Fur color in mice is affected by a gene with two alleles. The B allele causes dark brown fur and is completely dominant over the b allele that causes light brown fur. What is the fur color of a mouse with the genotype, Bb

Since this is heterozygote dominant state(Bb), and B allele is dominant over the b allele brown(b) ,hence the fur color must belong to the dominant allele which is dark brown.

Explanation:

Generally a dominant gene is the type that manifest itself from generations to generation irrespective of the presence of the recessive gene.

If there are two allele for fur colors (dark and light brown)

Therefore B(Dark brown) is a dominant allele,assuming with genotype BB.

while the recessive light brown (bb)

Therefore if the F1 generation is crossed the offspring are

Bb,Bb,Bb,Bb. These are hetero zygote dominants

Therefore the phenotype is dark brown.

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