Dihybrid Genetics Problems

Image produced by Dawn M Turner

1.In mice the gene for coat colour has two forms. The allele for coloured coat is dominant to the allele for albino. There are two forms for the gene controlling whiskers, as well, straight is dominant to bent.

Consider a cross between two mice heterozygous for both these genes.

a.What proportion of the offspring would you expect to be albino?
b.What proportion would have bent whiskers?
c.What would be the expected ratio of offspring phenotypes with respect to both alleles?


2.If a maize plant heterozygous for the alleles for pigmy and crinkly-leaf (both recessive to normal size of plant and normal leaf) is self-pollinated and 208 seeds are subsequently collected and germinated, how many would you expect to show:

a.crinkly leaves?
b.normal size?
c.normal leaves and normal size?
d.normal leaves and pigmy size?

3.If pollen from the original maize plant in the previous question was used to pollinate a plant which showed both pigmy size and crinkly leaves, what offspring phenotypes would be expected and in what proportions?


4.In rabbit coats, spotted is dominant to solid colour and black is dominant to brown. A brown, spotted rabbit is mated with a solid, black one and all the offspring are black and spotted.

What are the genotypes of their parents?
What would be the appearance of the F2 generation if two of these F1 black, spotted rabbits were mated? (Show complete diagram)


5.In guinea pigs, black hair is dominant to brown hair and short hair is dominant to long hair. What would be the genotypes of the following?

a.A black, long haired variety, homozygous for both traits?
b.A brown, short haired variety, heterozygous for hair length?
c.A brown, long haired variety?


6.In fruit flies, normal wings are dominant to vestigial wings and grey bodies are dominant to black bodies.

In a dihybrid cross, a pure bred grey, normal winged fly is crossed with a black, vestigial winged fly. What will be the phenotype and genotype of the F1 offspring? If two flies from the F1 generation are mated together, the F2 generation gives the following results:

31 grey body, normal wings
10 black body, vestigial wings
1 grey body, vestigial wings
2 black body, normal wings

Use a complete dihybrid diagram to show what the expected F2 generation would have been and then explain clearly why the actual results differ from the expected. Include in you explanation why such a small number of grey, vestigial and black, normal varieties occurred.


7.In peas, the allele for tall (T) plants is dominant to the allele for dwarf (t) plants and the allele for round (R) peas is dominant to that for wrinkled (r) peas.

Find both phenotypic ratios and the genotypic ratios for the results of each of the following crosses:

a. TtRr x TtRr
b. Ttrr x ttrr
c. ttRr x Ttrr
d. TTrr x ttRR