Multiple Alleles & Dihybrid Crosses
1) Multiple alleles control the coat colour of rabbits. A grey colour is produced by the dominant
allele C. The Cch allele produces a silver-grey colour, called chinchilla, when present in the
homozygous condition, CchCch. When Cch is present with a recessive allele, a light silvergrey colour is produced. The allele Ch is recessive to both the full colour-allele and the
chinchilla allele. The Ch allele produces black a white colour and black extremities. The
colouration is called Himalayan. An allele Ca is recessive to all alleles. The Ca allele results
in a lack of pigment, called albino. The dominance hierarchy is C > Cch > Ch > Ca. Below
are the possible genotypes and phenotypes:
Phenotypes
Genotypes
Full colour
CC, CCch, CCh, CCa
Chinchilla
CchCch
Light grey
CchCh, CchCa
Himalayan
ChCh, ChCa
Albino
CaCa
a) Indicate the genotypes and phenotypes of the F1 generation from the mating of a
heterozygous Himalayan rabbit with an albino rabbit.
b) The mating of a full-coloured rabbit with a light-grey rabbit produces two full-coloured
offspring, one light-grey offspring, and one albino offspring. Indicate the genotypes of
the parents.
c) A chinchilla rabbit is mated with a light-grey rabbit. The breeder knows thet the lightgrey rabbit had an albino mother. Indicate the genotypes and phenotypes of the F1
generation from this mating.
d) A test cross is performed with a light-grey rabbit, and the following offspring are noted:
five Himalayan rabbits and five light-grey rabbits. Indicate the genotype of the light-grey
rabbit.
3) In guinea pigs, colour of coat is determined by at least three alleles. Yellow coat is
determined by the homozygous genotype YY, white by the homozygous genotype WW, and
cream by the heterozygous genotype YW. Determine the expected genotype and phenotype
ratio of the F1 generation which would result from a cross between:
(a) Two cream coloured guinea pigs;
(b) A yellow coated and a cream coated animal
4) What gametes are possible from the individuals with the following alleles? Remember to
state your genotypic and phenotypic ratios!
(a) AaBb
(b) CCDd
(c) EeFF
(d) GGHH
(e) JjKk
5) In cats, the allele for black fur is dominant to the allele for brown fur, and the allele for short
hair is dominant to the allele for long hair. Show the genotypes and corresponding
phenotypes of the following crosses:
a. a diheterozygous male with a heterozygous black fur, long hair female;
b. a homozygous black fur, heterozygous short hair male, heterozygous black fur, long
hair female;
c. a diheterozygous male with a dihomozygous recessive female.
d. What proportion of the offspring from the cross shown in part b would be expected to
have black fur with short hair?
6) Dark hair is dominant over blonde hair, and a widow's peak hairline is dominant over a
straight hairline. If two people who were both heterozygous for both traits had children, what
is the phenotype ratio for the possible offspring? Don’t forget to assign your alleles!
Multiple Alleles & Dihybrid Crosses
7) For each of the following crosses remember that haemophilia, muscular dystrophy (MD), and colour
blindness are recessive sex-linked traits in humans. In sex linked traits men will always express the
trait if they carry it on the X chromosome. Women can express the trait only if it is found on both X
chromosomes. Women have two normal phenotypes: homozygous normal and carrier. Men have
only one normal phenotype because they have only one X chromosome. Haemophilia for example:
Normal Male: XHY, Haemophiliac Male XhY, Normal Female: XHXH & XHXh (carrier), Haemophiliac
Female XhXh.
a) A woman that is a carrier of haemophilia marries a haemophiliac man. What is the probability that
their first child will be a haemophiliac?
b) A haemophiliac woman has a mother who is phenotypically normal. What are the genotypes of
her mother and her father?
c) What is the probability that a normal vision woman who marries a man who is also colour blind,
will have a daughter who is colour blind?
d) A phenotypically normal man who has a brother with MD marries a homozygous normal woman.
What is the probability that any of their children will have MD?