Chapter 24 : An Adventure in Dog Hair, Part II: Yellow Labs

Abstract:

This chapter highlights the need for the adventure in dog hair, in order to understand how the yellow color is produced. The pigments are synthesized in cells called melanocytes. The phaeomelanins are the family of red and yellow pigments. The exact color of the phaeomelanin depends on the enzymes available for its synthesis, as is the case with the color of the eumelanin (black or brown). In Labrador retrievers, the phaeomelanin is yellow. Yellow Labs are yellow because the receptor for melanocortin 1 (MC1) does not work, so even though the dogs' bodies produce MC1 and have functional genes for TYRP2 and TYRP1, the signal to produce the enzymes is never transmitted to the melanocyte. The gene that causes yellow coat color in Labrador retrievers is actually a nonfunctional allele of the gene for MC1R. Black and brown Labs have at least one functional allele, so their melanocytes receive the hormone signal to make TYRP2 and TYRP1. In the case of yellow Labs, the expression of the black or brown genotype is altered by the alleles of a completely separate gene. Geneticists call this phenomenon epistasis.