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 allele for black fur (B) and one for white fur (b), yet its fur is black. The allele whose trait appears when paired with a different allele is dominant, so the dominant allele is for black fur.

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