A-Locus Horse (Agouti)

A-Locus (Agouti)

In horses, black coat colour is a trait linked to the gene for the Agouti Signalling Protein (ASIP), also known as the Agouti Locus (A-Locus). Normally, ASIP is in part responsible for the distribution of dark and light pigment across the body. For instance, black colouration in wild horses is limited to the primitive markings on the legs, muzzle and back. However, a mutation to the gene, causing a disruption of ASIP, results in a wholly black coat colour.

The black colour resulting from the A-Locus mutation can be modified by other genes. For instance, the presence of a white spotting allele results in white spots on a black coat, and the presence of the Dun allele on a black coat makes a grullo or mouse-grey horse.

Inheritance

The wild-type allele of the A-Locus is designated as ‘A’. The recessive mutant allele responsible for black coat colour is designated as ‘a’. Horses with two mutated alleles (a/a) are solid black, whilst horses A/a or A/A will be agouti. This final coat colour will be further influenced by the presence of other coat colours and patterns.

Relevant tests

• P907