Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
Answers to self-assessment questions
Genotypes
of sperm
X N
Y
Genotypes of eggs
X N X N
X N
normal female
X N Y
normal male
X N X n
X n
carrier female
X n Y
male with
colour
blindness
This can happen if the woman is
heterozygous. The affected child will be male.
c Yes, if the mother has at least one allele for
colour blindness, and the father has colour
blindness.
15a Male cats cannot be tortoiseshell, because a
tortoiseshell cat has two alleles of this gene.
As the gene is on the X chromosome, and
male cats have one X chromosome and one
Y chromosome, then they can only have one
allele of the gene.
b
Parental
phenotype
Parental
genotype
tortoiseshell
female
X CO X CB
orange
male
X CO Y
Gametes X CO or X CB X CO or Y
Genotypes of eggs
X CO
X CB
chromosomes carrying the A or a alleles
behave quite independently from those
carrying the D or d alleles. This means that
allele A can end up in a gamete with either
D or d, and allele a can also end up in a
gamete with either D or d. Thus independent
assortment is responsible for the fact that
each parent can produce four different types
of gamete, AD, Ad, aD and ad.
b Random fertilisation means that it is equally
likely that any one male gamete will fuse with
any female gamete. This results in the 16
genotypes shown in the square. Thus random
fertilisation is responsible for the different
genotypes of the offspring and the ratios in
which they are found.
17a AaBb and Aabb in a ratio of 1:1
b GgHh, Gghh, ggHh and gghh in a ratio of
1 : 1 : 1 : 1
c All TtYy
d EeFf, Eeff, eeFf and eeff in a ratio of 1 : 1 : 1 : 1
18a They would all have genotype GgTt and
phenotype grey fur and long tail.
b Grey long, grey short, white long, white short
in a ratio of 9 : 3 : 3 : 1.
19 Let T represent the allele for tall stem, t the
allele for short stem, L G the allele for green
leaves and L W the allele for white leaves.
Parental
genotype
TTL G L G
ttL G L W
Genotypes
of sperm
X CO
X CO X CO
orange
female
X CB X CO
tortoiseshell
female
Gametes TL G tL G or tL W
Offspring
TtL G L G and TtL G L W
in a ratio of 1 : 1
Y
X CO Y
orange
male
X CB Y
black
male
The kittens would therefore be expected to be
in the ratio of 1 orange female : 1 tortoiseshell
female : 1 orange male : 1 black male.
16a Independent assortment results from
the random alignment of the pairs of
homologous chromosomes on the equator
during metaphase I. It ensures that the
20a If B represents the allele for black eyes
and b represents the allele for red eyes,
L represents the allele for long fur and l
represents the allele for short fur, then the
four possible genotypes of an animal with
black eyes and long fur are: BBLL, BbLL, BBLl
and BbLl.
b Perform a test cross – that is, breed the
animal with an animal showing both
recessive characteristics. If the offspring
Cambridge International AS and A Level Biology © Cambridge University Press 2014