Question

A mouse from a true-breeding population with normal gait was crossed to a mouse displaying an odd gait called “dancing.” The F1 animals all showed normal gait. If dancing is caused by homozygosity for the recessive allele of a single gene, what proportion of the F2 mice should be dancers if you cross two F1 mice?

25%

50%

75%

0%

33%

Answer 1

The proportion of the F2 mice should be dancers when you cross two F1 mice = 25%

Let's assume a gene code for the gait character.

G - Dominant allele

g - Recessive allele

• First cross:

Here a true-breeding mouse with normal gait was crossed with a mouse with a dancing trait ( dancing is caused by homozygosity for the recessive allele).

parental genotypes: GG (Normal) gg (Dancing)

gametes: G g

F1 :

	G
g	Gg(Normal gait)

F1   Gg (Normal gait)

(From the F1 progeny, we can understand that normal gait is dominant over dancing).

• Second cross :

The crossing of two F1 mice

F1 F1

Parental genotypes: Gg Gg

Gametes: G g G g

F2:

	G	g
G	GG (Normal)	Gg (Normal)
g	Gg (Normal)	gg (Dancing)

Here, out of 4 chances, only one becomes a dancer. i.e., probability of dancers in F2 = 1/4 = 25%

The proportion of the F2 mice should be dancers = 25%