Seal Brown (horse) - Genetics of Seal Brown

Genetics of Seal Brown

The appearance of a horse's skin, eye, and coat-color is determined by pigment chemicals called melanins. Two types of melanins are used by mammals, such as horses and humans: eumelanin, which is visually black to brown, and phaeomelanin, visually red to yellow. Specialized cells in the skin and eyes, melanocytes or pigment cells, produce melanins and deposit them into the skin and hair using complex chemical reactions. The instructions for these chemical reactions are genetically encoded as DNA, and are therefore inherited.

See also: Introduction to genetics and Equine coat color genetics

One protein with an important role in eumelanin production is Melanocortin 1 receptor (MC1R). The gene which encodes a functional MC1R protein occupies the Extension locus, or chromosomal position, and is symbolized by the capital E. A mutation, or change, in the equine MC1R gene, that resulted in a non-functioning MC1R protein, was identified in 1996. This form of the gene is symbolized by the lowercase e or sometimes Ee. Since each horse has two copies of the MC1R gene, one from each parent, horses with one "broken" copy (e) can still produce eumelanin in the hair. However, if a horse has two copies of the "broken" allele and therefore no copies of the functional E allele, the horse is completely incapable of depositing black pigment in the hair. Such horses are chestnut, or at least, are not black, bay, or seal brown.

The gene with the greatest role in the seal brown coat phenotype is Agouti signalling peptide (ASIP) or simply Agouti. The functional Agouti signalling peptide (protein) acts as a switch between red-yellow phaeomelanin and black-brown eumelanin. It is ASIP that is responsible for the alternate banding of dark and light on animal hair, although it is also responsible for whole-body effects. ASIP attaches to MC1R molecules to temporarily prevent the latter from continuing the production of eumelanin, and so phaeomelanin is produced in its stead. Although an individual horse can only have two copies of the Agouti gene, there are four alleles or options:

• A+ (wildtype, dominant) highly restricted black points such as those seen in Przewalski's horses.
• A (mutation, 2o dominant) restricted black points, responsible for standard bay.
• At (mutation, 3o dominant) poorly restricted black points, responsible for seal brown.
• a (mutation, recessive) unrestricted black points, uniformly black coat.

The A and At alleles can be identified by DNA test. To be seal brown, a horse must have at least one copy of the functional MC1R gene (E/E or E/Ee) and must have one of the following genotypes at the Agouti locus: At/At or At/a.

Bay horses also possess at least one copy of the functional MC1R gene (E/E or E/Ee) but will possess one of the following genotypes at the Agouti locus: A/A, A/At, or A/a. While they may possess one copy of the At allele, its effects are not visible when a copy of the dominant Agouti allele is present.