A black mouse mates with a brown mouse, and all the offsprings are black.(a) Why are no brown offsprings produced?
(b) If two of the black offsprings mate with each other what kind of offspring would you expect and in what proportions? Give reason for your answer.

When a black mouse (BB) mates with a brown mouse (bb), all the offsprings are black 

(a) because black colour gene of the mouse is dominant over brown colour gene. Thus, the presence of B does not allow b to express itself.

(b) Mating of pure black mouse with pure brown mouse produces all black mouse in F1 generation (Bb, heterozygous).

When these black mouse are allowed to undergo mating among themselves, 

                                                 Bb     X     Bb

it will produce BB, Bb, Bb, and bb offsprings.

Thus, genotypic ratio of the offsprings in F2 generation will be BB: Bb: bb = 1: 2: 1; While the phenotypic ratio fo the offsprings in their F2 generation will be Black: brown = 3: 1. 

(BB and Bb will express the black colour gene, and bb will express brown color in mouse)

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Updated on: 21-Mar-2023

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