50 Black fur color: a dominant trait
Black fur color is dominant over brown
Most of us are familiar with the labrador retriever dog breed, such as the chocolate lab seen in Figure 8. But have you ever thought about what makes this dog brown? The difference between brown and black coat color in dogs is caused by a mutation in the TYRP1 gene. The TYRP1 gene provides instructions for making an enzyme called tyrosinase-related protein 1. This enzyme is required to produce a pigment called eumelanin. Eumelanin is a dark colored pigment.
The TYRP1 gene is located on chromosome 11 in dogs (Parker, 2001).
A group of scientists who were interested in determining what caused the difference between black and brown coats sequenced the DNA within the protein-coding region of the TYRP1 gene (Schmutz, 2002). They identified three variations in the DNA making up the TYRP1 gene between brown dogs and black dogs. These variations in DNA sequence are examples of different alleles of the TYRP1 gene.
Table 1: Variations in the TYRP1 allele that lead to brown color in dogs. Data from Schmutz, 2002.
| Location | Black DNA sequence | Brown DNA sequence | Effect on protein |
| exon 2 | TGT | CGT | changes a cysteine amino acid to a serine |
| exon 5 | CAG | TAG | introduces a premature stop codon which results in 330 amino acids instead of 512 amino acids in the protein |
| exon 5 | CCT | — (deleted) | deletion of a proline amino acid |
All of these variations in the DNA sequence are predicted to cause a change in the amino acid sequence of the TYRP1 protein. These changes affect the production of eumelanin pigment, which is black in color. When eumelanin is not being produced correctly, the dog appears brown instead of black.
Like other diploid organisms, dogs all have two copies of the TYRP1 gene (one from their male parent, one from their female parent). Dogs that are homozygous for the black allele (dogs that have two copies of the black allele) are obviously going to be black in color. Dogs that are homozygous for the brown allele are obviously going to be brown. Dogs that are heterozygous (dogs that have one black allele and one brown allele) appear black. This means that the black allele is dominant over the brown allele. Remember that dominant alleles cover up recessive alleles. If there is one dominant allele present, the dog will appear black. The brown allele is recessive to the black allele. There must be two copies of the recessive brown allele present in order for the dog to appear brown.
Remember that genotypes can be abbreviated with a single letter and that the letter which is chosen is typically the first letter of the dominant trait. In this case, the letter “B” is used to represent the dominant black allele, while “b” represents a recessive brown allele.
The reason that the black allele is dominant over the brown allele in this specific situation is because the black allele produces functional TYRP1 protein, while the brown allele does not. The prese