Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
Answers to self-assessment questions
Man and the child with blood group B: I B I o
Woman and the child with blood group A: I A I o
Child with blood group O: I o I o
11 Suitable symbols for these four alleles could be:
C A to represent the agouti allele
C g to represent the chinchilla (grey) allele
C h to represent the Himalayan allele
C a to represent the albino allele
a C A C A agouti
C A C g agouti
C A C h agouti
C A C a agouti
C g C g chinchilla
C g C h chinchilla
C g C a chinchilla
C h C h Himalayan
C h C a Himalayan
C a C a albino
b i The genotype of the albino parent must
be C a C a , as the allele C a is recessive to
everything else. Each of the offspring will
therefore get a C a allele from this parent.
The offspring are all chinchilla, so their
genotypes must all be C g C a . This means
that the chinchilla parent must have
given a C g allele to each offspring, so the
chinchilla parent almost certainly has the
genotype C g C g . If it had any other allele in
its genotype, this would be expected to
show in the phenotype of its offspring.
When the C g C a offspring are crossed, they
will produce genotypes C g C g , C g C a and
C a C a in a ratio of 1 : 2 : 1; that is, a ratio of
chinchilla to albino in a ratio of 3 : 1. This
is close enough to the actual ratio of 4
chinchilla to 2 albino, as with these very
small numbers it is unlikely for ratios to
work out exactly.
ii Following similar reasoning to that in i
above, the agouti rabbit probably has the
genotype C A C h , and the Himalayan parent
the genotype C h C h . This would produce
agouti and Himalayan offspring in a ratio
of 1 : 1.
iii As chinchilla rabbits are produced in the
first generation, at least one of the agouti
parents must carry a chinchilla allele. As
a Himalayan rabbit is produced in the
Parental
phenotype
Parental
genotype
second generation, one of the original
parents must carry a Himalayan allele.
The first cross is therefore:
agouti
C A C h
agouti
C A C g
Gametes C A or C h C A or C g
Offspring
C A C A
agouti
C A C h
agouti
C A C g
agouti
C h C g
chincilla
The second cross, between two chinchilla
rabbits with the genotype C h C g , would be
expected to produce offspring with the
genotypes C h C h , C h C g and C g C g in a ratio of
1 : 2 : 1,so giving the phenotypic ratio of 3
chinchilla to 1 Himalayan.
12 Parental phenotype female male
Parental genotype XX XY
Gametes X X or Y
Offspring
XX
female
XY
male
13a No. The son will receive a Y chromosome
from his father, which cannot carry a
haemophilia allele.
b Yes. The man could pass on his haemophilia
allele to a daughter, who could then pass it
on to a son.
14a Suitable symbols could be:
X N allele for normal colour vision
X n allele for red-green colour blindness
X N X N normal female
X N X n carrier (normal) female
X N Y normal male
X n Y male with colour blindness
b
Parental phenotype normal normal
Parental genotype X N X n X N Y
Gametes X N or X n X N or Y
Cambridge International AS and A Level Biology © Cambridge University Press 2014