81 Problems Involving Dihybrid Inheritance

Key Idea: Dihybrid crosses involve the inheritance of two genes. The problems below will give you an opportunity to practise solving dihybrid crosses.

1. In cats, the following alleles are present for coat characteristics: black (B), brown (b), short (L). long (l), tabby (T), blotched tabby (tb). Use the information to complete the dihybrid crosses below:

(a) A black short haired (BBLl) male is crossed with a black long haired (Bbll) female. Determine the genotypic and phenotypic ratios of the offspring:

Genotype ratio:

Phenotype ratio:

(b) A tabby, short haired male (TtbLl) is crossed with a blotched tabby, short haired (tbtbLl) female. Determine ratios of the offspring:

Genotype ratio:

Phenotype ratio:

2. In humans, two genes affecting the appearance of the hands are the gene for thumb hyperextension (curving) and the gene for mid-digit hair. The allele for curved thumb, H, is dominant to the allele for straight thumb, h. The allele for mid digit hair, M, is dominant to that for an absence of hair, m.

(a) Give all the genotypes of individuals who are able to curve their thumbs, but have no mid-digit hair:

(b) Complete the Punnett square (right) to show the possible genotypes from a cross between two individuals heterozygous for both alleles:

(c) State the phenotype ratios of the F1 progeny:

(d) What is the probability one of the offspring would have mid-digit hair?

3. A plant with orange-striped flowers was cultivated from seeds. The plant was self-pollinated and the F1 progeny appeared in the following ratios: 89 orange with stripes, 29 yellow with stripes, 32 orange without stripes, 9 yellow without stripes. Call orange O and stripes S.

(a) Determine which alleles responsible for the phenotypes observed are dominant and which are recessive:

(b) Determine the genotype of the original plant with orange striped flowers: