A mouse has alleles for both black fur and white fur The mouse has because the dominant allele, B, is always expressed.

A mouse with alleles for both black fur and white fur has two different alleles for the trait. An organism with two different alleles is heterozygous. The dominant B allele is expressed, but that does not make the mouse homozygous; homozygous means having two matching alleles.

AI-written and checked against the verified answer.