In labrador retrievers, black fur is dominant to yellow fur. how is it possible that 2 retrievers with black fur reproduce puppies with yellow fur

 Two Labrador retrievers with black fur can reproduce puppies with yellow fur only when their genotypes are heterozygous dominant for black fur that is- Bb and Bb.

Exaplanation-

As per the given information in the question, black fur is dominant (B) to yellow fur (b). A dominant trait ( depicted by capital letter) masks the expression of recessive trait.

There can be two possible genotypes for black fur that is BB ( homozygous dominant) and Bb ( heterozygous dominant) whereas yellow fur ( which is a recessive trait) can only have one genotype that  bb ( homozygous recessive).

When 2 Labrador retrievers with heterozygous dominant genotype ( Bb) are crossed, they is 25% chance of producing a puppy with yellow fur ( bb).

Refer punnett square.

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