The offspring between 2 black mice show the ratio of 3 black mice to 1 white mouse ...

a) which trait is dominant?

b) what must be the genotypes of the parents?

Answer: A) Black trait is dominant

B) The genotypes of the parents must be Bb

Explanation: Let B represent the allele for black colour and b represent the allele for white colour. If the offspring between 2 black mice show the ratio of 3 black mice to 1 white mouse, it then means that the two parents are heterozygous for black colour, that is each of them have one allele for black colour (B) and one allele for white colour (b). They both have a genotype of Bb.

Bb x Bb = BB, Bb, Bb, and bb.

This means 1BB : 2Bb : 1bb

BB is black, Bb is black while bb is white which gives a ratio of 3 black mice to one white mouse. For Bb to manifest as black, it means that B is dominant to b.