Notes on Determination of Colour

Lorraine Shelton

Cat color is dependent on the interaction between a variety of different genes. The purpose of this presentation will be to expand upon the typical cat breeder understanding of feline genetics as an alphabet soup of lower case and upper case letters and present the chemistry and embryology involved in creating a cat's color. Receptors, ligands, cellular communication, and enzymes define a cat color. The melanin pigmentation metabolic pathway and the development and migration mechanisms of melanocyte (pigment producing cells) are getting better defined with every passing year of research in this field.

There is a receptor on the surface of melanocytes called Melanocortin Receptor 1 (MC1-R). When a protein called Melanocyte Stimulating Hormone (MSH) interacts with this receptor, eumelanin (black or brown) pigment is produced. When there is no MSH present (the gene for MSH is the current best guess for the "orange" mutation on the X chromosome), only phaeomelanin (orange) pigment can be produced by the melanocyte.

Agouti signaling protein (ASIP) competes with MSH for a place in MC1-R. When it displaces MSH, phaeomelanin is produced. This creates a hair with a black tip (when MSH is in the receptor) and yellow at the roots (when ASIP is in the receptor). This is the mechanism that creates our tabby cats. The interactions between MC1-R, MSH, and ASIP are reversible, creating banding on the hairs, as eumelanin production cycles with phaeomelanin production.

When ASIP is defective (the "A" to "a" mutation), MSH stays in the receptor and only eumelanin is produced. This creates the coloration of a "solid" cat.

Tyrosinase is the primary enzyme needed to create pigment of any type from the amino acid tyrosine. When this enzyme (created at the C locus) is defective, you get the genetic variants cb, cs, and c (in decreasing order of function of the enzyme). This creates cats with pigment only at cooler areas of the skin and, in its most extreme form (c), albino cats.

Further down the metabolic pathway is the enzyme Tyrosinase Related Protein-1 (TRP-1). If this enzyme (created at the B locus) is broken, the eumelanin pigment doesn't get fully formed and you get chocolate (b) or cinnamon (bl) coloration.

Lastly, there is a protein that causes the pigment to be distributed evenly in the growing hair (D). When it is broken (d), pigment deposition is disrupted, and there is a "stuttering" of pigment deposition in the growing hair.

The tabby pattern genes create proteins involved in communication between the melanocyte cells and surrounding cells during migration to the skin and hair follicles. The white and white spotting genes create proteins essential for melanocyte development and distribution throughout the skin, eyes, hair follicles, and a layer of cells in the inner ear essential for sound hearing.

This presentation will also describe the various DNA tests currently available to breeders, how they can be used in a breeding program, and how breeders can contribute to future genetic research.