Question

When true-breeding mice with brown fur and short tails (BBtt) were crossed to truebreeding mice with white fur and long tails (bbTT), all F1 offspring had brown fur and long tails. The F1 offspring were crossed to mice with white fur and short tails. What are the possible phenotypes of the F2 offspring?

Answer 1

F2 phenotypes: brown fur, short tails; brown fur, long tails; white fur, short tails; and white fur, long tails.

Answer 2

Brown fur, long tail (1)

Brown fur, short tail (1)

White fur, long tail (1)

White fur, short tail (1)

Step-by-step explanation:

This is a cross involving two genes, one coding for fur colour and the other for tail length in mice. B and b are alleles for brown and white fur respectively while T and t are alleles for long and short tails respectively. According to the first cross in the question where a brown short-tailed mice (BBtt) was crossed with a white long-tailed mouse (bbTT) to produce a brown long offspring (BbTt), it shows that the brown fur allele (B) and Long tail allele are dominant over the white fur allele and short tail allele respectively. Since, their expression is masked.

Hence the F1 offsprings genotype is BbTt. When BbTt are crossed with a white fur and short-tailed mouse (bbtt), each parent produces the following gamete:

BbTt: BT, Bt, bT and bt

bbtt: bt, bt, bt, bt

Combining these gametes in a punnet square will produce 16 possible offsprings with only four different genotypes in the ratio 1:1:1:1.

The four genotypes with their Phenotypes are:

BbTt: brown fur, long tail (4)

Bbtt: brown fur, short tail (4)

bbTt: white fur, long tail (4)

bbtt: white fur, short tail (4)