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Chapter 24 : Mendelian Genetics at the Molecular Level: An Example of Epistasis
The black/brown coloration of Labrador retrievers is determined by whether their melanocytes can synthesize TYRP1 and thus convert brown eumelanin to black eumelanin. The yellow coat color is based on the ability of follicle melanocytes to receive a hormone signal that induces production of TYRP2 and TYRP1. These enzymes, which convert dopaquinone to the brown and black pigments, respectively, are not constitutively produced in melanocytes. Instead, the melanocytes receive a signal from the hormone melanocortin 1 (MC1) (also referred to as melanocyte-stimulating hormone), which causes the cell to produce the enzymes. The hormone signal is transmitted when the hormone binds to a membrane receptor protein, melanocortin 1 receptor (MC1R). The melanocytes of yellow Labs cannot receive the hormone signal to produce TYRP2 and TYRP1. Researchers took skin biopsies and used PCR to determine the protein-coding sequence of the TYRP1 gene. They found two common variants that could account for brown coat color—a premature stop codon in exon 5 and the deletion of a proline, also in exon 5—in addition to one less common variant.