Horse

The Agouti gene controls the distribution of black pigment and determines whether a horse will have a bay or black coat. The effect of the agouti gene is to be combined with the extension gene to obtain the three basic coat colours. Horses carrying the E allele of the extension gene are black if they have two copies of the a allele of the agouti gene and bay if they have at least one copy of the A allele of the agouti gene.

The Extension (red factor) gene codes for the MC1R protein which produces the black pigment (eumelanin). The dominant allele E produces a black pigment in the coat and the recessive allele e produces a red pigment. A horse with two copies of the e allele will have a chestnut coat. A horse with one or two copies of the E allele will have a bay or black coat. The effect of the extension gene is to be combined with the agouti gene (HORSE-AG) to obtain the three basic coat colours.

Lavender Foal Syndrome (LFS) is a genetic disease that causes severe neurological symptoms. Affected foals are unable to stand, have seizures, show hyperextension of the limbs, have involuntary eye movements and have a pink (lavender) coat. Affected foals die soon after birth and are usually euthanised.

The Overo horse has a specific white spotted pattern. These horses are heterozygous for a mutation (one copy of the allele) which is lethal in the homozygous state (two copies of the mutated allele) as white foals die from complications due to abnormalities of the intestinal tract.

The Gray gene causes a progressive depigmentation of the coat. Grey horses can be born with any colour and then gradually turn grey and finally almost completely white (at approximately 6 to 8 years of age). The HORSE-GREY test is used to differentiate horses that are not carriers of the Grey mutation from horses that will gradually turn grey. The test does not differentiate heterozygous horses from homozygous horses.

Champagne is a dilution of coat colour, which lightens the pigments in the coat from black to classic champagne, a bay coat becomes amber champagne and a chestnut coat is diluted to gold champagne. Champagne horses are born with unpigmented skin that develops black spots during the first days after birth.

The cream gene dilutes the basic coat colours (chestnut, bay and black) and leads to these coat colours: palomino, buckskin and smoky black if only one copy of the cream gene is present and cremello, perlino and smoky cream if two copies are present.

The dun is a dilution of the coat colour characterised by a lightening of the coat, with the head, lower legs, mane and tail not being diluted. Often the dun is also characterised by primitive markings such as a dark dorsal stripe. The dun dilutes a chestnut coat into a chestnut dun, a bay coat into a bay dun, and a black coat into a steel gray.

The Tobiano pattern is a pattern of white spots characterised by a white coat on the spine and four white legs below the knees. The spots are usually regular and distinct as oval or round patterns. 

Silver is a dilution of the coat colour that dilutes the black pigment and gives it a silvery appearance. Silver dilution is also associated with an inherited eye syndrome known as multiple congenital eye anomalies (MCOA).