 

crosses, and sex-linked genes (6.1, 6.2) 

• D3.3 explain the concepts of genotype, 

phenotype, dominance, incomplete 

dominance, codominance, recessiveness, 

and sex linkage according to Mendelian 

laws of inheritance (6.1, 6.2) 

• D3.4 describe some genetic disorders 

caused by chromosomal abnormalities 

or other genetic mutations in terms of 

chromosomes aff ected, physical eff ects, 

and treatments (6.1, 6.2) 

The inherited traits of an individual are the result of a complex 

array of genetic interactions. As genetics research continues to 

advance, we have a better understanding of these complexities. 

A signifi cant advancement is the Human Genome Project. In 2003, 

a team of over 2000 researchers, working in laboratory groups around 

the world, completed the Human Genome Project. For this project, 

numerous images like the one shown here were analyzed. Th is photo 

shows the products of chemical reactions that are used to identify 

the nucleotide sequence of a piece of DNA. Scientists used these to 

determine, base by base, the DNA sequence of the human genome. 

Other goals of the Human Genome Project included identifying 

all of the human genes and making them available for study. Because 

such scientifi c goals have consequences for society, there are also 

groups of researchers that explore and monitor the ethical and social 

impacts of these scientifi c achievements. 

240 MHR • Unit 2 Genetic Processes 

Complex Patterns of Inheritance

Launch Activity 

Assembling a Mini-Genome 

Th e 46 chromosomes that make up our genome contain over 

3 000 000 000 base pairs. Each chemical reaction that is used to determine 

the sequence of DNA can only provide the sequence of a few hundred 

bases at a time. Th erefore, to determine the DNA sequence of the human 

genome, scientists all over the world worked together to analyze millions 

of DNA sequencing reactions. Th ey then assembled the DNA sequence 

of the human genome by piecing together the much smaller fragments of 

sequences. In this activity, you will model how scientists did this. 

gel lanes 

ultraviolet light 

T T TTTT T T TT T TT T TT TT T T TT T 

140 

T 

A AAAA AAAA A A A A A AAA AA A A A AA A A 

A C C CCC CC C C CCC C C C C CC 

C 

G G G G G G G GG GGG 

150 160 170 180 190 200 210 220 

gel 

detector computer 

output 

The products of a DNA sequencing reaction are modified so they are visible 

under ultraviolet light. They are then separated in each lane of a gel-like material. 

The information in each lane is sent to a computer, which provides output 

in the form of a printout of the sequence of bases in a piece of DNA. Recall 

that nucleotides are often identified by their bases. For these data, red bands 

represent thymines, green bands represent adenines, blue bands represent 

cytosines, and black bands represent guanines. 

Materials 

• paper DNA fragments 

• tape 

Procedure 

1. Obtain the sequence of DNA that you are to work with from 

your teacher. 

2. With your classmates, construct one continuous segment of 

sequenced DNA from your individual fragments by matching 

overlapping sections and taping them into place. 

Questions 

1. How did you decide how to match and link the fragments together? 

2. How important was it to collaborate and discuss your results with 

other class members in order to obtain the full sequence? 

3. How important do you think it was for scientists to develop a 

systematic and organized approach to sequencing the human genome? 

How do you think computers played a role? 

Chapter 6 Complex Patterns of Inheritance • MHR 241

Key Terms 

SECTION 

6.1 

incomplete dominance 

codominance 

heterozygous advantage 

continuous variation 

polygenic trait 


a condition in which 

neither allele for a gene 

completely conceals the 

presence of the other; it 

results in intermediate 

expression of a trait 

Figure 6.1 When red 

(C R C R ) flowers and white 

(C W C W ) flowers of the 

snapdragon are crossed, 

the resulting offspring have 

an intermediate phenotype, 

pink flowers (C R C W ). In 

the F 2 generation, all three 

phenotypes are observed. 


Beyond Mendel’s Observations of Inheritance 

Much of today’s genetics research uses sophisticated technologies to study cellular 

processes at the level of individual molecules and atoms. In addition, international 

research collaborations and multi-million-dollar budgets are now common. Th ink of 

what a stark contrast this is to Mendel’s experiments. It is astounding that Mendel’s 

basic and, at times, simple observations led him to infer patterns of inheritance that 

still form the basis of our current understanding of heredity. 

As more sophisticated experimental technologies became available, scientists realized 

that patterns of inheritance are more complicated than what Mendel proposed. Some 

patterns result in phenotypes that are between dominant and recessive phenotypes. 

Other patterns result in phenotypes that are created when both alleles for a trait are 

equally expressed. 

Incomplete Dominance 

Incomplete dominance describes a condition in which neither of the two alleles for the 

same gene can completely conceal the presence of the other. As a result, a heterozygote 

exhibits a phenotype that is somewhere between a dominant phenotype and a recessive 

phenotype. One example is the fl ower colour of snapdragons (Antirrhinum majus). 

As you can see in Figure 6.1, a cross between a true-breeding red-fl owered plant and 

a true-breeding white-fl owered plant produces off spring with pink fl owers in the 

F 1 generation. If the F 1 plants are allowed to self-fertilize, the F 2 generation will 

include off spring with all three phenotypes—red, pink, and white. Th e Punnett square in 

Figure 6.1 predicts that all three phenotypes will be observed in the F 2 generation in a ratio 

of 1:2:1 (red:pink:white), which is what is observed experimentally. In true Mendelian 

inheritance, we would have predicted a phenotypic ratio of 3:1. Nevertheless, the alleles 

for fl ower colour do segregate according to Mendel’s law of independent assortment. 

When representing incomplete dominance, upper-case and lower-case letters are 

not usually used to represent the alleles, since neither allele is dominant over the other. 

One way to represent incomplete dominance is by using superscripts. In the example of 

snapdragon fl ower colour, both alleles aff ect the colour of the fl ower, C. Th e two alleles 

are represented as superscripts, R for red (C R ), and W for white (C W ). Lower-case 

letters are only used to represent a recessive allele. 

P generation F 1 generation F 2 generation 

red 

× 

white 

C R C R 

C W C W 

gametes 

C R 

C W 

self-fertilization 

of F 1 offspring 

pink 

C R C W 

C R 

C W 

C R C W 

C R C R 

C R C W 

C R C W 

C W C W

Incomplete Dominance and Human Disease 

Th ere are many examples of genetic disorders in humans that exhibit incomplete 

dominance. For example, there is a genetic disorder, called familial hypercholesterolemia, 

that prevents tissues from removing low-density lipoproteins (LDL) 

from the blood and causes very high levels of cholesterol in the bloodstream. In the 

majority of cases, the disorder is due to a mutation in the LDLR gene. LDL particles 

transport molecules like cholesterol throughout the body. Th e mutated version of the 

LDLR gene no longer produces a protein that interacts with LDL particles and removes 

them from the bloodstream. Th is disorder has an autosomal dominant inheritance 

pattern. So, an individual only requires one allele of the mutated form of the gene to 

show symptoms of the disorder. However, if the allele for the normal form of the gene 

is present, symptoms of the disease will not be as severe. People who are homozygous 

dominant for the trait have six times the normal amount of LDL in their blood and 

may have a heart attack by the age of 2. Heterozygotes have about twice as much 

cholesterol in their blood and may have a heart attack by the age of 35. 

Scientists are now fi nding that identifying the patterns of inheritance for many 

traits is not as straightforward as fi rst thought. Today’s more accurate techniques 

are showing that, in some cases, what had been identifi ed as a dominant inheritance 

pattern may actually be incomplete dominance. As a result, an individual who is 

heterozygous for a trait is not exactly the same as an individual who is homozygous 

dominant for the trait. 

Codominance 

Codominance is a situation in which both alleles are fully expressed. A roan animal is 

an excellent, visible example of codominance. A roan animal is a heterozygote in which 

both the base colour and white are fully expressed. If you look closely at the individual 

hairs on a roan animal, such as the cow in Figure 6.2, you will see a mixture of red 

hairs and white hairs. One allele is expressed in the white hairs, and the other allele is 

expressed in the red hairs. 

Figure 6.2 A roan cow is the product of a mating between a red cow and a white cow. The red 

and white hairs may be present in patches, as shown here, or be completely intermingled. 

codominance the 

condition in which 

both alleles for a trait 

are equally expressed 

in a heterozygote; both 

alleles are dominant 


heterozygous 

advantage a survival 

benefit for individuals 

who inherit two 

different alleles for the 

same trait 

Figure 6.3 Normal red 

blood cells are flat and 

disk-shaped. Sickle-shaped 

cells are elongated and 

“C” shaped. 

Learning Check 

Sickle Cell Anemia 

Sickle cell anemia is one of the most thoroughly studied genetic disorders. Although it 

is oft en described as being the result of autosomal recessive inheritance, it is actually 

an example of codominance. Sickle cell anemia is caused by a specifi c form of the gene 

that directs the synthesis of hemoglobin. Hemoglobin carries oxygen in the blood. 

Th e hemoglobin molecule that is made in individuals with the sickle cell allele leads 

to a C-shaped (or sickled) red blood cell. Th ese misshaped red blood cells, like the 

one shown in Figure 6.3, do not transport oxygen eff ectively because they cannot pass 

through small blood vessels. Th is leads to blockages and tissue damage. 

Th e allele for normal hemoglobin is represented as Hb A , and the allele for sickle 

cell hemoglobin is represented as Hb S . As shown in Figure 6.4, individuals who are 

homozygous (Hb S Hb S ) have sickle cell anemia. Individuals who are heterozygous 

(Hb A Hb S ) have some normal and some sickled red blood cells. Th ese individuals are 

said to have the sickle cell trait, but they rarely experience any symptoms. In fact, 

having the sickle cell trait can be an advantage, because these heterozygotes are more 

resistant to malaria. Malaria is a life-threatening disease caused by a parasite that 

is transmitted to humans through mosquito bites. Th e parasite infects the liver and 

eventually the red blood cells. Th e sickling of red blood cells is thought to prevent the 

parasites from infecting the cells. Resistance to malaria is very benefi cial in certain 

parts of Africa, where deadly epidemics can occur. Th e sickle cell trait is an example 

of the principle of heterozygous advantage, which describes a situation in which 

heterozygous individuals have an advantage over both homozygous dominant and 

homozygous recessive individuals. 

sickle cell 

trait 

Hb A Hb A 

normal 

Hb A Hb S 

sickle cell 

trait 

1. Distinguish between incomplete dominance and 

codominance. 

2. Why do geneticists use notations like CW and CR to 

describe incomplete or codominant alleles instead of 

using W and w or R and r ? 

3. A plant that produces white fl owers is crossed with 

a plant that produces purple fl owers. Describe the 

phenotype of the off spring if the inheritance pattern 

for fl ower colour is 

a. incomplete dominance 

b. codominance 


Hb A Hb S 

sickle cell 

trait 

Hb S Hb S 

sickle cell 

anemia 

sickle cell 

trait 

Hb A 

Hb S 

Hb A 

Hb A Hb A 

Hb A Hb S 

Figure 6.4 When a man and a woman are both heterozygous for the sickle cell 

gene, there is a one in four chance that they will have a child with sickle cell anemia. 

Hb S 

Hb A Hb S 

Hb S Hb S 

4. Th e frequency of the appearance of the sickle cell 

allele in human populations is much higher in 

Africa than in most other areas of the world. What 

has been proposed to explain this observation? 

5. Provide two pieces of evidence that support the idea 

that some inheritance patterns are more complex 

than those originally proposed by Mendel. 

6. Scientists fi rst thought that sickle cell anemia was 

inherited as an autosomal recessive allele. What led 

them to identify the true inheritance pattern of 

the disease?

Multiple Alleles 

Th e traits you have studied so far have all been controlled by one gene with two alleles, 

such as the fl ower colour in pea plants. Many traits in humans and other species are 

the result of the interaction of more than two alleles for one gene. A gene with more 

than two alleles is said to have multiple alleles. As you know, any individual has only 

two alleles for each gene—one allele on each homologous chromosome. However, many 

diff erent alleles for a gene can exist within the population as a whole. 

Human Blood Groups 

Do you know what blood type you are? In humans, a single gene 

determines a person’s ABO blood type. Th is gene determines what type 

of an antigen protein, if any, is attached to the cell membrane of red blood 

cells. An antigen protein is a molecule that stimulates the body’s immune 

system. Th e gene is designated I, and it has three common alleles: I A , I B , 

and i. As shown in Figure 6.5, the diff erent combinations of the three alleles 

produce four diff erent phenotypes, which are commonly called blood 

types A (I A I A homozygotes or I A i heterozygotes), B (I B I B homozygotes or 

I B i heterozygotes), AB (I A I B heterozygotes), and O (ii homozygotes). Th e 

I A allele is responsible for the presence of an A antigen on the red blood 

cells. Th e I B allele is responsible for the presence of the B antigen, and 

the i allele results in no antigen. Of the three alleles that determine blood 

type, one (i ) is recessive to the other two, and the other two (I A and I B ) 

are codominant. 

Rabbit Coat Colour 

Another example of multiple alleles involves coat colour in rabbits, as shown in 

Figure 6.6. Th e gene that controls coat colour in rabbits has four alleles: agouti 

(C ), chinchilla (c ch ), Himalayan (c h ), and albino (c). In that order, each allele 

is dominant to all the alleles that follow. Th e order of dominance sequence can 

be written as C > c ch > c h > c, where the symbol > means is dominant to. 

chinchilla 

agouti 

Figure 6.6 Rabbits have multiple alleles for coat colour, with four possible phenotypes. 

Predict the possible genotypes for each rabbit. 

Possible alleles from male 

l A 

or 

l B 

or 

i 

Possible alleles from female 

lA lB or or i 

l A l A l A l B l A i 

l A l B l B l B l B i 

l A i l B i ii 

blood types A AB B O 

Figure 6.5 Different combinations 

of the three I alleles result in four 

different blood types: type A, 

type B, type AB, and type O. 

albino 

Himalayan 


Figure 6.7 There are seven 

different alleles for clover 

leaf pattern. 

Sample Problem 

v/v 

vl/vl vh/vh vf/vf vba/vba Using a Punnett Square to Analyze Inheritance of Multiple Alleles 

Problem 

If a man has type O blood and a woman has type B blood, what possible blood types 

could their children have? If this couple has six children, all with type B blood, what 

could you infer about the woman’s genotype? 

What Is Required? 

You are asked to determine all possible blood types of the children and the possible 

genotype of the mother based on all the children having type B blood. 

What Is Given? 

Th e man has blood type O, the woman has blood type B. 

Plan Your Strategy Act on Your Strategy 

Determine the possible genotypes of 

the man and the woman. 

Make Punnett squares for all the 

possible combinations of genotypes. 

List all the possible genotypes and 

phenotypes of the children. 

What could be the mother’s genotype 

based on the children being type B? 

Since the man has blood type O, his genotype must be ii. 

Th e woman has blood type B, so her genotype could be 

either I B I B or I B i. 

father 

father 

mother 

I B I B 

i I B i I B i 

i I B i I B i 

mother 

I B i 

i I B i ii 

i I B i ii 

Th e children could have genotype I B i, resulting in type B 

blood, or genotype ii, resulting in type O blood. 

Th e mother`s genotype is most likely I B I B . 

Check Your Solution 

Th e only genotype that produces type O blood is ii. To have type B blood, the woman must 

have at least one I B allele. Her second allele could be either I B or i. Since all of the children 

had to receive an i allele from their father, they must have inherited an I B allele from their 

mother. Since all of the children have type B blood, the mother is most likely I B I B . 


Clover Leaf Patterns 

Th e pattern on the leaves of the clover plant is also controlled by multiple alleles. While 

a single gene is responsible for clover leaf pattern, there are seven diff erent alleles for 

the pattern. Varying combinations of these result in 22 diff erent patterns that can be 

expressed in clover leaves. Patterns for the seven homozygous combinations of alleles 

are shown in Figure 6.7. 

vb/vb vby/vby

Practice Problems 

1. If a man has type AB blood and a woman has type 

A blood, what possible blood types could their 

children have? 

2. A baby has blood type AB. If the baby’s mother 

has blood type B, what blood type(s) could the 

father have? 

3. A couple just brought home their new baby from 

the hospital. Th ey begin to suspect that the hospital 

switched babies, and the baby they brought home is 

not theirs. Th ey check the hospital records, and fi nd 

that the man’s blood type is B, the woman’s blood 

type is AB, and the baby’s blood type is O. Explain 

why the parents are or are not justifi ed in their 

concern about this baby. 

4. Four children have the following blood types: A, B, 

AB, and O. Could these children have the same two 

biological parents? Explain. 

5. Some of the off spring of a chinchilla rabbit and a 

Himalayan rabbit are albino. What are the genotypes 

of the parents? 

6. A chinchilla rabbit with genotype cchch is crossed 

with a Himalayan rabbit with genotype chc. What 

is the expected ratio of phenotypes among the 

off spring of this cross? 

7. Could a mating between a chinchilla rabbit and 

an albino rabbit produce a Himalayan rabbit? 

Explain your reasoning. Your answer should include 

reference to the genotypes and phenotypes of the 

parents and the off spring. 

Environmental Effects on Complex Patterns of Inheritance 

Environmental conditions oft en aff ect the expression of traits. For example, 

some genes are infl uenced by temperature. Th e dark colour in Himalayan 

rabbits, shown in Figure 6.8, is on the cooler parts of their bodies: the 

face, ears, tails, and feet. In these animals, dark colouring is the 

result of a gene that is only active below a certain temperature. 

One way to study the eff ect of the environment on expression 

of traits is to study genetically identical organisms placed 

in diff erent surroundings. For example, identical twins are 

genetically identical. Diff erences in the activity of their 

genes can be due to environmental eff ects. 

Figure 6.8 The dark ears, nose, feet, and tails 

of Himalayan rabbits are thought to be caused 

by lower body temperature in these areas. 

8. In one family, all three siblings have type B blood. 

a. Use Punnett squares to show how two diff erent 

sets of parent genotypes could produce this result. 

b. Which of the two sets of potential parents in your 

answer to (a) is more likely to be the parents of 

these siblings? Explain why. 

9. In dogs, coat colour is determined by the interaction 

between three alleles. Th e allele AS produces a dark 

coloured dog, a y produces a sandy coloured dog, and 

at produces a spotted dog. Th e order of dominance 

is AS > a y > at . Determine the following from the 

pedigree below. 

a. the genotypes of the parents (I-1 and I-2) 

b. the probability of an off spring from the mating 

between individuals II-2 and II-3 having spots 

c. the possible genotypes of individual II-1 

I 

II 

1 

1 

2 

2 

3 

Key 

dark 

coloured 

sandy 

coloured 

spotted 

10. A dark coloured dog is mated with a sandy coloured 

dog. Th e litter of puppies includes a dark puppy, a 

sandy puppy, and a spotted puppy. Use a Punnett 

square to determine the possible genotypes of the 

off spring and the parents. Note: Use the information 

about dog coat colour inheritance from question 9 

to answer this question. 

SuggestedInvestigation 

Plan Your Own Investigation 

6-A, Environmental 

Influences on the 

Production of Chlorophyll 



a range of variation in 

one trait resulting from 

the activity of many 

genes 

polygenic trait a trait 

that is controlled by 

more than one gene 

Polygenic Inheritance 

Mendel carefully selected plants that had very diff erent heights so there would be 

no question about phenotypes. However, there are traits that exhibit continuous 

variation. Th ese are traits for which the phenotypes vary gradually from one extreme 

to another. 

Some examples of traits that show continuous variation include height and skin 

colour in humans, ear length in corn, and kernel colour in wheat. Continuous traits 

cannot be placed into discrete categories because they vary over a continuum. For 

example, height in humans varies over a wide range of values. People cannot be 

categorized as only short or tall. 

Traits that exhibit continuous variation are usually controlled by more than one 

gene. For some traits this can involve several genes. Traits that are controlled by many 

genes are called polygenic traits. A group of genes that all contribute to the same trait 

is called a polygene. Each dominant allele contributes to the trait. Recessive alleles do 

not contribute to the trait. For skin colour, the more dominant alleles a person has, 

the darker their skin. Th e graph in Figure 6.9 shows that there are more intermediate 

phenotypes than extreme phenotypes. 

Frequency 

Skin Colour 

AaBbcc 

AaBbCc 

aaBbCC aaBBCC 

AabbCc AAbbCc AAbbCC 

aaBbCc AabbCC AABBcc 

Aabbcc AAbbcc AABbcc AaBbCC AaBBCC 

aaBbcc aaBBcc aaBBCc AaBBCc AABbCC 

aabbcc aabbCc aabbCC AaBBcc AABbCc AABBCc AABBCC 

0 1 2 3 4 5 6 

Number of Dominant Alleles 

Figure 6.9 This graph shows possible shades of skin colour from three of the sets of alleles that 

determine this trait. 

Predict the effect of more gene pairs on the possible phenotypes. 

Activity 6.1 Identifying a Polygenic Trait 

A polygenic trait is one that is controlled by more than 

one gene and shows continuous variation. In this activity, 

you will choose one human trait that you hypothesize is 

controlled by more than one gene and shows continuous 

variation. You will then collect data from your classmates to 

test your hypothesis. 

Materials 

• ruler or measuring tape (if necessary) 

• graph paper 

Procedure 

1. In your group, choose one human trait that you think 

is polygenic. Make sure your choice is one for which 

data can be easily and respectfully collected from your 

classmates. 


2. Construct a data table to organize your data. Keep in 

mind that you will be measuring a particular trait and 

recording the number of times that measurement 

of the trait occurs. 

3. Collect your data from your classmates. 

4. Create a line graph of your data. Your graph should 

refl ect the actual measurements you took and the 

frequency of the values that you measured. 

Questions 

1. Do your data support your hypothesis that the trait 

you selected is polygenic? Explain. 

2. How could this activity be improved to provide a 

clearer picture of the inheritance pattern of the trait 

you selected?

STSE 

Quirks & 

Quarks 

THIS WEEK ON QUIRKS & QUARKS 

with BOB MCDONALD 

Selecting for Genetic Defects 

Most scientists agree that certain inherited 

traits are favoured when they improve 

chances for survival. But what if improved 

chances for survival are due to a mutation 

associated with hereditary deafness? Bob 

McDonald interviewed Dr. David Kelsell, 

Professor of Human Molecular Genetics at 

Queen Mary College, University of London, 

to discuss this question. 

Good News and Bad News 

Scientists have known which gene is 

associated with most cases of hereditary 

deafness since 1996. A specifi c mutation 

in gene Cx26 (Connexin 26) is the culprit. 

People carrying one copy of the gene with 

the deafness mutation have normal hearing, 

while people with two copies are deaf. 

Because the deafness mutation in Cx26 is 

found in many human populations around 

the world, Dr. Kelsell‘s team suspected it must 

convey some kind of survival advantage. 

Outer Ear Middle Ear Inner Ear 

Curiously, it does seem to. Individuals with 

the deafness mutation also had skin that 

was marginally thicker than the skin of 

people who do not have the mutation. 

Tests were conducted on the mutated 

skin cells to see whether the deafness 

mutation helped skin form a better barrier 

against bacterial invasions, and whether the 

aff ected skin cells healed diff erently. Results 

showed that the thicker skin could off er 

better protection and that healing could 

occur much more quickly. 

Therefore, while there is a risk of deafness 

if two copies of the mutation are inherited, 

one copy seems to provide better protection 

against skin diseases. As the Quirks host said, 

“How is it that one gene can aff ect two such 

diff erent and seemingly unrelated things— 

deafness and thickness of skin?” “This,” said 

Dr. Kelsell, “is one of the great mysteries.” 

tympanic membrane 

(or eardrum) 

cochlea 

The protein product of the Cx26 gene is needed 

for movement of potassium ions between cells 

in the cochlea of the inner ear. This movement 

of ions is needed for proper hearing. 

QUESTIONS 

Related Career 

Human molecular geneticists 

study genetic processes in 

humans, particularly how genes 

function in human disease. 

These scientists are referred to as 

“molecular” geneticists because 

they look at the structure 

and function of genes at the 

molecular level. For example, 

they look for the eff ects of a 

genetic mutation by studying 

the mutant protein that is 

formed from it and how it aff ects 

processes in the body. 

1. Is deafness due to a Cx26 mutation inherited by 

an autosomal recessive or autosomal dominant 

pattern? Explain your answer. 

2. Explain why the deafness mutation of Cx26 is an 

example of heterozygous advantage. 

3. Use the Internet or print resources to fi nd out more 

about the work of human molecular geneticists. 

What essential skills would you need in order to 

work in this fi eld? 


Section 6.1 REVIEW 

Section Summary 

• Incomplete dominance leads to the expression of an 

intermediate phenotype. In the case of codominance, 

both alleles are fully expressed. 

• Although an individual has only two alleles for any 

gene, multiple alleles for a gene may exist within the 

population. 

Review Questions 

1. K/U A white-fl owered plant is crossed with a 

red-fl owered plant. What is the likely mode of 

inheritance if the off spring produced are 

a. plants with pink fl owers? 

b. plants with fl owers that are red and white spotted? 

2. K/U Describe a human genetic disorder that results 

from incomplete dominance. Explain why it is 

classifi ed as incomplete dominance. 

3. T/I In radishes, colour is controlled by two alleles 

that show incomplete dominance. When pure-breeding 

red radishes are crossed with pure-breeding white 

radishes, purple radishes are produced. 

a. Provide the genotypes for the three colours of 

radishes. 

b. What is the phenotypic ratio expected when two 

purple radishes are crossed? 

4. T/I A farmer crosses a black rooster with a white 

hen. Of the seven off spring, three are black, three are 

speckled black and white, and one is white. 

a. What can you infer about the inheritance patterns 

of the alleles for white and black feathers? 

b. Given the inheritance pattern you described in 

part (a), what are the expected genotypes and 

phenotypes of the off spring produced by a cross 

between a speckled hen and a black rooster? 

5. C Th e colour of an organism is controlled by one 

gene with two alleles: an allele that produces a blue 

colour and an allele that produces a yellow colour. 

Using genetic notations, describe the diff erences in 

genotypes and phenotypes of the organisms produced 

by crossing a true-breeding blue organism with a 

true-breeding yellow organism for the following three 

inheritance patterns. Use drawings in your answers. 

a. blue is dominant over yellow 

b. blue and yellow are incompletely dominant 

c. blue and yellow are codominant 


• Environmental conditions can infl uence the expression of 

certain traits. 

• Polygenic traits are controlled by more than one gene 

and can usually be identifi ed by continuous variation in 

phenotype. 

6. T/I Th e following pedigree shows the inheritance 

pattern of sickle cell anemia in a family. Known 

carriers of the sickle cell gene are noted. However, not 

all individuals have been tested for the sickle cell allele. 

I 

II 

III 

1 

1 

2 

2 

3 

1 

4 

Key 

normal 

phenotype, 

but not 

tested 

sickle cell 

trait (carrier) 

sickle cell 

anemia 

phenotype 

a. Determine the genotype of each individual in the 

pedigree. If there are any you cannot be certain of, 

explain why. 

b. Determine the probability that individuals II-3 and 

II-4 will have another child with sickle cell anemia. 

7. T/I A chinchilla rabbit is crossed with a Himalayan 

rabbit, producing an albino rabbit. 

a. Determine the genotypes of the parents. 

b. Identify other phenotypes expected from this cross 

and give the predicted phenotypic ratios. 

8. A Your friend has bred her female albino rabbit 

with her male Himalayan rabbit. “I’m hoping I’ll get 

some agouti rabbits,” she says. What are her chances 

of getting an agouti rabbit? Explain. 

9. C “Human ABO blood grouping is an example 

of the eff ects of multiple alleles, codominance, and 

dominance/recessiveness.” Use a table or graphic 

organizer to explain this statement. 

10. A Siamese cats that spend their lives indoors tend 

to have lighter-coloured fur than Siamese cats that live 

outdoors. What genetic process could account for this 

change? 

11. K/U What evidence is there that skin colour in 

humans is a polygenic trait?

SECTION 

6.2 

Cross a plant with purple 

flowers and long pollen to 

a plant with red flowers 

and round pollen. 

Observe the phenotypes 

of the F 1 offspring. 

Allow the F 2 offspring 

to self-fertilize. 

Observe the phenotypes 

of the F 2 offspring. 

Inheritance of Linked Genes 

As you have learned, there is no apparent interaction between non-homologous 

chromosomes during meiosis. Th e movement of each pair of homologous chromosomes 

is independent of the movement of other pairs of homologous chromosomes. Th is agrees 

with Mendel’s law of independent assortment. Recall that this law states that the alleles 

for a gene segregate independently of the alleles for other genes during gamete formation. 

However, Walter Sutton’s research showing that alleles segregate in the same way that 

homologous chromosomes do implies a very important point: alleles on the same 

chromosome do not assort independently. Th erefore, they do not follow the Mendelian 

inheritance patterns that have been discussed in this unit. It turns out that some genes 

are inherited together. Th erefore, some traits are oft en inherited together or are “linked.” 

Linked Genes 

In 1905, William Bateson and Reginald Punnett carried out the fi rst study that showed 

the movement of alleles that are on the same chromosome. Using sweet peas, they 

tracked the inheritance pattern of two traits: fl ower colour and pollen shape. Th ey 

knew that purple fl owers were dominant to white fl owers, and that long pollen shape 

was dominant to round pollen shape. Th eir results are shown in Figure 6.10. All four 

phenotypes that are predicted using a Punnett square were present in the F 2 generation. 

However, there were far more of the phenotypes from the parental generation. Th is 

suggested that the gametes produced by the parental generation, PL and pl, tended to 

assort together rather than independently when producing the F 2 off spring. Genes that 

do not assort independently are oft en called linked genes. 

purple flowers, 

long pollen 


long pollen 


long pollen 


long pollen 

Phenotype 

× 

× 

Red flowers, 

round pollen 


long pollen 


long pollen 

purple flowers, purple flowers, red flowers, 

long pollen round pollen long pollen 

15.6 : 1.0 : 1.4 : 

Figure 6.10 A dihybrid cross between two sweet pea plants does not 

produce the expected phenotypic ratio of 9:3:3:1. These results support the 

theory that alleles on the same chromosome do not assort independently. 

Identify Provide the genotypes of the F2 offspring. 


long pollen 

red flowers, 

round pollen 

4.5 

Key Terms 

linked genes 

sex-linked trait 

linked genes genes 

that are on the same 

chromosome and that 

tend to be inherited 

together 

Genotype 

PPLL × ppll 

PpLl 

Meiosis 

PL and pl gametes—more frequent 

Pl and pL gametes—less frequent 

Fertilization 

F 2 offspring having phenotypes of 

purple flowers, long pollen or red 

flowers, round pollen occurred more 

frequently than expected from Mendel’s 

law of independent assortment. 


Figure 6.11 In most of the 

gametes formed, there is 

no crossing over—they 

maintain the linkage of the 

alleles. In a small minority 

of gametes, crossing 

over occurs and alleles of 

previously linked genes 

become unlinked. 

Describe why alleles 

of genes that are closer 

together on a chromosome 

are more likely to remain 

linked during meiosis. 


Crossing Over and the Inheritance of Linked Genes 

A chromosome may contain up to a few thousand genes. All of the genes on any one 

chromosome are called a linkage group because they tend to be inherited together. 

However, linked genes do not always stay linked—researchers have found that they 

segregate on a regular basis. Th is is due to the process of crossing over, which you 

learned about in Chapter 5. Recall that crossing over occurs in prophase I of meiosis, 

when non-sister chromatids exchange pieces of chromosomes. 

Suppose you are studying two genes that are on the same chromosome and, 

therefore, linked. Crossing over between homologous chromosomes can occur. As 

shown in Figure 6.11, this will result in the alleles of the linked genes no longer being 

on the same chromosome. Th e alleles of the previously linked genes are now unlinked. 

Th is means that they will migrate into diff erent gametes. Th e result is that instead of 

two types of gametes being produced, four diff erent types of gametes will be produced 

in diff ering proportions. Th ere are fewer gametes with the recombined alleles because 

crossing over is a random event and it occurs infrequently. 

A 

B 

no crossing over 

during meiosis 

97% 

a 

b 

A 

B 

four types of gametes in unequal proportions 

a 

b 

crossing over 

during meiosis 

a 

B 

3% 

recombinant gametes 

Using Gene Linkage for Chromosome Mapping 

Scientists have discovered that alleles for a given pair of linked genes will separate 

with a predictable frequency and that this frequency is diff erent for diff erent pairs of 

linked genes. Th e frequency depends on how close the alleles of the linked genes are 

positioned on a chromosome. Crossing over occurs more frequently between alleles that 

are far apart on a chromosome than between alleles that are close together. Th erefore, 

a given pair of linked genes will separate more frequently than the alleles for another 

pair of linked genes if their alleles are farther apart on the chromosome. Th is process 

of determining the relative locations of genes on chromosomes is called chromosome 

mapping. Th ese types of linkage studies are useful for mapping chromosomes in species 

that reproduce rapidly and produce many off spring, such as plants and insects. But 

chromosome mapping is not useful in mapping human chromosomes. Chromosome 

mapping of humans only became possible when modern techniques that allow 

scientists to directly see the chromosomes became available. 

A 

b


7. What are linked genes? 

8. How are linked genes found experimentally? 

9. What is chromosome mapping? How is gene linkage 

used in chromosome mapping? 

10. Suppose that two individuals with the genotype AaBb 

are crossed, and the phenotypic ratio produced is 

about 3:1 (A_B_:aabb). Are the genes for the two 

traits linked? Explain. 

Sex-linked Inheritance 

An American biologist named Th omas Hunt Morgan, shown in Figure 6.12, originally did 

not accept Sutton’s chromosome theory of inheritance. In the early 1900s, Morgan chose 

to do research on the fruit fl y, Drosophila melanogaster, to develop a new and alternative 

theory. Morgan chose this organism because it is economical to maintain, reproduces 

rapidly, and has traits that are fairly easy to characterize. As Morgan collected data, 

however, his results soon convinced him that Sutton’s theories were correct. Nevertheless, 

Morgan’s meticulous research provided additional information about genetic inheritance. 

In 1910, Morgan discovered an unusual white-eyed male among his fl y population. 

He crossed the white-eyed male with a normal red-eyed female. All the F 1 generation 

had red eyes. Th is seemed to indicate that normal red eyes are dominant to the 

white-eye mutation. When Morgan crossed a male and female from the F 1 generation, 

however, the results surprised him. All the females of the F 2 generation had red eyes, 

half the F 2 males had red eyes, and half the F 2 males had white eyes. Th e discovery that 

the gene for eye colour was connected to gender led Morgan to conclude that the gene 

for eye colour is located on the X chromosome. 

Like humans, female fruit fl ies have two X chromosomes, while males have one 

X chromosome and one Y chromosome. Th e fruit fl y F 1 data indicated that the 

white-eye phenotype is recessive, since it was masked in all of the off spring in that 

generation. How did white eyes reappear in only the male fruit fl ies in F 2, but remain 

masked in the female fl ies? Th e answer lies in the sex-linked genes—the genes that are 

located on the X and Y chromosomes. 

Traits that are controlled by genes on either the X or Y chromosome are called 

sex-linked traits, because they are linked to the genes that determine sex. Th ey are 

identifi ed by their diff erent rates of appearance between males and females. 

A 

female 

male 

11. Some traits are described as being due to sex-linked 

genes. Use your knowledge of chromosomes to 

explain what this means. 

12. Many genetic tests are based on analyzing genes that 

are linked to alleles that cause disease. Explain how 

testing for a linked gene could lead to an incorrect 

diagnosis. 

B 

sex-linked trait 

a trait controlled by 

genes on the X or 

the Y chromosome 

Figure 6.12 (A) Drosophila 

melanogaster traits that are 

often studied include eye 

colour and wing size and 

shape. Males and females 

can be easily identified. 

(B) Thomas Morgan’s 

ground-breaking research 

into the genetics of fruit 

flies was recognized 

in 1933, when he was 

awarded the Nobel Prize in 

physiology or medicine. 


SuggestedInvestigation 

ThoughtLab Investigation 

6-B, Sex-Linked Crosses 

white-eyed 

male 

X r Y 


Sex-linked Genes 

Th e X and Y chromosomes, although paired together during meiosis and for 

karyotyping purposes, have very little homologous DNA. Th e X and Y chromosomes 

in humans have only a few genes in common. Th e human X chromosome is estimated 

to contain about 2000 genes, while the Y chromosome contains fewer than 100. Th e 

most important genes are the sex-determination genes. For all other genes on the X 

chromosome, females have two copies, while males have only one. Th is allows for the 

diff erence in the expression of traits for genes that are found on the X chromosome, 

which are oft en called X-linked genes. By comparison, only a few genes are known to 

be Y-linked, because there are signifi cantly fewer genes on the Y chromosome. When 

considering sex-linked traits, the allele on the sex chromosome is shown as 

a superscript to an X or a Y. 

The Red and White Eyes of Fruit Flies 

Red and white eyes were the fi rst sex-linked trait explored by Morgan. Th e possible 

genotypes and phenotypes in both males and females are listed in Table 6.1. XR indicates red eyes, which is the dominant phenotype, and Xr indicates white eyes, 

which is the recessive phenotype. Notice that female fl ies may be a carrier for the 

white-eye phenotype. However, if the allele for white eyes is present in males, it will 

always be expressed. Th is means that X-linked traits are exhibited more oft en in males. 

Punnett squares can be used to predict the outcome of crosses that involve sex-linked 

traits. Figure 6.13 represents some of the crosses that Morgan studied. 

Table 6.1 Possible Genotypes and Phenotypes for Drosophila Eye Colour 

Genotype Phenotype 

XRXR Female with red eyes (homozygous dominant) 

XRX r 

Female with red eyes (heterozygous, carrier for the white-eyed allele) 

X r X r 

Female with white eyes (homozygous recessive) 

XRY Male with red eyes 

X r Y Male with white eyes 

X r 

Y 

X R 

X R X r 

X R Y 

red-eyed 

female 

X R X R 

X R 

X R X r 

X R Y 

F 1 male 

X R Y 

X R 

Y 

X R 

X R X R 

X R Y 

F 1 female 

X R X r 

Figure 6.13 In Morgan’s experiment on tracking the inheritance pattern of a sex-linked trait, 

the white-eye phenotype was passed from the father in the P generation through the daughter 

in the F1 generation. 

Predict the genotype and phenotype ratios of the offspring created by crossing a white-eyed 

male and a heterozygous female. 

X r 

X R X r 

X r Y

Sex-linked Traits in Humans 

Some examples of sex-linked traits in humans are listed in Table 6.2. As you can see, 

many are genetic disorders. If a disorder is X-linked dominant, aff ected males pass the 

allele only to daughters, who have a 100 percent chance of having the disorder. Females 

can pass an X-linked dominant allele to both sons and daughters, all of whom will have 

the disorder. Most sex-linked inherited traits in humans are X-linked recessive traits. 

Th erefore, while the male only needs to inherit one allele to be aff ected, the female 

must inherit both alleles to be aff ected. Th us, X-linked recessive traits aff ect more males 

than females in a family. 

Table 6.2 Sex-linked Traits in Humans 

I 

II 

III 

Condition Inheritance Pattern Description 

Red-green colour vision 

defi ciency (CVD) 

Duchenne muscular 

dystrophy 

X B Y 

X B Y 

X B X B 

X-linked recessive Cannot distinguish between certain 

shades of red and green 

X-linked recessive Progressive weakening of muscles and 

loss of coordination 

Hemophilia X-linked recessive Cannot produce a necessary blood 

clotting factor 

Adrenoleukodystrophy X-linked recessive A build-up of fatty acids that causes 

progressive brain damage and death 

X-linked severe combined 

immunodefi ciency (SCID) 

X-linked recessive Decreased immune response due to 

low white blood cell counts 

X-linked hypophosphatemia X-linked dominant Soft ening of bone, which leads to bone 

deformity 

Hairy ears Y-linked Hair grows on the outside of the ears 

Colour Vision Defi ciency: An X-linked Recessive Trait 

In humans, there are inherited forms of colour vision defi ciency (CVD). Individuals 

aff ected by CVD have varying degrees of diffi culty distinguishing between diff erent 

colours or shades of colours. One form, called red-green CVD, is an X-linked recessive 

disorder. Individuals with red-green CVD have diffi culty distinguishing between 

shades of red and green. To track the inheritance patterns of sex-linked traits in 

humans, pedigrees are oft en used. Th e inheritance pattern of red-green CVD in one 

family is shown in Figure 6.14. 

X B X B 

X B X b 

X B X b 

X b Y 

X b Y 

X B Y 

X b Y 

X b X b 

Key 

X B X B = normal female 

X B X b = carrier female 

X b X b = CVD female 

X B Y = normal male 

X b Y = CVD male 

Figure 6.14 An X-linked recessive trait like CVD will affect more males than females in a family. 


Figure 6.15 Great Britain’s 

Queen Victoria was a carrier 

for hemophilia. 


Using Punnett Squares to Analyze Sex-linked Inheritance Patterns 

Problem 

Determine the probability that a woman who is a carrier for hemophilia and a man 

without hemophilia will have a child with hemophilia. 


You need to determine the possible genotypes and phenotypes of the off spring to 

determine if any of the children could have hemophilia. 


You know the phenotypes of the parents, and you know that the pattern of inheritance 

is X-linked recessive. 


Assign letters to represent each allele 

for the trait, and then determine the 

genotypes for the parents based on an 

X-linked recessive inheritance pattern. 

Use a Punnett square to predict the 

genotypes of the off spring. 

Since the inheritance pattern is X-linked recessive, 

• let X h = allele for hemophilia 

• let X H = allele for normal blood clotting 

Th e female is a carrier, so her genotype is X H X h . 

Th e male is unaff ected, so his genotype is X H Y. 

male 

X H 

Y 

female 

X H X h 

Complete the Punnett square. female 

X H X h 

Determine the predicted phenotypes 

of the off spring, and the probability of 

producing a child with hemophilia. 

Hemophilia: A Common Sex-linked Trait in Humans 

Hemophilia is a condition that aff ects the body’s ability to produce proteins involved 

in blood clotting. People with hemophilia can suff er serious blood loss from simple 

cuts and bruises. Hemophilia is an X-linked recessive trait that aff ects more than 

3000 individuals in Canada. 

Hemophilia is oft en referred to as the royal disease because it spread among the 

royal families of Europe, through the descendents of Great Britain’s Queen Victoria, 

shown in Figure 6.15. Queen Victoria was a carrier who passed the allele on to some of 

her off spring. Arranged marriages among royalty of Europe were very common until 

the twentieth century. Pedigree analyses can trace the allele for hemophilia throughout 

the royal families of Spain, Russia, and Prussia. 

male 

X H X H X H X H X h 

Y X H Y X h Y 

Th ere is a 25 percent chance of having a child with 

hemophilia (X h Y). All other genotypes produce a child 

with normal blood clotting. 


To check your solution, ensure that the genotypes of the parents accurately represent 

the phenotypes, and that all possible combinations of gametes have been made. 

256 MHR • Unit 2 Genetic Processes


Determining Sex-linked Inheritance Patterns in a Pedigree 

Problem 

Th e pedigree on the right shows the inheritance of red-green 

CVD in a family. Identify the genotype of each family 

member represented in the pedigree. How does the inheritance 

pattern in the pedigree support X-linked inheritance? 


You need to determine the genotype of each individual and 

describe the evidence for X-linked inheritance. 


You know that the pattern of inheritance is X-linked 

recessive, and you have the phenotype of each of the 

individuals (the pedigree). 


Assign letters to represent each allele. Identify possible 

genotypes for each of the phenotypes based on an X-linked 

recessive inheritance pattern. 

Assign all possible genotypes, according to the information 

in the pedigree. 

• At this point, you cannot be certain of the genotypes for 

individuals I-1, II-1, and II-3. Since they are unaff ected 

females, the possible genotypes are X C X c and X C X C . 

Complete the pedigree with genotypes that you can infer 

based on the data in the pedigree. 

• You know that individual II-3 must be X C X c to produce 

a son who has CVD. 

• Individual II-1 must be X C X c . Th e X chromosome she 

received from her father is X c and, since she is unaff ected, 

she must have received X C X c from her mother. 

• You cannot be certain of the genotype of I-2 because 

both genotypes are possible (X C X c and X C X C ), given the 

genotypes of the off spring. 

Describe how the inheritance pattern supports X-linked 

recessive inheritance. 

• let X c = allele for CVD 

• let X C = allele for normal vision 

X C X C = unaff ected female X C Y = unaff ected male 

X C X c = female carrier X c Y = male with CVD 

X c X c = female with CVD 


To check the pedigree, ensure that all the off spring genotypes are possible given the 

genotypes of the parents. 

I 

II 

III 

I 

II 

III 

I 

II 

III 

X C Y 

1 

X C Y 

1 

1 

X c Y 

1 

? X ? 

C Y 

1 2 3 4 

X C X c 

X C Y 

X C Y 

2 3 4 

1 

X C Y 

? 

2 

2 

X c Y 

X C Y 

1 2 3 4 

X C Y 

X c Y 

X C Y 

X C X c 

X c Y 

2 3 4 

? 

X C Y 

Th e allele for CVD is passed from the grandfather (I-1) 

through his unaff ected daughter (II-3) to her aff ected son 

(III-4). Th is pattern is indicative of X-linked recessive 

inheritance. As well, more males are aff ected than females, 

which also indicates X-linked recessive inheritance. 

1 

1 2 3 4 

2 

2 3 4 


Practice Problems 

11. A woman who is a carrier for CVD and a man who 

has CVD decide to have children. 

a. Determine the genotypes of these two people. 

b. What is the expected ratio of genotypes and 

phenotypes among their children? 

12. Th e mother and father of a boy who has CVD both 

have normal colour vision. Use a Punnett square to 

explain how this can occur. 

13. A woman with hemophilia and a man without 

hemophilia decide to have children. What is the 

probability that their sons will have hemophilia? 

14. Nystagmus is a condition in which involuntary eye 

movement leads to poor vision. Th is condition is 

caused by an X-linked recessive allele. Suppose that 

a man and woman, both with normal vision, have 

two children. Th e boy is aff ected with nystagmus, 

and the girl is unaff ected. 

a. Determine the genotype of the parents. 

b. Is it possible to determine the genotypes of the 

children? Why or why not? 

15. A woman has X-linked hypophosphatemia, which 

aff ects bone development. She marries a man with 

normal bone structure. If the woman’s father also 

has normal bone structure, what is the probability 

that the woman and her husband will have a child 

with the disorder? 

16. A true-breeding tan-bodied female fruit fl y is 

crossed with a yellow-bodied male. All of the 

off spring in F1 have tan bodies. In the F2 generation, 

all the females have tan bodies, 50 percent of the 

males have tan bodies, and 50 percent of the males 

have yellow bodies. 

a. Describe the pattern of inheritance for body 

colour in fruit fl ies. Explain your answer. 

Figure 6.16 In cats, the 

alleles for black or orange 

coat are carried on the 

X chromosome. 


b. Determine the genotypes of the fl ies described in 

the F2 generation. 

c. What is the probability of producing tan off spring 

from a yellow female and a tan male? 

17. Given the pedigree below, determine whether 

the pattern of inheritance of this trait is X-linked 

recessive, X-linked dominant, or Y-linked dominant. 

Explain your answer. 

I 

II 

III 

1 2 3 4 

1 2 

1 2 3 4 

18. In one breed of dog, a mutant gene that causes 

hearing impairment is found on the Y chromosome. 

What are the possible phenotypes of off spring from 

each of the following crosses? 

a. a male dog whose father is hearing impaired and 

a female dog whose father is not hearing impaired 

b. a female dog whose father is hearing impaired and 

a male dog whose father is not hearing impaired 

19. Suppose you have one homozygous dominant 

red-eyed female fl y and one white-eyed male 

fl y. What steps would you follow to produce a 

white-eyed female fl y? 

20. Th e allele for short fi ngers is dominant to the allele 

for long fi ngers. What is the genotype of a male who 

has CVD and long fi ngers? If all of his children have 

normal vision and short fi ngers, what is the likely 

genotype of the children’s mother? 

Barr Bodies: Inactive X Chromosomes 

Since females carry two X chromosomes and males only one, why is there no diff erence 

in the expression of X-linked genes between males and females? Th e answer is that 

every cell has only one functioning X chromosome. In every female cell, one of the 

X chromosomes is inactive. Th e inactive X chromosome is condensed tightly into 

a structure known as a Barr body. At an early stage of embryonic development, one 

X chromosome in each cell is deactivated. Which X chromosome is deactivated can 

vary among cells. One visible eff ect of one X chromosome being inactive is the calico, 

or tortoiseshell, coat colour in cats, shown in Figure 6.16. In heterozygous females, 

roughly 50 percent of the cells have an active X chromosome with the allele for black 

coat colour, and 50 percent of the cells have an active X chromosome with the allele for 

orange coat colour. Th is results in a tortoiseshell coat with patches of both black and 

orange. Th e patches of white are the result of the interaction with a diff erent gene.



• Alleles of diff erent genes that are on the same 

chromosome do not assort independently. Th ese genes 

are said to be linked and their associated traits tend to be 

inherited together. 


1. T/I Design an experimental procedure that you 

could follow to determine whether two plant genes are 

linked. 

2. K/U Describe how the process of crossing over 

of non-sister chromatids can aff ect linked genes. 

3. K/U What experimental evidence would lead 

scientists to suspect that two genes are linked? 

4. T/I A chromosome contains three genes, P, Q, 

and R. Th e percentage of gametes produced that have 

the genes separated due to crossing over is shown in 

the table below. 

Linked Genes 

Gametes with 

Genes 

Unlinked Genes (%) 

P and Q 5 

P and R 18 

Q and R 13 

From these data, identify the gene pair with alleles 

that are closest together on the chromosome. Explain 

your answer. 

5. C Draw a diagram that shows how crossing over 

can cause linked genes to become unlinked. 

6. K/U List two features of Drosophila melanogaster 

that make this species a good choice for the study of 

sex-linked inheritance. 

7. T/I A woman with regular vision and a man with 

regular vision have three children, one of whom 

has CVD. 

a. What can you conclude about the genotypes of 

the parents? 

b. What sex is the child who has CVD? How do 

you know? 

8. K/U Describe the possible genotypes of the parents 

of a woman who has hemophilia. Explain your answer. 

• Sex-linked traits are expressed in diff erent ratios by male 

and female off spring because they are determined by the 

segregation of X and Y chromosomes. 

• Although sex-linked genes are linked to the X and Y 

chromosomes, Punnett squares can be used to predict 

genotypes and phenotypes. 

9. A Explain how a girl with Turner syndrome could 

have red-green CVD, even though both of her parents 

have normal vision. 

10. T/I Th e following pedigree was given to a group of 

students to analyze. Th ey believe it indicates X-linked 

recessive inheritance. Do you agree or disagree? 


I 

II 

III 

1 

2 

1 2 

1 

3 

4 

2 

3 4 

5 

6 

5 6 

11. K/U How do pedigrees for autosomal recessive traits 

and X-linked recessive traits diff er? 

12. C A boy has Duchenne muscular dystrophy. His 

mother’s brother also has this disorder. Th e boy’s father 

and his two younger sisters do not appear to be 

aff ected by the disease. Draw a pedigree to illustrate 

the inheritance of Duchenne muscular dystrophy in 

this family. What is the probability that his sisters are 

carriers of the disease? 

13. K/U Th e symptoms associated with X-linked 

dominant diseases are oft en more severe in males. 

Explain. 

14. C Draw a sample pedigree to illustrate inheritance 

of hemophilia in a family. Make sure that your 

pedigree refl ects that particular inheritance pattern. 

15. A Some women are heterozygous for an X-linked 

genetic disorder that results in a non-uniform 

distribution of sweat glands on their skin. Th ese 

women have patches of skin that lack sweat glands and 

patches of skin that have sweat glands. How can the 

Barr body cause this phenomenon? 


Key Terms 

SECTION 

6.3 

bioinformatics 

genomics 

genetic profi le 

Figure 6.17 The Human 

Genome Project achieved 

many milestones and has 

provided a springboard 

for decades of future 

research. Nevertheless, 

this project would not have 

been possible without 

several essential preceding 

discoveries—including 

Mendel’s studies of pea 

plants. 

1865 

1900 00 1913 

1944 1953 1966 

1990 1991 1992 1995 1996 

the Human 

Genome Project 

is launched 

ethical, legal, and 

social implications 

(ELSI) program 

founded 

Gregor Mendel 

discovers discove laws 

of genetics gene 

rediscovery covery 

of Mendel’s Mendel’s 

work ork 

first US genome 

centres established 


The Future of Genetics Research 

Genetics research is continually changing and developing in response to new 

discoveries. Many genetics researchers now focus on obtaining more and more detailed 

information. In addition to wanting to know the sequences of genes that are associated 

with certain inherited traits, investigators want to know how those genes play a role 

in determining those particular traits. Looking for answers to these types of questions 

has led to the development of more sophisticated technologies and equipment, and 

has resulted in new scientifi c fi elds of study. In addition, many studies now require the 

collaboration of scientists from very diff erent disciplines, such as biology, chemistry, 

physics, sociology, bioethics, and political science. 

The Human Genome Project 

In the opener for this chapter, you were introduced to the the Human Genome Project. 

Determining the DNA sequence of the human genome is considered to be one of the 

most pivotal contributions to science ever made. Nevertheless, achieving this scientifi c 

landmark depended on many discoveries that came before it. Figure 6.17 highlights only 

a small number of developments since Mendel’s work that formed the foundations of 

this project. 

An important component of the 13-year Human Genome Project was determining the 

DNA sequences of other organisms. Th is allows scientists to make comparisons between 

species and learn even more about important features of genomes. Overall, identifying 

the genome sequences of humans and many other organisms allows for a much more 

comprehensive understanding of biological systems. Th is knowledge will have a wide 

range of applications in fi elds such as human health, agriculture, and the environment. 

the first linear 

map of genes 

is produced roduced 

rapid-data-release 

guidelines 

established 

US Equal Employment 

Opportunity Commission 

issues policy y on 

genetic discrimination mination 

in the workplace lace 

DNA A as the hereditary 

here 

material is identified identifie 

the structure 

of of DNA D is 

determined 

det 

yeast 

(Saccharomyces 

cerevisiae) 

genome sequenced 

the genetic code 

is identified 

1997 

Escherichia coli 

genome 

sequenced

What’s in Our Genome? 

In addition to determining the actual sequence of the nucleotides in the human genome, 

scientists had to make sense of the sequence. Trying to make sense of the sequence 

can be compared to reading a book written in a language nobody knows or understands. 

Imagine the genome as words in a book written without capitalization, punctuation, 

or breaks between words, sentences, or paragraphs. Also, suppose there are strings of 

additional letters scattered randomly between and within sentences. Figure 6.18 shows 

how a page from such a book might look. To understand what is written, you have to 

decode the jumbled text. Similarly, scientists had to decode the sequence of our DNA 

to learn about the human genome. When the Human Genome Project began, there 

was a great deal that was not known about our genome. For example, it was not known 

how many genes humans actually had and how much of our DNA is part of those genes. 

Aft er sequencing the entire human genome, scientists observed many things that 

surprised them. Some of these discoveries include the following: 

• Only about 2 percent of the nucleotides in the human genome make up our genes 

and code for all the proteins in the body. 

• Th e estimated 25 000 total number of genes is much less than scientists predicted. 

Previous estimates were between 80 000 and 140 000. 

• Over 50 percent of our DNA consists of stretches of repeating sequences. 

• Th ere is very little genetic variation within our species. About 99.9 percent of the DNA 

sequence is almost exactly the same in all people. 

Having the sequence of the human genome only represents a starting point. It is 

like being given the pages of an instruction manual for the human body. Th e next steps 

involve fi guring out how to interpret all of the information and use that to understand 

how everything works together. Scientists agree that this process will take many more 

years of research. 

methods for determining the 

sequence of DNA are developed 

Human 

1972 1977 1983 1989 

1990 2003 

Genome Project 

re recombinant 

DN DNA technology 

is 

developed 

1999 2000 2001 2002 2003 

full-scale human 

genome sequencing begins 

sequence of first human 

chromosome 

(chromosome 22) 

completed 

free access to genome 

information established 

fruit fly 

( (Drosophila melanogaster) 

genome sequenced 

mustard sstaard 

cress 

(Arabidopsis oop 

psis thaliana) 

genome e sequenced 

cystic fibrosis 

gene is identified 

first human disease 

gene—for Huntington 

disease—is mapped 

draft version 

of human 

genome sequence ce 

published 

sequences of mouse, rat, an and 

rice genomes completed 

Figure 6.18 Decoding the 

DNA sequence of the human 

genome is like figuring out 

where the punctuation and 

capitalization must go to 

understand what is written 

on this page. 

human genome 

sequence 

completed 


ioinformatics a 

field of study that 

deals with using 

computer technology 

to create and analyze 

large databases of 

information 

Figure 6.19 A chemist 

named Margaret Dayhoff 

is considered to be the 

founder of bioinformatics. 

In this activity, you will join the worldwide community 

of scientists who explore information stored in the many 

on-line databases that are available. 

Materials 

• computer with Internet access 

Procedure 

1. Choose one of the genetic disorders provided by your 

teacher. 

The Development of Bioinformatics 

In the Launch Activity at the beginning of this chapter, you simulated the work 

required to piece together the sequence of a small fragment of DNA. Imagine 

doing this work by hand for the over three billion base pairs of the human genome. 

Sequencing the human genome and the genomes of other organisms generated 

exceptionally large amounts of data that needed to be organized and shared among 

labs around the world. A new fi eld of study, called bioinformatics, arose from this need. 

Bioinformatics is a branch of biology that deals with applying computer technology to 

create and maintain databases of information that can be analyzed to better understand 

biological processes. 

Bioinformatics is a relatively new branch of biology. American chemist Margaret 

Dayhoff , shown in Figure 6.19, is the founder of bioinformatics. Her work, which 

began in the late 1940s, involved creating a computerized protein and DNA sequence 

database—the fi rst bioinformatics project. Today’s bioinformatics exists because of 

simultaneous advances in three areas: techniques to sequence biological molecules such 

as DNA and proteins, computer database soft ware to sort and store massive amounts of 

genetic information, and communication technology to share information around the 

world effi ciently. Today, there are many on-line genetics databases available that allow 

easy access to vast amounts of genetic information by all members of the public—not 

just scientifi c researchers. 

Bioinformatics is just one of a number of newly developed fi elds, all of which 

involve using computers to study biological problems. For example, computational 

biology involves developing mathematical models and computer simulations of 

biological processes. 

Activity 6.2 Accessing Genetic Information 

2. Use the Internet to access the website that you will be 

using. Your teacher will provide a demonstration to help 

you get started. 


13. What were some achievements of the Human 

Genome Project? 

14. How much of the human genome is actually used to 

code for proteins? 

15. List three types of technologies that contribute to 

developing the tools used in bioinformatics. 


3. Spend some time looking at the diff erent databases 

that are available from this site. What diff erent types of 

information about a genetic disorder can be obtained 

from them? 

4. Choose three or four types of information that are 

available about the genetic disorder that you selected. 

Questions 

1. Summarize the information you collected on the genetic 

disorder you investigated. 

2. Based on your experience with the on-line databases, 

what was the most eff ective way of obtaining the 

information that you were looking for? 

16. Explain how bioinformatics contributed to the 

Human Genome Project. 

17. Why was development of the Internet crucial for the 

Human Genome Project? 

18. Describe an experiment that requires bioinformatics.

Genomics: The Study of Genomes 

Just as genetics is the study of genes, genomics is the study of genomes and how genes 

work together to control phenotype, as illustrated in Figure 6.20. Although some traits 

are determined by only one gene, most traits involve multiple genes. To understand 

how an individual gene produces a specifi c phenotype, researchers such as Mendel and 

Morgan chose one gene and studied it and its phenotype across many individuals. A 

signifi cant advantage that came from the Human Genome Project was the ability to 

consider multiple genes and the genome as a whole. Th is allows scientists to study the 

interactions among many genes and how they all contribute to a phenotype. Computer 

technology and fi elds such as bioinformatics play a vital role in this by allowing 

scientists to analyze large amounts of information from a variety of sources. 

Although there is considered to be little variation in the sequence of the human 

genome, it is important to keep in mind that the 0.1 percent diff erence represents 

potential for variation in about three million nucleotides. Some of this variation is 

associated with many diseases. Scientists believe that almost all human diseases have 

a genetic component, either directly or indirectly. Comparing genome sequences has 

been particularly useful in studying the genetic basis for many human diseases, such 

as cancer. For example, bioinformatics and computational biology have been used to 

compare the DNA sequences of certain regions of the genome in individuals aff ected by 

a particular type of cancer with the DNA sequences of the same regions in those who 

are not. Diff erences in DNA sequence indicate a potential genetic basis for the disease. 

While this represents a good starting point for the study of the genetics of a disease, 

scientists are discovering that many diseases are the result of a complex array of factors, 

and studying them requires more elaborate methods. 

phenotypes 

are expressed 

CGTTCTC TATTAACA... 

GCAAGAGATAATTGT... 

three billion DNA base pairs 

in the cell nucleus 

Figure 6.20 Genomics is the study of how an organism’s genome contributes to its phenotype. 

genomics the study 

of genomes and the 

complex interactions 

of genes that result in 

phenotypes 

thousands of different proteins 

are produced in trillions of cells 



Linking Genetic Variations to Disease 

In previous sections, you learned about diseases that are associated with a mutation 

or mutations in a single gene, such as sickle cell anemia. Many other diseases, such as 

cancer, stroke, heart disease, diabetes, and asthma, are infl uenced by a combination of 

environmental and genetic factors. 

Many scientists consider determining what variations in DNA sequence contribute 

to diff erent diseases to be one of the best opportunities to understand the complex 

causes of many human diseases. Th e most common type of variation between people is 

diff erences in individual nucleotides, as shown in Figure 6.21. For example, one person 

may have a C at a certain location, while another person may have a T. Th is type of 

genetic variation is called a single nucleotide polymorphism, or SNP (pronounced 

“snip”). A SNP can act as a marker for a gene or be associated with a gene if it is 

genetically linked to it. Recall that sequences of DNA are genetically linked when 

they are physically close to each other on a chromosome and tend to be inherited 

together. For example, if a SNP is common among people with high blood pressure, 

that could provide a marker for the location of a gene that is involved in the disease. 

However, there are almost 10 million diff erent SNPs that commonly occur in the 

human genome. Testing all of these is not feasible. Nevertheless, SNPs that are near 

each other on a chromosome tend to be inherited together. Th ese regions of genetically 

linked variations are called haplotypes. Certain tag SNPs can uniquely identify these 

haplotypes. Since there are far fewer of these types of SNPs, they can be used as a basis 

for comparing genetic variations and identifying genes that infl uence the health of an 

individual. 

In 2002, an international group of researchers from Canada, the United States, 

Japan, China, Nigeria, and the United Kingdom collectively began the International 

HapMap Project. Th e major aim of this project is to develop a haplotype map 

(HapMap) of the human genome, which represents a map of the variations in the 

human genome. Th is can then be used by other scientists to identify the genetic basis 

for many human diseases. 

Chromosome 1 

Chromosome 1 

Chromosome 1 

Chromosome 1 

SNP 

A A C A C G C C A . . . . 

A A C A C G C C A . . . . 

A A C A T G C C A . . . . 

A A C A C G C C A . . . . 

SNP 

T T C G G G T C . . . . 

T T C G A G T C . . . . 

T T C G G G T C . . . . 

T T C G G G T C . . . . 

Haplotype map of the human genome 

SNP 

A G T C G A C C G . . . . 

A G T C A A C C G . . . . 

A G T C A A C C G . . . . 

A G T C G A C C G . . . . 

Figure 6.21 A haplotype map is constructed by identifying single nucleotide polymorphisms 

(SNPs) among a number of individuals. 

Beyond the Genome Sequence 

Analysis of the data generated from the Human Genome Project will continue for many 

decades. A signifi cant part of that research involves more than working at the level of the 

DNA sequence. For example, the fi eld of proteomics began when scientists recognized 

how important it is to understand the products of our genes—proteins. Based on the 

term genome, the term proteome was developed to refer to all of the proteins in an 

organism. Research studies in proteomics focus on studying the three-dimensional 

shape of proteins and eventually determining the functions of all the cellular proteins.

Studying Gene Expression 

Today, many scientists are studying what regulates the expression of genes. Th at is, they 

look at what infl uences whether a particular protein is produced from a certain gene 

and, if so, how much of the protein is made. An individual’s phenotype is the result 

of which genes are active—are being expressed—and which genes are inactive, or not 

being expressed. While all cells of an individual have the identical genetic material, 

the same genes are not expressed in the same way in every type of cell. For example, 

diff erences in gene activity can exist between healthy cells and cells of diseased tissue, 

such as cells of cancerous tumours. 

Scientists now realize that some factors that aff ect gene expression can be inherited, 

but they are not due to changes in DNA sequence. Epigenetics is the study of how 

changes in the inheritance of certain traits or phenotypes are based on changes 

to gene function and not to changes in DNA sequence. Epigenetics diff ers from 

evolution because there is no change to the DNA sequence of a gene and epigenetic 

changes are not necessarily permanent. Epigenetic changes represent a response to an 

environmental condition that may be reversed once that condition changes, or soon 

aft er the change. Th e term epigenome refers to cellular material that is not part of the 

genome but that infl uences whether a gene is “turned on” or “turned off .” Identifying 

epigenetic factors is believed to be a next major frontier in biological sciences. 

Studying Gene Expression Using Microarrays 

A very important method that is used to study diff erences in gene activity is DNA 

microarray technology. In this technique, DNA is placed as spots on a glass plate, 

called a microarray plate. One slide can contain thousands of spots of DNA that 

correspond to certain parts of a genome, and that contain diff erent genes. Figure 6.22 

shows an example of a microarray plate. Th is technique allows scientists to study the 

activity of up to thousands of genes at a time, under particular conditions. Studying the 

activity of so many genes at once tells scientists which genes are active or inactive under 

certain conditions, and gives them information on how this activity is co-ordinated 

among diff erent genes. 

Figure 6.22 DNA microarrays allow scientists to see the activity of genes 

under certain conditions. The colour of each circle on a microarray plate like 

this one corresponds to the activity of a gene in the DNA spot on the plate. 


genetic profile the 

complete genotype of 

an individual, including 

various mutations 

Figure 6.23 This altered 

representation of the 

caduceus—a common 

symbol for medical 

practice—illustrates the 

link between genetics and 

treatment of disease. 


Genetic Information: Public Benefits and Concerns 

Some of the most important benefi ts of genetic research are in the area of human 

medicine. Figure 6.23 illustrates this link between genetics and medical treatment. 

Studying the human genome as a whole may make it possible to develop drugs that are 

tailored to the expression of the genes associated with particular disorders, and to the 

unique genome of a patient. 

In the future, researchers hope to use established links between genetic variation 

and risk of disease to provide better medical advice to patients. If the cost of DNA 

sequencing continues to decrease, individuals may have access to their genetic 

profi le—their complete genotype, including all of the various mutations linked to 

disease. Currently, doctors are only able to make generalized risk assessments based on 

medical history. Armed with a genetic profi le, however, genetic counsellors and doctors 

will be able to provide more specifi c risk assessments, design individualized prevention 

plans, and design genetically precise treatment programs. 

What Can Happen to Information from a Genetic Profi le? 

Establishing genetic profi les for individuals, and making these profi les available to 

health-care providers, also creates ethical concerns. For example, 

• Could insurance companies deny coverage to people who have a genetic 

predisposition for a particular disease? 

• Could potential employers have access to an individual’s genetic profi le and use it in 

assessing whether to hire the person? 

• Should researchers be allowed to use the genetic profi les of individuals to help them 

better understand the link between genome and phenotype? 

Th e central issue in all of these ethical questions is who should have access to the 

information in a genetic profi le. 

Ownership of Genetic Information 

All the data gathered through the Human Genome Project is publicly available. Having 

access to the data made it possible for scientists to share what they learned about 

human genetics. In other areas of genetics research, however, the relationship between 

public and private information is more complex. 

In 2005, the National Geographic Society and the IBM company jointly launched 

the Genographic Project. Th is project uses DNA samples provided by hundreds of 

thousands of volunteers around the world to learn more about the migrations of 

ancient peoples. Using high-tech genetics tools and computer facilities, DNA sequences 

of the individuals are analyzed to better understand human genetic roots and how we 

all “connect” at the level of our DNA. 

Studies such as the Genographic Project can contribute valuable information to 

researchers in many fi elds. But who owns the genetic information? For example, should 

companies have the right to sell DNA information to other companies without the 

permission of the people who provided the samples? Should companies that use DNA 

in medical research be required to share the results of their work with the individuals 

or communities whose genetic information was used? 

Some people argue that genetic information is a natural resource that belongs 

to everyone. Others believe that genetic information about a person belongs only to 

that person. In addition, many think that if companies cannot earn a profi t from their 

research, there is little incentive for them to invest in genetic studies. In the world of 

genetics research, where is the boundary between public and private property?



• Th e complete DNA sequence of the human genome was 

determined as part of the Human Genome Project. 

• Th e fi eld of bioinformatics arose from the need to share 

and maintain the large quantities of data collected from 

genomic research. It also provides tools for analyzing 

genomic data. 


1. K/U Th e Human Genome Project involved 

sequencing the genomes of other organisms as well as 

of humans. Provide two reasons for why this was done. 

2. C In this section, the human genome was 

compared to a book. Illustrate how the parts of a 

book—pages, paragraphs, sentences, words, and 

letters—can be used to represent chromosomes, 

chromatids, genes, and nucleotides. 

3. K/U Describe three things about the human genome 

that scientists learned from the Human Genome 

Project. 

4. K/U Although the Human Genome Project is 

complete, research based on its fi ndings continues. 

Describe two areas of current research that developed 

from it. 

5. A Th e Human Genome Project cost billions of 

dollars to complete. Do you think it was worth it? 

Provide reasons that support your opinion. 

6. T/I Th e two pictures below show scientists 

conducting genetic research in labs. Th e photo on the 

left was taken in the 1980s, and the photo on the right 

was taken in the 2000s. Describe how these photos 

refl ect the changes in genetic research that took place 

over this time period. 

• Th ere is still much to be learned from the data generated 

from the Human Genome Project, particularly in 

identifying genes that are associated with human health. 

• Current and future research in genomics may allow 

scientists to tailor preventative and curative treatments 

for individual patients based on their specifi c genetic 

profi les. However, ethical questions about who owns an 

individual’s genetic information continue to be debated. 

7. T/I Describe why you think the fi eld of 

bioinformatics was given that name. Provide a suitable 

alternative name for this fi eld of science. 

8. K/U What is genomics? Describe the type of research 

that is involved and how it may help society. 

9. K/U What is the HapMap project? What is its main 

goal? 

10. T/I Explain how epigenetics suggests that the traits 

we inherit may not be due only to the DNA we receive. 

11. K/U Describe how DNA microarray technology is 

used to study gene expression. 

12. C Determining a genetic profi le can have its 

benefi ts and its risks. Use a table to list as many 

benefi ts and risks as you can. 

13. C Should people be encouraged to have their 

genetic profi les determined, since this might prevent 

them from developing certain illnesses? Justify your 

answer using examples. 

14. A Imagine that you have been hired by an 

international organization that establishes practices 

for scientists to follow when doing genetics research. 

Your job is to develop a policy on the collection and 

ownership of genetic information. 

a. What are some of the issues you should consider? 

b. Based on the issues you listed, decide where you 

stand on those issues and develop a policy that 

refl ects that stance. 

c. Briefl y summarize how your policy will balance 

public and private interests. 

15. C Write a paragraph expressing your opinion on 

whether employers should have to provide a work 

environment that suits a person’s genetic profi le. 


Plan Your Own 

INVESTIGATION 

Skill Check 

✓ Initiating and Planning 

✓ Performing and Recording 

✓ Analyzing and Interpreting 

✓ Communicating 

Safety Precautions 

• Wash your hands when you have 

completed this investigation 

Suggested Materials 

• seeds (Brassica rapa, radish, 

or bean) 

• labels 

• paper towels 

• water 

• shoe boxes 

• petri dishes 

• graduated cylinder 

• light source 

Go to Organizing Data in a Table in 

Appendix A for help with designing 

a table for data. 

Go to Constructing Graphs in Appendix A 

for information about making graphs. 


6-A 

Environmental Infl uences 

on the Production of Chlorophyll 

Chlorophyll is the molecule that allows plants to capture light energy from the 

Sun and use the energy to produce food in the form of sugars. Chlorophyll 

is also the pigment that gives leaves their green colour. Plants that produce 

chlorophyll appear green. If the production of chlorophyll is “turned off ,” the 

plant will become pale yellow, or even white. Th e production of chlorophyll is 

under genetic control. 

Working in groups and using the materials provided, you will design and 

conduct an investigation to test the infl uence of light on the production of 

chlorophyll. Your investigation must enable you to draw conclusions about each 

of the following. 

• What is the minimum duration of exposure to light required to turn on the 

production of chlorophyll? 

• Is the triggering event reversible? Th at is, does chlorophyll production start 

and stop as environmental conditions change? 

When chlorophyll is no longer present, a green plant will become pale yellow or even 

white. This is similar to what happens to many trees during the autumn. In the spring 

and summer, tree leaves appear green because chlorophyll is being produced. With 

the change in environmental conditions that accompanies autumn, chlorophyll is no 

longer produced and other pigments in the tree leaves become visible. This results in 

the yellow, orange, and red “fall colours” of some trees.

Pre-Lab Questions 

1. Describe the genotype of the organisms you should use 

that will allow you to test the eff ect of the environment 

on phenotype. 

2. What is the diff erence between qualitative and 

quantitative data? 

3. Diff erentiate among independent, dependent, and 

controlled variables. 

Question 

How does light infl uence the production of chlorophyll in 

germinating plants? 

Hypothesis 

Formulate a hypothesis to explain how light infl uences the 

activity of the genes responsible for chlorophyll production. 

Use this hypothesis as the basis of your experimental design. 

Plan and Conduct 

1. With your group, brainstorm several methods you 

could use to test your hypothesis given the materials 

provided. As a group, select one method for your 

experimental design. 

2. Identify the independent, dependent, and controlled 

variables, and the type of data you will collect. 

3. As you prepare your procedure, be sure to consider the 

time required for each step. 

4. Prepare the data table you will use to record your 

observations. Decide what form (such as the type of 

graph) you will use to present your results. 

5. Review your procedure with your teacher. Do not 

begin doing the investigation until your teacher has 

approved your group’s procedure. 

6. Record your observations in your table. Make notes 

about any fi ndings that do not fi t in your data table. 

Record any questions that come up as you conduct 

your investigation. 

Analyze and Interpret 

1. Did your observations support or refute your 

hypothesis? Explain. 

2. Did your investigation allow you to draw conclusions 

about the inheritance of the genes that are involved in 

the production of chlorophyll? Why or why not? 

3. Identify the variables you considered when designing 

your investigation. Explain why you needed to consider 

each variable to obtain scientifi cally valid results. 

Conclude and Communicate 

4. State your conclusions about the relationship between 

exposure to light and the activity of the genes that are 

involved in the production of chlorophyll. 

Extend Further 

5. INQUIRY Could a diff erent hypothesis be consistent 

with the results of your investigation? How could you 

design an investigation to test this diff erent hypothesis? 

6. RESEARCH What social benefi t could come from 

understanding the eff ect of light on chlorophyll 

production? 


ThoughtLab 

INVESTIGATION 

Materials 

Skill Check 

Initiating and Planning 

✓ Performing and Recording 

✓ Analyzing and Interpreting 

✓ Communicating 

• data on crosses 

Go to Organizing Data in a Table in 

Appendix A for help with designing 

a table for data. 

6-B 

Sex-linked Crosses 

A B 

Th omas Morgan used Drosophila melanogaster, the common fruit fl y, extensively 

in his studies of sex-linked traits. In this investigation, you will model Morgan’s 

experiments using Drosophila melanogaster and use your results to confi rm 

sex-linked inheritance for the trait you chose to study. 

Pre-Lab Questions 

1. How is a sex-linked recessive trait distinguished from an autosomal 

recessive trait? 

2. Describe the genotype of the P generation that could be used to model 

Morgan’s studies of sex-linked genes in Drosophila. 

3. What phenotype is expected in the F1 generation produced from the cross 

described in question 2? 

Question 

How are sex-linked traits inherited in Drosophila melanogaster? How do actual 

results compare with theoretical ratios? 

Organize the Data 

1. Choose one trait from the table below (eye colour, eye shape, or body 

colour) to investigate. 

Common Sex-linked Traits in Drosophila melanogaster 

Trait Phenotype 1 Phenotype 2 

Eye colour White Red 

Eye shape Round Bar 

Body colour Black Yellow 

Two forms of eye colour in fruit flies are white and red (A). Eye shape can be round (A) or appear as narrow bars (B). 

270 MHR • Unit 2 Genetic Processes

Part I: The F 1 Generation 

2. Determine the genotype of the fl ies use for the P 

generation. 

3. Construct a table to record your results. 

4. Use a computer simulation program or obtain results 

for the F1 generation from your teacher. Record the 

results in your table. 

5. Before beginning Part II, complete the Analysis section 

of the investigation for Part I. 

Part II: The F2 Generation 

6. Determine the genotype of the fl ies for the F1 cross. 

7. Construct a table to record your results. 

8. Use a computer simulation program or obtain results 

for the F2 generation from your teacher. Record the 

results in your table. 


Part I 

1. From the data you recorded on the appearance of the 

fl ies in the F1 generation, which trait is dominant? 


2. Given your response to question 1, form a hypothesis 

about the phenotypic ratio that you will observe in the 

F2 generation. 

Part II 

3. Calculate an actual phenotypic ratio of the F2 generation from your results. 

A B 

Two forms of body colour in fruit flies are black (A) and yellow (B). 

Conclude and Communicate 

4. Describe the inheritance pattern for the trait you 

studied in this investigation. 

5. How does the actual phenotypic ratio you obtained 

compare to the theoretical phenotypic ratio? Account 

for any diff erences. 

Extend Further 

6. INQUIRY In this investigation, you tracked the 

inheritance pattern of one sex-linked trait. Design an 

investigation that would track the inheritance of one 

of these traits and the autosomal trait of normal versus 

vestigial wings. Describe the results you expect. 

7. RESEARCH Drosophila melanogaster has been used 

extensively in genetics research. What other traits have 

been studied in Drosophila? On which chromosomes 

are the genes for these traits located? 


Chapter 6 

Section 6.1 

SUMMARY 

Beyond Mendel’s Observations of Inheritance 

Some patterns of inheritance are more complex 

than those fi rst proposed by Mendel. These include 

codominant and incomplete dominant inheritance 

patterns. In addition, for some traits multiple alleles 

for a gene can exist within the population. 

KEY TERMS 

codominance 


heterozygous advantage 

Section 6.2 


polygenic trait 

Inheritance of Linked Genes 

Some traits are inherited together, due to linked genes. 

Gene linkage includes sex-linked genes, which are on 

the sex chromosomes. 

KEY TERMS 

linked genes sex-linked trait 

Section 6.3 

The Future of Genetics Research 

Current and future research in genetics involves 

studying how phenotypes result from complex 

interactions between genes and gene products. 

KEY TERMS 

bioinformatics 

genetic profi le 

genomics 


KEY CONCEPTS 

• Incomplete dominance leads to the expression of an 

intermediate phenotype. In the case of codominance, 

both alleles are fully expressed. 

• Although an individual has only two alleles for any gene, 

multiple alleles for a gene may exist within the population. 

• Environmental conditions can infl uence the expression of 

certain traits. 

• Polygenic traits are controlled by more than one gene 

and can usually be identifi ed by continuous variation in 

phenotype. 

KEY CONCEPTS 

• Alleles of diff erent genes that are on the same chromosome 

do not assort independently. These genes are said to be 

linked and their associated traits tend to be inherited 

together. 


and female off spring because they are determined by the 


• Although sex-linked genes are linked to the X and Y 

chromosomes, Punnett squares can be used to predict 

genotypes and phenotypes. 

KEY CONCEPTS 

• The complete DNA sequence of the human genome was 

determined as part of the Human Genome Project. 

• The fi eld of bioinformatics arose from the need to share 

and maintain the large quantities of data collected from 

genomic research. It also provides tools for analyzing 

genomic data. 

• There is still much to be learned from the data generated 

from the Human Genome Project, particularly in identifying 

genes that are associated with human health. 

• Current and future research in genomics may allow 

scientists to tailor preventative and curative treatments for 

individual patients based on their specifi c genetic profi les. 

However, ethical questions about who owns an individual’s 

genetic information continue to be debated.


REVIEW 

Knowledge and Understanding 

Select the letter of the best answer below. 

1. Th e seed colour of a particular species of plant 

is inherited through incomplete dominance. If a 

true-breeding plant with blue seeds is crossed with 

a true-breeding plant with yellow seeds, what is the 

expected seed colour of the off spring? 

a. yellow 

b. green 

c. blue 

d. yellow and blue spots 

e. You cannot predict seed colour from the 

information given. 

2. Roan cows are the result of a codominant inheritance 

pattern. In roan cows, the allele for white hair and the 

allele for red hair are both expressed. Which of the 

following is the most appropriate representation for 

codominant alleles? 

a. Let W = allele for white hair, and let R = allele for 

red hair. 

b. Let W = allele for white hair, and let r = allele for 

red hair. 

c. Let w = allele for white hair, and let R = allele for 

red hair. 

d. Let CW = allele for white hair, and let CR = allele for 

red hair. 

e. Let Cw = allele for white hair, and let CR = allele for 

red hair. 

3. A man with blood type O and a woman with blood 

type AB have a child. Which of the following are 

possible blood type(s) for the child? 

a. O only 

b. AB only 

c. A or B 

d. A, B, or O 

e. A, B, AB, or O 

4. Skin colour in humans ranges from very dark to very 

light. Which of the following most likely describes how 

skin colour is inherited? 

a. principle of dominance 

b. incomplete dominance 

c. codominance 

d. polygenic inheritance 

e. environmental infl uence 

5. Th e following pedigree follows the inheritance pattern 

of sickle cell anemia in a family. What is the sex, 

genotype, and phenotype of individual II-5? 

I 

II 

III 

1 2 3 4 

1 2 

3 

1 

4 5 

a. unaff ected female, HbAHbS b. aff ected female, HbAHbS c. unaff ected male, HbSHbS d. aff ected male, HbSHbS e. unaff ected male, HbAHbA 6. An X-linked dominant allele is inherited from a 

heterozygous female by 

a. all of her sons 

b. half of her sons 

c. all of her daughters 

d. none of her daughters 

e. all of her children 

7. Which of the following most accurately describes the 

fi eld of genomics? 

a. the study of haplotypes 

b. the study of how DNA is copied 

c. the study of how genes interact to produce a 

phenotype 

d. the study of how genomes are formed 

e. the study of the inheritance pattern of genes 

8. How has DNA microarray technology revolutionized 

the study of gene activity? 

a. Gene expression in cells can now be studied. 

b. Th e proteins produced by genes have been 

discovered. 

c. Many genes can be studied at the same time. 

d. Th e human genome has been completely sequenced. 

e. All of the proteins produced in a cell can now be 

studied. 

Chapter 6 Complex Patterns of Inheritance • MHR 273 

2 

5



Answer the questions below. 

9. A plant that produces white fl owers is crossed with a 

plant that produces red fl owers. Describe the pattern 

of inheritance if the fl owers produced are 

a. pink 

b. red and white spotted 

c. all red 

10. What is the predicted phenotypic ratio in the F2 generation if two alleles are inherited by incomplete 

dominance? 

11. What is heterozygous advantage? Provide an example. 

12. Describe how multiple alleles infl uence inheritance of 

a trait. Provide an example. 

13. Height is an example of a polygenic trait. What aspect 

of height suggests this? 

14. What are linked genes? Explain why their inheritance 

is not according to the law of independent assortment. 

15. Parents who do not have symptoms of Duchenne 

muscular dystrophy have a son with Duchenne 

muscular dystrophy. Which parent has passed the 

disease to their son? Explain your answer. 

16. What is a person’s genetic profi le? What are some 

ethical issues concerning access to this information? 

Thinking and Investigation 

17. A man with straight hair has two children with a 

woman who has curly hair. One child has straight hair, 

and one has wavy hair. What pattern of inheritance for 

hair type does this suggest? 

18. Use the pedigree below to answer the following 

questions. Th e letters in the symbols indicate the blood 

type of each individual. 

a. Determine the blood types of individuals I-4 

and I-6. 

b. Individual III-2 and a man with blood type AB have 

four children. Will any of these children have blood 

type O? Explain. 

I 

II 

III 

A AB B ? O ? 

1 

AB 

1 

B 

2 

2 3 4 

A 

AB 

O O O A B 

A 

O 

3 4 

5 6 

1 2 3 4 5 


5 6 

19. In foxes, a pair of alleles, CP and Cp , interact as follows: 

• CPCP is lethal, usually during an embryonic stage 

• CPCp produces platinum-coloured fur 

• CpCp produces silver foxes. 

Could a fox breeder establish a true-breeding variety 

of platinum foxes? Explain. 

20. A man with type B blood and a woman with type 

AB blood have children. What blood types are possible 

among their children? What would tell you that the 

man is heterozygous for type B blood? 

21. A woman with type AB blood has a child with the 

same blood type. What are the possible genotypes of 

the father? 

22. What could be a genetic reason for the black area of fur 

forming aft er a cold pack has been placed on the back 

of this Himalayan rabbit? 

23. Explain why genes that are far apart on a single 

chromosome may be inherited as though they are on 

diff erent chromosomes. 

24. A horse breeder fi nds that one of his stallions has a 

genetic defect that aff ects the production of sperm. 

Th e gene associated with this trait is located on the Y 

chromosome. What is the possibility that the stallion’s 

female off spring could pass on this trait to their sons? 

Explain. 

25. Fruit fl ies can have normal wings or stunted wings. 

In an investigation, you mate several normal-winged 

females with a male that has stunted wings. In the F1 generations, only the males have stunted wings. What 

can you conclude from this investigation? 

26. Suppose that the fi rst dihybrid crosses Mendel 

performed had involved traits controlled by closely 

linked genes. 

a. How would Mendel’s results have diff ered from the 

results he obtained for a dihybrid cross involving 

non-linked genes? 

b. What hypothesis might Mendel have developed to 

explain his results? 

c. What investigation could Mendel have conducted to 

test this hypothesis? What would he have observed?

Communication 

27. Rudy and Maria are expecting a baby. Th ey have 

normal vision, but both of their fathers are colour 

vision defi cient (CVD). Th eir mothers have normal 

vision. 

a. Draw a pedigree for their family. 

b. What is the probability that the baby will be a girl 

with CVD? 

c. What is the probability that the baby will be a boy 

with normal vision? 

28. Th e closer genes are together on a chromosome, the 

more likely they will assort together. Illustrate this 

concept with a model or diagram. 

29. Variability and diversity of living organisms 

result from the distribution of genetic 

materials during the process of meiosis. Mendel 

proposed the idea that all genes assort independently, 

producing off spring with a variety of traits whose 

distribution can be predicted mathematically. William 

Bateson and Reginald Punnett found that not all genes 

do assort independently. Develop a diagram that shows 

independent assortment and how linked genes 

contradict this theory. 

30. Genetic and genomic research can have 

social and environmental implications. 

Identify a potential scientifi c outcome of genomics 

research. Develop an illustration showing the possible 

social implications of achieving that outcome. 

31. In this chapter, DNA sequences in a genome are 

compared to letters strung together in a book. Develop 

another analogy for DNA, chromosomes, genes, and 

nucleotides. Illustrate your analogy with a diagram or 

model. 

32. Use a graphic organizer to summarize the uses of 

bioinformatics in genetics research. 

33. Th ere are many benefi ts to genetics research, but there 

are also signifi cant ethical concerns. Use a concept map 

to illustrate some of the benefi ts and concerns that are 

associated with the diff erent genetics research topics 

discussed in this chapter. 

34. Summarize your learning in this chapter using 

a graphic organizer. To help you, the Chapter 6 

Summary lists the Key Terms and Key Concepts. Refer 

to Using Graphic Organizers in Appendix A to help 

you decide which graphic organizer to use. 

Application 

35. A farmer wants to breed a variety of taller corn. 

a. How can the farmer use variation in the height of 

the current corn plants to produce taller corn plants? 

b. Will the farmer’s work be most eff ective if height in 

corn plants is determined by polygenic inheritance, 

multiple alleles, or codominant alleles? Explain. 

c. Th e farmer fi nds that many of the tallest corn plants 

are also very susceptible to a particular disease. 

How could the farmer design an investigation to 

fi nd out if the genes for height are linked to the 

genes that cause susceptibility to the disease? 

d. If these genes are linked, what steps could the farmer 

take to create a breed of corn that is taller and more 

disease-resistant than the current corn crop? 

36. Figure 6.17 provides a summary of some important 

discoveries in genetics research, including the Human 

Genome Project. 

a. Research one development or discovery that is 

in the fi gure, including an aspect of the Human 

Genome Project. Choose a subject that you have not 

learned about in this unit. 

b. As part of your research, fi nd out about at least one 

individual who is associated with the discovery or 

invention. 

c. Summarize your fi ndings and develop a 

presentation that you could present to the 

class or another general audience. Make sure 

your presentation includes a discussion on 

the importance of the discovery in terms of its 

contribution to scientifi c research. 

37. Genome Canada was established in 2000 to develop a 

national program for fi nancial support of genomic and 

proteomic research in Canada. 

a. Choose a research project that is funded by Genome 

Canada and that is listed on the Genome Canada 

website. 

b. Write a brief description that summarizes what 

the project is studying. Include the names of the 

individuals associated with the project and at what 

institution(s) they work. 

c. Research Genome Canada’s GE3LS program. What 

does this acronym stand for and what are the main 

objectives of this program? Develop an argument for 

or against the importance of having such a program. 


Chapter 6 SELF-ASSESSMENT 


1. K/U Incomplete dominance occurs when 

a. one allele masks the expression of the other allele 

b. one trait is masked by the presence of another trait 

c. both alleles are expressed when the alleles occur 

together 

d. an intermediate phenotype is expressed when the 

alleles occur together 

e. an unpredictable phenotype is expressed when the 


2. K/U Which of the following is an example of 

codominance? 

a. A plant with green seeds is crossed with a plant with 

white seeds; the off spring produce white seeds. 

b. Individuals who are heterozygous for sickle cell 

disease produce both normal and sickle-shaped 

red blood cells. 

c. A red snapdragon crossed with a white snapdragon 

produces pink snapdragons. 

d. Th ere are many genes that control eye colour. 

e. A litter of kittens oft en display a wide variety of 

traits. 

3. T/I A man who is homozygous for blood type A 

and a woman who is homozygous for blood type B 

have a child. Which of the following could be the 

child’s genotype? 

a. IAi b. IAIA c. IBi d. IBIB e. IAIB 4. K/U Which two terms are most relevant to the 

inheritance of human blood types? 

a. incomplete dominance and codominance 

b. codominance and multiple alleles 

c. incomplete dominance and multiple alleles 

d. codominance and polygenic inheritance 

e. dominance and codominance 

5. K/U Traits that exhibit continuous variation are 

usually 

a. controlled by one gene 

b. the result of codominance 

c. dominant 

d. polygenic 

e. aff ected by the environment 


Use the following information to answer questions 6 and 7. 

Th e gene that controls coat colour in rabbits has four alleles: 

agouti (C), chinchilla (cch ), Himalayan (ch ), and albino (c). 

Th e order of dominance is C > cch > ch > c. 

6. K/U What is the phenotype of a rabbit with the 

genotype cchc? a. agouti 

b. chinchilla 

c. chinchilla and albino mix 

d. Himalayan 

e. albino 

7. T/I If a rabbit with the phenotype cchch is crossed 

with an albino rabbit, what is the probability of 

producing a Himalayan rabbit? 

a. 0 percent 

b. 25 percent 

c. 50 percent 

d. 75 percent 

e. 100 percent 

8. K/U How can linked genes become “unlinked”? 

a. During meiosis, they sort independently. 

b. During crossing over, they are separated. 

c. During anaphase, they segregate to opposite poles 

in the cell. 

d. During mutation, the genes are separated. 

e. During DNA replication, the genes are rearranged. 

9. T/I Hemophilia is an X-linked recessive disorder. 

If a female with hemophilia and a male without 

hemophilia had children, what is the predicted 

percentage of children who would have hemophilia? 

a. 0 percent 

b. 25 percent 

c. 50 percent 

d. 75 percent 


10. K/U Which of the following statements about the 

Human Genome Project is false? 

a. It involved sequencing the human genome. 

b. It identifi ed coding and non-coding sections of 

DNA. 

c. It involved sequencing the genome of common 

representative organisms. 

d. It identifi ed genes in the human genome. 

e. It determined the functions of the genes in the 

human genome.

Use sentences and diagrams as appropriate to answer the 

questions below. 

11. T/I In radishes, colour is controlled by two alleles, 

one for red colour and one for white colour. 

Inheritance of these alleles shows incomplete 

dominance. Th e photographs below show the 

phenotype for each possible colour: red, purple, and 

white. What phenotypic ratio would you expect from 

crossing two heterozygous radish plants? 

12. T/I A student crosses a true-breeding plant that 

produces green seeds with a true-breeding plant that 

produces yellow seeds. Predict the possible off spring 

when 

a. the allele for green seeds is dominant to the allele 

for yellow seeds 

b. the allele for green seeds is codominant with the 

allele for yellow seeds 

c. the alleles for green and yellow seeds are 

incompletely dominant 

13. C Blood type ABO is determined by three alleles. 

Draw a diagram that shows how blood type is 

determined by a combination of the three alleles. 

14. A Investigating environmental eff ects on gene 

expression is an important aspect of genetics research 

on plant crops. Explain why, using an example of a trait 

to illustrate your answer. 

15. C Draw a diagram that illustrates the concept of 

linked alleles of genes. In your diagram, show how they 

can become unlinked. 

16. C A female fruit fl y that is homozygous dominant 

for red eyes is crossed with a white-eyed male fruit fl y. 

Use a Punnett square to predict the genotype(s) and 

phenotype(s) of their off spring. 

Self-Check 

If you missed 

question... 

Review 

section(s)... 

17. T/I Th e pedigree below illustrates the sex-linked 

inheritance pattern of a trait in a family. 

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 

6.1 6.1 6.1 6.1 6.1 6.1 6.1 6.2 6.2 6.3 6.1 6.1 6.1 6.1 6.2 6.2 6.2 6.2 6.2 6.2 6.3 6.3 6.3 6.3 6.3 

I 

II 

III 

1 

2 

1 2 

1 

3 

2 

4 

3 4 

5 

6 

5 6 

a. Explain how this pedigree shows sex-linked 

inheritance. What type of sex-linked inheritance is 

it? Explain. 

b. From the pattern of inheritance you determined in 

part (a), determine the genotype of II-2. 

c. Based on your answer to part (a), determine the 

probability that individuals II-1 and II-2 would have 

an aff ected child. 

18. C Duchenne muscular dystrophy aff ects many 

more males than females. Explain why and draw a 

pedigree to illustrate its inheritance pattern. 

19. K/U Explain why males cannot be carriers of an 

X-linked trait. 

20. K/U Explain how Barr bodies account for the patchy 

colours of female calico cats. 

21. K/U Why did the Human Genome Project include 

the sequencing of other organisms? 

22. C “Decoding the human genome can be compared 

to reading a book in a language that nobody knows or 

understands.” Explain this statement using diagrams or 

a graphic organizer. 

23. A What is genomics research? How can it be used 

to improve human health? 

24. A What is bioinformatics? Describe a scientifi c 

study that uses bioinformatics. 

25. A Th e human genome has long stretches of DNA 

that do not code for proteins. Describe how the 

variation between individuals in these regions can 

be useful to study. 


Unit 2 Project 

An Issue to Analyze 

Analyzing the Risks and Benefi ts of GMOs 

For many years farmers have used reproductive technologies such as selective breeding to 

produce the strongest and most profi table plants and animals possible. Now genetic engineering 

allows scientists to manipulate genomes. All of the organisms shown below have been genetically 

modifi ed to become transgenic organisms. Recall that a genetically modifi ed organism, or GMO, 

is one whose genome has been altered. A transgenic organism is produced when this alteration 

involves the insertion of a gene from another organism. Th e merits of producing such organisms 

are continually debated. Th e long-term eff ects that GMOs have on humans who are exposed to 

them, on the modifi ed organisms themselves, and on the environment are not yet known. 

Assume the role of a science writer who contributes to an on-line magazine that is a 

forum for discussions on the impact of GMOs on society. Choose a GMO and research its 

application(s). Analyze the benefi ts and risks associated with the use of this organism and develop 

recommendations regarding its application(s). 

What are the issues related to the use of genetically modifi ed organisms, and do the benefi ts 

outweigh the possible risks? 

Initiate and Plan 

1. Select one GMO and its application(s). Some options 

for you to consider are 

• organisms genetically modifi ed to be more resistant 

to disease 

• organisms genetically modifi ed to be more resistant 

to very cold temperatures 

• organisms genetically modifi ed to allow for 

easier harvesting 

Most canola plants grown in western 

Canada have had a bacterial gene 

inserted into their genomes that makes 

them resistant to herbicides. 


This Enviropig has had a bacterial gene 

inserted into its genome that enables it 

to break down a phosphorus-containing 

compound in feed. 

• organisms genetically modifi ed to improve medical 

treatment for humans (for example, plants or animals 

used in pharmaceutical production) 

• organisms genetically modifi ed to provide alternative 

and/or higher yield food products for human 

consumption 

• organisms genetically modifi ed to improve 

environmental conditions 

These GloFish® have had a gene from sea 

anemones inserted into their genomes 

that makes them glow in the dark.

Perform and Record 

2. Research your chosen GMO. Focus your initial research 

on the scientifi c technology associated with the 

development of the organism, and on the regulatory 

processes that must be followed. 

3. Consider the following questions to guide your 

research: 

• What type of genetic modifi cation have scientists 

made to the organism? Does this modifi cation 

involve inserting a gene from another organism, to 

produce a transgenic organism? 

• What government regulatory bodies are involved in 

reviewing the research, development, and use of the 

organism (for example, local municipal government, 

provincial government, Health Canada, Environment 

Canada, U.S. Food and Drug Administration)? 

• At what stage of research and development is 

the genetic engineering of the organism and its 

application(s)? Is research still at a preliminary 

stage? Has research and development received some 

kind of government approval? Has the organism 

or its products received government approval for 

commercial use? 

4. Research the economic, political, societal, ethical, and 

environmental issues related to the application(s) of the 

GMO that you have selected. 

5. Consider the following questions to guide your research: 

• Who stands to benefi t the most from the application(s)? 

• What is the most signifi cant benefi t? 

• What is the most signifi cant risk to the environment 

(if any), and to society as a whole? 

• What, if any, long-term benefi ts and risks have citizens 

or scientists identifi ed regarding the use of this GMO? 

• What are the sources of the information you have 

gathered? How trustworthy and credible are your 

sources? How do you know? 


1. Prepare a risk-benefi t analysis that outlines the risks 

and benefi ts associated with the development of the 

GMO and its application(s). Refer to Analyzing STSE 

Issues in Appendix A for help with how to do 

this analysis. 

2. Make recommendations about whether development 

and use of the GMO should continue as is, should 

stop, or requires stricter regulations. Support your 

recommendations using specifi c examples from your 

risk-benefi t analysis. 

Communicate Your Findings 

3. Choose a form of communication to convey your 

recommendations that is appropriate for an on-line 

magazine (such as a web page, blog, podcast, or 

Internet video). 

Assessment Criteria 

Once you complete your project, ask yourself these 

questions. Did you… 

✓ K/U select an appropriate GMO? 

✓ K/U describe the scientifi c and technical principles 

related to the technology and the regulatory 

processes that must be followed? 

✓ A identify the economic, political, societal, 

ethical, and environmental issues related to 

the technology? 

✓ A make recommendations based on specifi c 

examples from the risk-benefi t analysis of whether 

use of the GMO should continue as is, be stopped, or 

be under stricter regulations? 

✓ C organize your research using an appropriate 

format and appropriate academic documentation? 

✓ C select a format for your recommendations 

that is appropriate for the audience and purpose? 

✓ C use scientifi c vocabulary appropriately? 

Unit 2 Project • MHR 279

UNIT 2 



SUMMARY 

• Genetic and genomic research can have social and 

environmental implications. 

• Variability and diversity of living organisms result from 

the distribution of genetic materials during the process 

of meiosis. 


Cell Division and Reproduction 

KEY IDEAS 

• Chromosomes in human somatic cells are organized into 

23 pairs. One pair is the sex chromosomes, and the other 

22 pairs are the autosomes. 

• Meiosis produces haploid gametes from diploid parent 

cells. It leads to genetic variation in gametes through the 

independent assortment of chromosomes and crossing 

over of genetic material. 

Patterns of Inheritance 

KEY IDEAS 

• Mendel’s monohybrid and dihybrid cross experiments 

demonstrated the existence of dominant and recessive 

forms of traits. 

• The combination of alleles in an individual is its genotype. 

The expression of the genotype in an individual is the 

phenotype. A dominant phenotype is expressed when 

a dominant allele is present. A recessive phenotype 

requires two copies of the recessive allele. 

• Punnett squares are used to study the genotypes and 

phenotypes of off spring. 

Complex Patterns of Inheritance 

KEY IDEAS 

• Some patterns of inheritance are more complex than those 

fi rst proposed by Mendel. These include codominant and 

incomplete dominant inheritance patterns. In addition, for 

some traits multiple alleles of a gene exist in the population. 

• Linked genes occur on the same chromosome and tend to 

be inherited together. However, crossing over can unlink 

these genes. 


and female off spring because they are governed by the 



Overall Expectations 

In this unit you learned how to… 

• evaluate the importance of some recent contributions to 

our knowledge of genetic processes, and analyze social 

and ethical implications of genetic and genomic research 

•investigate genetic processes, including those that occur 

during meiosis, and analyze data to solve basic genetic 

problems involving monohybrid and dihybrid crosses 

•demonstrate an understanding of concepts, processes, 

and technologies related to the transmission of hereditary 

characteristics 

• Errors during meiosis can result 

in changes to the structure and 

number of chromosomes. 

• Modern technologies allow 

scientists to manipulate the 

genetic make-up of organisms. 

This has led to many benefi ts. 

• Pedigrees provide information 

about the inheritance of genotypes 

and phenotypes of individuals 

across generations within a family. 

• Karyotyping, fl uorescence 

in situ hybridization (FISH), 

and gene testing are used to 

monitor chromosome structure, 

chromosome number, and 

disease-causing genes. 

• The Human Genome Project 

determined the complete DNA 

sequence of the human genome. 

Many new research fi elds and 

methods have developed from 

this project. 

• Current and future research in 

genomics may allow scientists 

to tailor medical treatments 

for individual patients based on their genetic profi les. 

However, ethical questions about who owns genetic 

information continue to be debated.

UNIT 2 


Knowledge and Understanding 


1. Which phase of meiosis is shown in the illustration 

below? 

a. prophase I 

b. prophase II 

c. metaphase I 

d. metaphase II 

e. interphase 

2. Which of the following statements best describes the 

diff erence between a daughter cell produced by mitosis 

and one produced by meiosis? 

a. A cell produced by mitosis is genetically identical to 

a cell produced by meiosis. 

b. A cell produced by mitosis has half the DNA 

content of a cell produced by meiosis. 

c. A cell produced by meiosis has half the DNA 

content of a cell produced by mitosis. 

d. A cell produced by mitosis is genetically altered due 

to crossing over, but a cell produced by meiosis is 

not. 

e. A cell produced by mitosis can produce an egg or 

sperm cell, but a cell produced by meiosis cannot. 

3. Which of the following processes contributes to genetic 

variation? 

a. cloning 

b. mitosis 

c. crossing over 

d. interphase 

e. cytokinesis 

4. A cross is performed between two pea plants, one with 

the genotype Tt, and the other with the genotype tt. If 

250 off spring are produced, approximately how many 

have the genotype Tt? 

a. 0 

b. 63 

c. 125 

d. 180 

e. 250 

5. Which of the following statements best describes an 

individual whose genetic make-up is shown below? 

a. Th e individual is a male with the correct number 

of chromosomes. 

b. Th e individual is a female with the correct number 

of chromosomes. 

c. Th e individual is a male with trisomy. 

d. Th e individual is a female with trisomy. 

e. Th e individual is a female with monosomy. 

6. Blue fl owers (B) is dominant to white fl owers (b). 

A true-breeding plant with blue fl owers is crossed with 

a true-breeding plant with white fl owers. Which of the 

following statements represents a result of this cross? 

a. Th e off spring all have the genotype Bb. 

b. Th e off spring are all homozygous recessive for blue 

fl owers. 

c. Th e off spring are all homozygous recessive for white 

fl owers. 

d. Th e off spring all have the phenotype bb. 

e. Th e off spring are all homozygous dominant for blue 

fl owers. 

7. What is the predicted phenotypic ratio of the off spring 

from a dihybrid cross between two individuals that 

are heterozygous for both traits? Assume that the two 

genes involved are not linked. 

a. 3:1 

b. 9:3:3:1 

c. 1:2:2:1 

d. 1:1:1:1 

e. 1:3 

Unit 2 Review • MHR 281

UNIT 2 


8. What is a key indicator of autosomal dominant 

inheritance? 

a. Th e trait is passed from father to son. 

b. Th e trait is passed from father through an 

unaff ected daughter to her sons. 

c. Th e trait skips generations. 

d. Two unaff ected parents have an aff ected child. 

e. Two aff ected parents have an unaff ected child. 

9. Incomplete dominance is expected when 

a. one allele prevents the expression of the other allele 

b. the expression of one allele is masked by the 

presence of another allele 

c. an intermediate phenotype is expressed when the 


d. both phenotypes are expressed when the alleles 

occur together 

e. the phenotypes are expressed randomly when the 


10. A man with blood type AB married a woman with 

blood type B who carries an allele for blood type O. 

What are the possible blood types of their children? 

a. O 

b. A and B 

c. A and AB 

d. B and AB 

e. A, B, and AB 

Answer the questions below. 

11. What happens during each phase of interphase? 

12. What is a karyotype and what is it used for? 

13. What are the important features that make 

chromosomes homologous pairs? Why are 

homologous chromosomes not identical? 

14. What are haploid and diploid cells? Where is each cell 

type found? 

15. What are the two essential outcomes of meiosis? 

Identify the phases of meiosis where these outcomes 

are achieved. 

16. Th e diploid cells of a fruit fl y (Drosophila melanogaster) 

contain four chromosomes. 

a. How many pairs of chromosomes does a diploid cell 

of a fruit fl y contain? 

b. How many chromosomes does a haploid cell of a 

fruit fl y contain? 

c. How many genetically distinct gametes can be 

produced from a parent? 


17. Mendel performed his ground-breaking genetic 

experiments using pea plants. List three characteristics 

of pea plants that helped Mendel obtain such conclusive 

results, and thus allowed him to develop his theory 

of inheritance. 

18. Describe what the terms dominant and recessive mean. 

How are they used to describe the forms of a trait at 

the genotype level and at the phenotype level? 

19. What are monohybrid and dihybrid crosses? How can 

Punnett squares be used to represent these crosses? 

20. What is meant by the phrase autosomal recessive 

inheritance? In your explanation, use an example of 

a genetic disorder that is inherited in this manner. 

21. Describe the chromosome theory of inheritance and 

the contribution that Walter Sutton’s research made to 

the development of this theory. 

22. Describe three types of genetic tests that are done and 

the information that each provides. 

23. Why is sickle cell anemia an example of codominant 

inheritance? 

24. Explain how a single gene may have multiple alleles. 

Include an example of a trait aff ected by multiple alleles 

in your explanation, and describe how multiple alleles 

aff ect phenotypes. 

25. Colour vision defi ciency (CVD) is a sex-linked trait. 

Explain why males cannot be carriers for CVD. 

26. Describe the role that bioinformatics played in the 

Human Genome Project. 

27. Describe the similarities and diff erences between 

mitosis and meiosis. 

28. What is the diff erence between a gene and an allele? 

Thinking and Investigation 

29. Errors can occur during meiosis that result 

in alterations to the number and structure of 

chromosomes. 

a. Describe the diff erent types of errors. 

b. What methods are used to detect and diff erentiate 

between these errors? 

30. How do artifi cial insemination and embryo transfer 

diff er in terms of controlling genetic variation? 

31. Compare and contrast oogenesis and spermatogenesis. 

List their similarities and their diff erences.

32. If black coat colour is dominant to white coat colour in 

an animal, what is the 

a. genotype of a homozygous black-coated animal? 

b. genotype of a homozygous white-coated animal? 

c. genotype of a heterozygous animal? 

d. genotypes of the gametes produced by each of the 

animals in parts (a) to (c)? 

33. Th e following data were obtained from an initial cross 

between a true-breeding round-seeded pea plant and a 

true-breeding wrinkled-seeded pea plant. 

a. Based on the data, what are the dominant and 

recessive forms of seed shape? Explain your answer. 

b. Do the data in the tables support the Mendelian ratio? 

Explain your answer, and any diff erences observed. 

Results for the F 1 Generation 

Trait Form Number of Off spring 

Plants with round seeds 175 

Plants with wrinkled seeds 0 

Results for the F2 Generation 

Trait Form Number of Off spring 

Plants with round seeds 154 

Plants with wrinkled seeds 49 

34. In humans, the allele for peaked hairline is dominant 

to the allele for smooth hairline. Is it possible for two 

adults with peaked hairlines to have a child with a 

smooth hairline? Explain. 

35. Copy and complete the table below in your notebook, 

given the information about pea plants in Table 5.1 and 

the following: 

T = tall plant G = green pod colour 

Y = yellow seed colour 

Gamete 

from Male 

Parent 

Gamete 

from Female 

Parent 

TY tY 

Gt gt 

Yg yg 

Genotype 

of Off spring 

Phenotype 

of Off spring 

36. In pea plants, the allele for purple fl owers is dominant 

to the allele for white fl owers and the allele for tall 

plants is dominant to the allele for short plants. Two 

pea plants that are heterozygous for both traits are 

crossed, producing 272 off spring. 

a. Provide the genotype of each parent. 

b. What are the genotypes of the gametes from each 

parent? 

c. What is the expected number of off spring that are 

short plants with white fl owers? 

37. Th e pedigree below traces a genetic disorder in a 

family. 

I 

II 

1 

2 

1 2 3 

a. Do you think the disorder has an autosomal 

dominant or autosomal recessive inheritance 

pattern? Why? 

b. Provide the genotypes and phenotypes for all 

individuals in this pedigree. Explain your answer. 

If there is a genotype you cannot be sure of, explain 

why. 

38. In snapdragons, the alleles for fl ower colour display 

incomplete dominance. 

a. A red-fl owered plant is crossed with a 

white-fl owered plant. What are the predicted 

genotypes and phenotypes of the off spring? Explain 

your answer. 

b. An off spring produced from the mating in part 

(a) is crossed with a white-coloured snapdragon. 

What are the predicted phenotypes and genotypes 

of the off spring? Include the phenotypic ratio of the 

off spring. 

39. From the following blood types, determine which baby 

belongs to which parents. Explain your answer. 

Baby 1 – blood type O 

Baby 2 – blood type B 

Mr. Jones – blood type A 

Mrs. Jones – blood type A 

Mr. Guttierez – blood type A 

Mrs. Guttierez – blood type AB 

40. Determine the probability of a hemophiliac child 

being born when neither the father nor the mother has 

hemophilia, but the mother’s father has hemophilia. Is 

there any chance that their daughters will be aff ected? 

Why or why not? 

41. How do epigenetics and genetics diff er? Provide two 

examples of investigations that illustrate the diff erences 

between these fi elds of study. 

Communication 

42. Draw an illustration that shows the relationship 

between DNA, chromatin fi bre, a chromosome, a gene, 

an allele, and homologous chromosomes. 


UNIT 2 


43. Summarize the process of meiosis in graphic 

form, illustrating the movement and number of 

chromosomes in each cell. 

44. Variability and diversity of living organisms 

result from the distribution of genetic 

materials during the process of meiosis. Crossing over 

and independent assortment play an important role in 

genetic recombination. Draw labelled diagrams to 

show how they provide genetic variation. 

45. Genetic and genomic research can have 

social and environmental implications. 

Th rough genetic modifi cation, some crop plants can 

be engineered to be more resistant to disease. Many 

organizations and citizen groups oppose the use of 

these crops. Choose a crop plant that has been 

genetically modifi ed to be more resistant to disease. 

Research the risks and benefi ts associated with this 

technology. Illustrate these benefi ts and risks in a 

pamphlet, poster, or graphic organizer. 

46. Use a diagram to illustrate how transgenic organisms 

are created. 

47. A Punnett square can be used to predict the possible 

outcomes of a genetic cross. Explain graphically 

how a Punnett square uses the laws of probability 

by diagramming a cross between two pea plants 

heterozygous for height (given that the allele for tall 

plants is dominant to the allele for short plants). 

Predict the genotypic and phenotypic ratios for the 

off spring based on the results of your Punnett square. 

48. Using Punnett squares, illustrate how someone could 

determine whether an organism with a dominant 

phenotype is heterozygous for that trait. 

49. Assume you write a monthly blog for an on-line 

magazine that provides information to the general 

public about various genetic disorders. You have been 

asked to write about Huntington disease. 

a. Provide a description of the genetic abnormality 

that causes Huntington disease and the inheritance 

pattern of the disorder. Also include a brief 

statement about the symptoms, diagnosis, and 

treatment options that are available. 

b. Write a brief paragraph describing your opinion 

on whether genetic testing for Huntington disease 

should be mandatory for family members when 

there is a family history of the disorder. Include 

valid points to support your argument. 

50. Draw a diagram that illustrates gene linkage. 


51. Since Mendel performed his experiments with pea 

plants, scientists have discovered that there are more 

complex patterns of inheritance. Use examples and 

diagrams to illustrate the diff erences among the 

following mechanisms: 

• dominance 

• incomplete dominance 

• codominance 

• sex-linked inheritance 

52. Draw a pedigree that could represent the inheritance 

of hemophilia in a family. When drawing the pedigree, 

ensure that you choose genotypes that will clearly 

illustrate the pattern of inheritance for hemophilia. 

Provide a brief rationale for why the pedigree shows 

the correct inheritance pattern. 

53. In this unit, you have learned how diff erent fi elds of 

study are applied to provide a better understanding 

of genetic processes and human disease. For example, 

bioinformatics was essential for the success of the 

Human Genome Project. Develop an illustration 

using one or two examples of diff erent fi elds of study 

or technologies that have worked together to provide 

a more complete understanding of a genetic process. 

Application 

54. Type 1 diabetes is managed eff ectively with synthetic 

insulin produced by bacteria. Why do scientists 

continue to research this disease in hopes of fi nding 

other treatments or a cure? 

55. What is the risk of relying on artifi cial insemination or 

embryo transfer to produce the off spring in a herd of 

animals? 

56. Stem cell research has led to many ground-breaking 

discoveries, as well as thought-provoking controversies. 

a. Describe some of the controversy surrounding stem 

cell research and how new research has managed to 

reduce the controversy. 

b. Research a development in regenerative medicine 

that has come from stem cell research in Canada. 

Describe what the research involved, as well as its 

potential benefi t to society. 

57. Scientists believe that most human diseases involve 

a complex array of interactions between genetic and 

environmental factors. Why is it not possible to follow 

a trait such as high blood pressure by performing 

a monohybrid cross, as done by Mendel with pea 

plants? Be sure to include both scientifi c and ethical 

considerations in your answer.

58. Many breeds of dogs are known for a high incidence of 

genetic disorders. German shepherd and Saint Bernard 

dogs, like the one shown below, are predisposed to 

developing a crippling condition called hip dysplasia. 

a. Why are purebred dogs more at risk for such 

conditions than mixed breeds? 

b. What advice would you give to dog breeders who 

want to maintain their dogs’ purebred pedigrees, 

but also want their dogs to be as healthy as possible? 

59. Cystic fi brosis is a genetic disorder that leads to the 

build-up of thickened mucus in the lungs and other 

organs. Individuals aff ected by cystic fi brosis are more 

susceptible to respiratory illnesses and must undergo 

physical therapy regularly to manage the symptoms 

of the disease. 

a. Describe the genetic basis of cystic fi brosis and its 

pattern of inheritance. 

b. How can a genetic counsellor help aff ected 

individuals and their families? 

c. One hope for a cure for cystic fi brosis is gene 

therapy. Describe how gene therapy could be used 

to cure cystic fi brosis, and the obstacles that must be 

overcome for gene therapy to provide that cure. 

60. Develop a plot for a movie or play that involves the use 

of gene therapy. Ensure that the application is accurate 

scientifi cally. 

61. Many organisms undergo a heat shock response when 

they are placed at higher temperatures than they 

normally live at. One part of this response involves 

increased expression of certain genes, which helps the 

organisms to cope with the higher temperature. 

a. Describe a technique that could be used to monitor 

this response in an organism. 

b. Saccharomyces cerevisiea, shown below, is a type of 

yeast that undergoes a heat shock response. Th is 

organism has been extensively used in genetics 

studies. Research the use of Saccharomyces cerevisiea 

in genetics studies. Provide a summary of why it is 

such a useful organism for this type of research. 

62. Bioinformatics has applications in many fi elds of study. 

a. Research how bioinformatics is playing an 

important role in cancer research. 

b. Identify a research group in Canada that is using 

bioinformatics as part of its studies in cancer 

research. 

c. Summarize the information you gather and present 

your fi ndings to the class, using a format of your 

choice. 

63. How can having your genetic profi le determined 

pose both potential risks and benefi ts? How has this 

development of genetics research brought to light the 

need for new social and political policies? 

64. Our knowledge in the areas of genetics and genomics 

has grown incredibly since 2000. Choosing one specifi c 

example, discuss how this research has increased our 

understanding of human health and disease. 


UNIT 2 

SELF-ASSESSMENT 


1. K/U Below is a list of characteristics of chromosomes. 

Which combination of characteristics is the same for 

each chromosome of a homologous pair? 

I – same size IV – same gene location 

II – same genes V – same mutations 

III – same alleles 

a. I and II d. I, II, and IV 

b. I and III e. I, II, and V 

c. I, II, and III 

2. K/U What is the diff erence between a karyotype for 

a normal male and a karyotype for a male with 

trisomy 21? 

a. A normal male has 20 chromosomes; a male with 

trisomy 21 has 21 chromosomes. 

b. A normal male has one X chromosome and one 

Y chromosome; a male with trisomy 21 has two 

Y chromosomes. 

c. A normal male has one X chromosome and one 

Y chromosome; a male with trisomy 21 has two 

X chromosomes. 

d. A normal male has 46 chromosomes; a male with 


e. A normal male has 46 chromosomes; a male with 


3. K/U Which of the following reproductive 

technologies produces off spring that are the most 

similar genetically? 

a. preimplantation genetic diagnosis 

b. in vitro fertilization 

c. artifi cial insemination 

d. embryo transfer 

e. embryo splitting 

4. K/U What is the goal of therapeutic cloning? 

a. to produce genetically identical organisms 

b. to produce multiple copies of genes for mass 

production 

c. to produce multiple copies of genes for further 

research 

d. to produce identical cells to treat disease 

e. to repopulate endangered species 

5. K/U What term describes the form of a trait that 

seemed to disappear in Mendel’s crosses of 

true-breeding pea plants? 

a. dominant d. heterozygous 

b. recessive e. sex-linked 

c. homozygous 


6. K/U A gene exists in two diff erent forms, T and t. 

Which allele(s) will be present in the gametes of a 

heterozygous individual? 

a. T d. Tt 

b. t e. TT or Tt or tt 

c. T or t 

7. T/I Th e allele for round seeds is dominant to the 

allele for wrinkled seeds in pea plants. Two pea plants 

that are heterozygous for seed shape are crossed. What 

is the probability of producing a plant with wrinkled 

seeds? 

a. 0 percent d. 75 percent 

b. 25 percent e. 100 percent 

c. 50 percent 

8. K/U In guinea pigs, black coat colour is dominant to 

brown coat colour, and short hair is dominant to long 

hair. How could you determine the genotype of a black 

short-haired guinea pig? 

a. Perform a cross between it and a brown guinea pig 

with either hair length and examine the off spring. 

b. Perform a cross between it and a brown 

short-haired guinea pig and examine the off spring. 

c. Perform a cross between it and a brown long-haired 

guinea pig and examine the off spring. 

d. Perform a cross between it and a black short-haired 

guinea pig and examine the off spring. 

e. Th e genotype cannot be determined. 

9. T/I Attached earlobes and albinism, a lack of skin 

pigment production, have autosomal recessive 

inheritance patterns. Using the pedigree below, 

determine the probability of individuals I-1 and I-2 

having a child with albinism and unattached earlobes. 

I 

II 

1 2 

= albino 

1 2 3 

Key 

= attached 

earlobes 

a. 6.25 percent d. 50 percent 

b. 12.5 percent 

c. 25 percent 


10. K/U Which of the following will inherit an X-linked 

allele from a heterozygous female? 

a. only her sons 

b. only her daughters 

c. half of her sons 

d. all of her daughters 

e. one-quarter of her daughters

Use sentences and diagrams as appropriate to answer the 

questions below. 

11. K/U Describe the two key outcomes of meiosis. 

12. T/I Errors that occur during meiosis are present in 

all cells of the body, whereas errors that occur during 

mitosis may occur in only a small number of cells. 

Explain why this occurs. 

13. C Using labelled diagrams, illustrate how meiosis 

contributes to genetic variation. 

14. A Prenatal testing can be used to determine 

genetic abnormalities. Describe a genetic disorder in 

terms of the source of the disorder, the chromosome(s) 

aff ected, the associated symptoms, and any prevention, 

diagnosis, or treatment options. 

15. A A variety of reproductive technologies, 

including cloning, artifi cial insemination, and in vitro 

fertilization, are used to control the genetic diversity of 

farm animals or plant crops. Choose one method and 

describe how it is used in this manner. 

16. T/I In tomatoes, round shape is dominant to pear 

shape. A student crossed a plant that had round 

tomatoes with a plant that had pear-shaped tomatoes 

and obtained the data below. What are the genotypes of 

the plants that were crossed? Explain your answer. 

Results of a Tomato Plant Cross 

Trait Forms Number of Off spring 

Pear-shaped tomatoes 33 

Round tomatoes 37 

17. T/I A long-haired cat and a short-haired cat have a 

litter of kittens. Th e litter has both short-haired and 

long-haired kittens. If the allele for short hair is 

dominant to the allele for long hair, determine the 

genotypes of the parents. Explain your answer. 

18. T/I In mice, the allele for black fur is dominant to 

the allele for brown fur, and the allele for deafness is 

recessive to the allele for normal hearing. If a mouse 

that is heterozygous for both traits is crossed with a 

homozygous black mouse carrying the deafness allele, 

determine the probability of producing a deaf black 

mouse. 

Self-Check 

If you missed 

question... 

Review 

section(s)... 

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 

4.1 4.2 4.3 4.3 5.1 5.1 5.2 5.2 5.3 6.2 4.2 4.1, 

4.2 

19. A Why is genetic testing usually not recommended 

until aft er a genetic counsellor looks at a family 

pedigree? 

20. C Gene therapy holds much promise for curing 

a number of diseases, including diabetes and cystic 

fi brosis. Th ere are also a number of ethical issues 

related to gene therapy. Using a graphic organizer of 

your choice, summarize the pros and cons associated 

with gene therapy. Refer to Using Graphic Organizers 

in Appendix A for help choosing a graphic organizer. 

21. C Using an organism with black hair and another 

organism with white hair as an example, illustrate the 

diff erence between incomplete dominance and 

codominance with a cross between these organisms. 

22. A Use your knowledge of blood types to match 

the baby with the correct set of parents. Explain your 

answer using Punnett squares. 

Baby 1 – blood type A 

Baby 2 – blood type O 

Mr. Rousseau – blood type AB 

Mrs. Rousseau – blood type B 

Mr. Sakic – blood type A 

Mrs. Sakic – blood type B 

23. T/I Duchenne muscular dystrophy is an example of 

a sex-linked recessive trait found on the X chromosome. 

a. Write the genotypes of a female carrier, a normal 

male, and an aff ected male. 

b. Determine the probability of a female carrier and 

a normal male having an aff ected child. 

c. Is it possible for the parents in (b) to have an 

aff ected daughter? Explain why or why not. 

24. K/U How will the HapMap project help scientists 

gain a better understanding of the genetic reasons for 

diff erent human diseases? 

25. K/U Describe the Human Genome Project and its 

achievements. How has the completion of this project 

been important for the advancement of genetics 

research? 

4.2 4.2, 

5.3 

4.3 5.2 5.2 5.2 5.3 5.3 6.1 6.1 6.2 6.3 6.3 

Unit 2 Self-Assessment • MHR 287

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