Coat color in Labrador retrievers is usually yellow, black, or chocolate brown. This is determined by alleles of the E gene and the B gene. Dogs with recessive e alleles (ee) are yellow, even when the B locus is functional. This means that the B gene is epistatic to the E gene.

Question

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The E gene is epistatic to the B gene because the expression of the B gene (which determines whether the coat is black or chocolate) is dependent on the E gene's expression. If the E gene has the recessive ee combination, the dog will be yellow regardless of the B gene's alleles.

Coat Color in Labrador Retrievers Is Usually Yellow, Black, or Chocolate