Question

In rabbits, black (B) coat colour is dominant over white (b) coat colour. Which Punnett square correctly represents a cross between a rabbit heterozygous for coat colour and a white rabbit?

A.

	B	b
B	BB	Bb
b	Bb	bb

B.

	B	B
b	Bb	Bb
b	Bb	Bb

C.

	B	b
b	Bb	bb
b	Bb	bb

D. None of the above.

Answer 1

Hint: In the Punnett square given above, black (B) coat is dominant over white (b) coat colour. There can be three conditions for a coat colour: homozygous black coat colour, heterozygous black coat colour, and homozygous white colour. When the cross between a heterozygous rabbit and a white rabbit is carried out the resulting phenotype ratio will be 1:1.

Complete answer:
According to the law of dominance, a trait is represented by the two different alleles of a gene in a heterozygous condition and by two same alleles in a homozygous condition.
• Here in the case of the mouse coat colour, there are two alleles of the colour which are black (B) and white (b). The black is dominant over the white colour.
• The heterozygous condition in mouse coat colour is represented as (Bb) whereas the homozygous white colour is represented as (bb).
• When the cross between heterozygous and homozygous white is carried out, the heterozygous one produces two gametes which are (B) and (b) in the same proportion i.e. 50% both. The homozygous white produces only one that is (b).
• Therefore, when the cross completes it results in the same ratio of heterozygous red and homozygous white. The ratio 1:1

Hence, the correct answer is option (C).

Note: According to the law of segregation, the two different alleles of a trait that are present in the heterozygous condition (Bb) do not get blended or mixed and they separate out and individually transfer during the gametogenesis. The gametes produced are both in the 50:50 ratio.