Suppose that a mouse with black fur has both an allele for black fur and an allele for white fur (Bb). Based on this information, you can conclude that the dominant allele must be for fur and

The mouse has one black-fur allele (B) and one white-fur allele (b), but its fur is black. In a heterozygous organism, the allele whose trait appears is dominant, so the dominant allele is for black fur.

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