Incomplete Dominance
Incomplete Dominance
Incomplete Dominance
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Complex Patterns of<br />
Heredity<br />
Ch. 11, 14<br />
p. 272-273, 350-351
<strong>Incomplete</strong> <strong>Dominance</strong><br />
Codominance<br />
Summary<br />
Traits appear blended<br />
(red + white = pink)<br />
Both traits are<br />
expressed equally<br />
(A + B = AB)
Multiple Alleles<br />
Polygenic Trait<br />
Summary<br />
More than two possible<br />
alleles, but only two<br />
present at a time.<br />
Single trait controlled<br />
by more than one gene.
<strong>Incomplete</strong> <strong>Dominance</strong><br />
• Heterozygous offspring show a phenotype<br />
that is in-between the phenotypes of the<br />
homozygous parents<br />
• Neither allele is dominant<br />
Snapdragon<br />
Flowers<br />
RED + WHITE =<br />
PINK
<strong>Incomplete</strong> <strong>Dominance</strong><br />
W<br />
W<br />
R R<br />
RW RW<br />
RW RW<br />
THE ALLELES REMAIN DISTINCT……ONLY THE<br />
PHENOTYPE APPEARS BLENDED.
Codominance<br />
•2 dominant alleles are expressed<br />
equally at the same time<br />
Ex:<br />
Blood types<br />
“Roan” cattle<br />
Red + White = Red & White<br />
ROAN
Homozygous<br />
Type A blood<br />
Codominance<br />
+<br />
Homozygous<br />
Type B blood<br />
=<br />
Type AB
<strong>Incomplete</strong> <strong>Dominance</strong><br />
Codominance<br />
Summary<br />
Traits appear blended<br />
(red + white = pink)<br />
Both traits are<br />
expressed equally<br />
(A + B = AB)
Multiple Alleles<br />
•Genes with 3 or more alleles<br />
•Only 2 alleles are present at a time<br />
Ex: Blood Types: A, B, AB, O
Blood Types<br />
Phenotype Genotypes<br />
Type A I A I A or I A i<br />
Type B I B I B or I B i<br />
Type AB I A I B<br />
Type O ii
Polygenic Traits<br />
• Traits controlled by two or<br />
more genes that interact<br />
• Ex: eye color, skin color,<br />
hair color, height
Multiple Alleles<br />
Polygenic Trait<br />
Summary<br />
More than two possible<br />
alleles, but only two<br />
present at a time.<br />
Single trait controlled<br />
by more than one gene.
Environmental Effects<br />
(copy this)<br />
• Phenotype is a combination of genetic<br />
and environmental influences<br />
–Example: Himalayan rabbit fur color<br />
depends on body temperature. The<br />
ears, nose, and feet are dark because<br />
they are cooler.
Human Genetics
Difficulties in Studying<br />
• Controlled<br />
experiments are<br />
not possible<br />
• Small numbers of<br />
offspring per<br />
generation<br />
• Long periods<br />
between<br />
generations<br />
Human Heredity
Sex Determination<br />
• Autosomes – body chromosomes – first 22<br />
pair<br />
– Same in both male and female<br />
• Sex Chromosomes – 1 pair, last pair<br />
– XX – Female<br />
– XY - Male
X X<br />
X XX XX<br />
Y<br />
XY XY<br />
XX = 50%<br />
XY = 50%<br />
You always have a 50% chance of having<br />
a girl and a 50% chance of having a boy!
Sex-Linked Traits<br />
• Traits controlled by recessive genes located on<br />
sex chromosomes<br />
• Usually found on the X chromosome<br />
• More common in males<br />
• Ex:<br />
– Hemophilia – missing a blood clotting protein, 1<br />
in 10,000 males<br />
– Color blindness – red/green is the most<br />
common, 1 in 10 U.S. males<br />
– Muscular dystrophy – progressive weakening<br />
and loss of skeletal muscle, 1 in 3000 U.S. males
Sex Linked Traits<br />
• Hemophilia – blood clotting<br />
enzyme<br />
is absent<br />
– Queen Victoria’s family<br />
affected
Sex Linked Traits<br />
• Red – Green Colorblindness – individuals<br />
cannot distinguish between these two colors
Sex Linked Traits<br />
• Duchene<br />
Muscular<br />
Dystrophy –<br />
wasting away of<br />
skeletal muscle
X R<br />
Y<br />
X r<br />
Y<br />
X R<br />
X R X R<br />
X R Y<br />
X R<br />
X R X r<br />
X R Y<br />
X r<br />
X R X r<br />
X r Y<br />
X r<br />
X r X r<br />
X r Y<br />
Females<br />
•X R X R = normal<br />
•X R X r = carrier<br />
•X r X r = disease<br />
Males<br />
•X R Y = normal<br />
•X r Y = disease
Sex – Limited Traits<br />
• Controlled by genes located in the<br />
autosomes<br />
• Only expressed in the presence of sex<br />
hormone<br />
• Only expressed in one gender<br />
• First seen at puberty (hormones<br />
produced in large enough quantities
• Examples:<br />
Sex – Limited Traits<br />
– Male bird’s<br />
colorful plumage<br />
– Beard growth in<br />
males<br />
– Milk production<br />
in females
Sex Influenced Traits<br />
• Traits that are caused by a gene whose<br />
expression differs in males and females<br />
• Also located on autosomes<br />
• Expressed in the presence of male or female<br />
hormones<br />
• Example: male pattern baldness<br />
– BB = normal<br />
– Bb = males bald, females normal<br />
– bb = males and females bald
Hereditary Disorders<br />
• Techniques for detecting genetic disorders<br />
– Amniocentesis – technique used that<br />
removes and studies amniotic fluid<br />
during pregnancy;<br />
– identifies only<br />
chromosomal disorders
Hereditary Disorders<br />
• Karyotyping – an<br />
enlarged photo of<br />
the chromosome<br />
pairs (map) to<br />
identify any<br />
abnormalities in the<br />
chromosomes
Pedigree Charts<br />
• Shows how a trait and the genes that control it<br />
are inherited within a family<br />
• Identifies the presence or absence of particular<br />
trait in members of each generation
Pedigree Charts<br />
• Males = Square<br />
• Females = Round<br />
• Generations = Roman Numerals (I, II, III, IV)<br />
• Individuals = Numbered or Named<br />
• Trait Expressed = Filled in<br />
• Normal = Empty<br />
• Carrier = Half filled