Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
QUESTION
16.15 One of the genes for coat colour in cats is sex linked.
The allele C O gives orange fur, whereas C B gives black
fur. The two alleles are codominant, and when both
are present the cat has patches of orange and black,
which is known as tortoiseshell.
a Explain why male cats cannot be tortoiseshell.
b Draw a genetic diagram to show the expected
genotypes and phenotypes of the offspring from a
cross between an orange male and a tortoiseshell
female cat. (Remember to show the X and Y
chromosomes, as well as the symbols for
the alleles.)
genotype AaDd
A
a
d
D
At metaphase of
meiosis I, the pairs
of homologous
chromosomes line
up on the equator
independently of
each other. For two
pairs of chromosomes,
there are two possible
orientations.
380
Dihybrid crosses
So far, we have considered the inheritance of just one gene.
Such examples are called monohybrid crosses. Dihybrid
crosses look at the inheritance of two genes at once.
You have already seen that, in tomato plants, there is a
gene that codes for stem colour. This gene has two alleles:
stem colour gene A = allele for purple stem
a = allele for green stem
where A is dominant and a is recessive.
A different gene, at a different locus on a different
chromosome, codes for leaf shape. Again, there are
two alleles:
leaf shape gene D = allele for cut leaves (jagged edges)
d = allele for potato leaves (smooth edges)
where D is dominant and d is recessive.
At metaphase of meiosis I, the pairs of homologous
chromosomes line up on the equator independently of
each other. For two pairs of chromosomes, there are two
possible orientations (Figure 16.18).
At the end of meiosis II, each orientation gives two
types of gamete. There are therefore four types of gamete
altogether.
What will happen if a plant that is heterozygous for
both of these genes (for stem colour and leaf shape) is
crossed with a plant with a green stem and potato leaves?
Figure 16.18 shows the alleles in a cell of the plant that
is heterozygous for both genes. When this cell undergoes
meiosis to produce gametes, the pairs of homologous
chromosomes line up independently of each other on
D
A
A
a
d
a
a
d
D
d
A
D
Figure 16.18 Independent assortment of homologous
chromosomes during meiosis I results in a variety of
genotypes in the gametes formed.
or
or
At the end of meiosis II, each orientation gives two types of
gamete. There are therefore four types of gamete altogether.
the equator during metaphase I. There are two ways
in which the two pairs of chromosomes can do this. If
there are many such cells undergoing meiosis, then the
chromosomes in roughly half of them will probably line
up one way, and the other half will line up the other way.
This is independent assortment. We can therefore predict
that the gametes formed from these heterozygous cells
will be of four types, AD, Ad, aD and ad, occurring in
approximately equal numbers.
A
d
A
a
D
a
D
D
d
d
A
a