Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
376
As there are equal numbers of each type of sperm
and each type of egg, the chances of each of these four
possibilities are also equal. Each time a child is conceived,
there is a one in four chance that it will have the genotype
Hb A Hb A , a one in four chance that it will be Hb S Hb S and
a two in four chance that it will be Hb A Hb S . Another way
of describing these chances is to say that the probability
of a child being Hb S Hb S is 0.25, the probability of being
Hb A Hb A is 0.25, and the probability of being Hb A Hb S
is 0.5. It is important to realise that these are only
probabilities. It would not be surprising if this couple had
two children, both of whom had the genotype Hb S Hb S
and so suffered from sickle cell anaemia.
Genetic diagrams
A genetic diagram is the standard way of showing the
genotypes of offspring that might be expected from two
parents. To illustrate genetic diagrams, let us consider
flower colour in snapdragons (Antirrhinum).
One of the genes for flower colour has two alleles,
namely C R , which gives red flowers, and C W , which
gives white flowers. The phenotypes produced by each
genotype are:
Genotype
C R C R
C R C W
C W C W
Phenotype
red
pink
white
What colour flowers would be expected in the offspring
from a red and a pink snapdragon?
Parental phenotypes red pink
Parental genotypes C R C R C R C W
all C or
Gametes
R C R C W
in equal proportions
Offspring genotypes and phenotypes:
Gametes from red parent
C R
C R C R
C R red flowers
Gametes from
pink parent
C R C
C W
W
pink flowers
Thus, you would expect about half of the offspring to
have red flowers and half to have pink flowers.
QUESTION
16.6 Red Poll cattle are homozygous for an allele
that gives red coat colour. White Shorthorn are
homozygous for an allele that gives white coat
colour. When crossed, the offspring all have a
mixture of red and white hairs in their coats,
producing a colour called roan.
a Suggest suitable symbols for the two alleles of
the coat colour gene.
b List the three possible genotypes for the coat
colour gene and their phenotypes.
c Draw genetic diagrams to show the offspring
expected from the following matings:
i a Red Poll with a roan ii two roans.
Dominance
In the examples used so far, both of the alleles in a
heterozygous organism have an effect on the phenotype.
A person with the genotype Hb A Hb S has some normal
haemoglobin and some sickle cell haemoglobin. A
snapdragon with the genotype C R C W has some red colour
and some white colour, so that the flowers appear pink.
Alleles that behave like this are said to be codominant.
Frequently, however, only one allele has an effect in a
heterozygous organism. This allele is said to be the dominant
allele, whereas the one that has no effect is recessive. An
example is stem colour in tomatoes. There are two alleles
for stem colour, one of which produces green stems, and
the other purple stems. In a tomato plant that has one allele
for purple stems and one allele for green stems, the stems
are exactly the same shade of purple as in a plant that has
two alleles for purple stems. The allele for purple stems is
dominant, and the allele for green stems is recessive.
When alleles of a gene behave like this, their symbols
are written using a capital letter for the dominant allele
and a small letter for the recessive allele. You are often
free to choose the symbols you will use. In this case, the
symbols could be A for the purple allele and a for the green
allele. The possible genotypes and phenotypes for stem
colour are:
Genotype
AA
Aa
aa
Phenotype
purple stem
purple stem
green stem