The genetics of Appaloosa horses

Spotting only shows in horses with at least one copy of the dominant Appaloosa spotting allele Ap^Ap, although it is then influenced by several other genes. Horses with this gene but no other spotting genes show at least one of the three minimal appaloosa characteristics. These are mottled skin pigment on the face and/or genitals, hooves striped with narrow vertical bands and white sclera of the eye. The skin pigmentation may not be present at birth, but the other two characteristics are. Pink and black mottled skin is one of the most indicative characteristics. It is always evident in the genital region and may also occur around the lips, muzzle, nostrils and/or eyes. An appaloosas eye is sometimes called the "human eye" due to the white sclera encircling the eye. Most other horses have a dark sclera.

Horses showing all three traits are said to be mottled. The expression of appaloosa characteristics is thought to be influenced by sex, possibly through an hormonal effect. There is some evidence that stallions show more patterning than mares. Jeanette Gower (1999) suggests the following phenotypes for horses with no other spotting genes:

Genotype at the appaloosa locus stallions mares

Ap⁺Ap⁺ solid solid

Ap^ApAp⁺ mottled 1 or 2 minimal characteristics

Ap^ApAp^Ap false snowflake mottled

Appaloosa horses, and spotted horses in general, come in an enormous variety of patterns. There are many names for the different patterns observed and some patterns are known by more than one name depending on the breed and the local customs. From a genetic perspective Jeanette Gower (1999) proposed a model based on four genetically distinct patterns. The presence of one or more of these patterns in various combinations provides for the array of patterns actually observed. I will follow this model since it seems to me the best available based on the current understanding of appaloosa genetics. The four patterns identified by Gower are snowcap or blanket (discussed below), dark spots, white spots and varnish. In the discussions of these it is assumed that there is at least one copy of the dominant appaloosa spotting allele Ap^Ap.

Snowcap (blanket)

Snowcaps have varying amounts of white, ranging from a white patch over the hips (snowcap or white blanket), to a blanket that extends over the rump and as far as the withers in front. Large snowcaps may even cover most of the body with color remaining on the head, legs, girth area, flanks, mane and tail. The snowcap pattern occurs at birth and changes little, if at all, during the life of the horse.

In the extreme form of snowcap the white may cover the whole body with perhaps just dark �flash� markings on legs. This form is called few spot, is true-breeding, and is homozygous for the incompletely dominant snowcap allele Sc^S. When crossed to non snowcap appaloosas the offspring will show a white blanket (snowcap), providing they also least one copy of the dominant appaloosa spotting allele Ap^Ap. When crossed to heterozygotes there will be a 50% chance of snowcap and a 50% chance of few spot.

It would seem that the Sc^S also causes night-blindness when homozygous, with few-spots night-blind to some degree. Whether this is a recessive effect or occurs to some degree in other snowcaps is uncertain. Since the Sc^S allele is affecting more than character it is said to show pleiotropy, although there is also a possibility that it is tightly linked to another gene affecting eyesight.

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