Dihybrid crosses
1. Assume that in a species of fish having large scales is dominant to having small scales and being brightly
coloured is dominant to being dull coloured .
Perform the following crosses.
a. A heterozygous large scale and heterozygous brightly coloured fish has 450 offspring with a small scale
heterozygous brightly coloured fish.
Calculate how many of the offspring would you expect (assuming that the gene for scale size and the gene for
colour are unlinked) to be?
i. large scales and brightly coloured
ii. large scale and dull coloured
iii. Small scale and brightly coloured
iv. Small scale and dull coloured
When the 450 offspring fish were counted for their combination of scale size and colour the following was
observed.
i. large scales and brightly coloured = 120
ii. large scale and dull coloured
= 145
iii. Small scale and brightly coloured = 115
iv. Small scale and dull coloured
= 70
What will you conclude when the observed numbers do not coincide with the expected numbers?
2. Do this one slowly as it is a little tricky
A man who has normal vision and is heterozygous for sickle cell anaemia has a child with a woman who is a
carrier for colourblindness who has sickle cell anaemia.
Use the following symbols:
XB = normal vision, Xb = colourblind allele,
HbA = normal allele for haemoglobin, HbS = mutated allele causing sickle cell anaemia
Calculate the chance that they will be
i.
Male with normal vision and homozygous normal for sickle cell anaemia
ii.
Male with normal vision and carrier for sickle cell anaemia
iii.
Male with normal vision and has sickle cell anaemia
iv.
Male with colourblindness and homozygous normal for sickle cell anaemia
v.
Male with colourblindness and carrier for sickle cell anaemia
vi.
Male with colourblindness and has sickle cell anaemia
vii.
Female and homozygous normal for vision and homozygous normal for sickle cell anaemia
viii.
Female and homozygous normal for vision and heterozygous for sickle cell anaemia
ix.
Female and homozygous normal for vision has sickle cell anaemia
x.
Female and carrier for colourblindness and homozygous normal for sickle cell anaemia
xi.
Female and carrier for colourblindness and heterozygous for sickle cell anaemia
xii.
Female and carrier for colourblindness has sickle cell anaemia
xiii.
Female and has colourblindness and homozygous normal for sickle cell anaemia
xiv.
Female and has colourblindness and heterozygous for sickle cell anaemia
xv.
Female and has colourblindness and has sickle cell anaemia