Genes often interact with one another. The term epistasis is applied to cases in which one gene alters the expression of another. The gene locus involved with production of melanin pigment:B(black) has two alleles. Another gene locus determines whether melanin produced is actually deposited in individual hairs as they grow (E) or not deposited(e). Any dog with at least one dominant allele B and one dominant E will be a lighter, chocolate Labrador. If a dog has the homozygous ee genotype it will be a yellow labrador, regardless of the alleles at the pigment locus. A. Give the possible genotypes for a dog with the black labrador phenotype B. Give the possible genotypes for a dog with the chocolate labrador phenotype  C. Give the possible genotypes for a dog with the yellow labrador phenotype 

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In this example, the gene for the deposition of pigment which is coded by the letter E/e is considered to be epistatic to the gene that codes for pigment B/b. Also, the terms gene and locus can be used interchangeably.

It is important to note that BB or Bb will code for a black lab since black (B) is dominant to brown (b).  The only way a chocolate lab will be produced is with the combination bb and provided that the gene for pigment deposition is either EE or Ee. Whenever a dog inherits the homozygous ee, they will NEVER deposit pigment in the hair due to epistasis and will result in a yellow lab.

Therefore, in question a, a black lab has the genotype BB or Bb combined with either EE or Ee. As long as one copy of each dominant gene is present, this will result in the appearance of the black phenotype in the offspring.

In question b, a chocolate lab can result when the dog has the following genotype--bb combined with either EE, or Ee. Chocolate color is recessive to black and in order for this pigment to be deposited, the dog must have at least one copy of the dominant gene E.

In question c, a yellow lab will occur anytime the locus for pigment deposition is homozygous ee. It doesn't matter whether the dog has the genotype BB, Bb, or bb on the locus for pigment color because no pigment will be deposited due to epistasis if the dog has the genotype ee.

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