The leopard complex is a group of genetically related coat patterns in horses. These patterns range from progressive increases in interspersed white hair similar to graying or roan to distinctive, Dalmatian-like leopard spots on a white coat. Secondary characteristics associated with the leopard complex include a white sclera around the eye, striped hooves and mottled skin. The leopard complex gene is also linked to abnormalities in the eyes and vision. These patterns are most closely identified with the Appaloosa and Knabstrupper breeds, though its presence in breeds from Asia to western Europe has indicated that it is due to a very ancient mutation.
Leopard complex patterns
Coat patterns in the leopard complex range from being hardly distinguishable from an unaffected coat, to nearly pure white. Unlike most other spotting patterns, the spotting and especially the white regions associated with the leopard complex tend to be symmetrical and originate over the hips. Furthermore, a certain amount of this inherited white patterning is present at birth. The amount of white, even if none is present at birth, often grows throughout the horse's life by gradual "roaning" which is not related to graying or true roan. Colored spots reflect the underlying coat color, be it black, chestnut, gray, or silver dun-buckskin. A number of factors, each separately, genetically controlled, interact to produce familiar patterns such as "snowflake," "leopard," and "fewspot".
Leopard spotting
A single, incomplete dominant gene (Lp) controls the presence of leopard-spotting in horses. A dominant gene requires only a single copy to produce an affected phenotype; an incomplete dominant gene produces a different result depending on whether one or two copies are present. A horse's genotype may be lp/lp (homozygous recessive), Lp/lp (heterozygous), or Lp/Lp (homozygous dominant). Horses without a dominant Lp gene do not exhibit leopard-complex traits, and cannot produce offspring with the Lp gene unless it is contributed by the other parent. Such horses are termed "non-characteristic" among Appaloosa horse aficionados. Horses with at least one Lp gene possess, at the very least, leopard-complex "characteristics":
skin that is mottled, speckled or blotchy around the muzzle, eyes, genitals, and anus; the remainder of the body may be primarily pigmented (gray or black in the absence of other genes), primarily unpigmented (pink or flesh-colored), or mottled, striped hooves, white sclera. The presence of regions of alternating pigmented and unpigmented skin may not definitively suggest the leopard gene. They may not be visible due to the effects of other genes. For example, extensive white markings on the face may mask the presence of mottling around the eyes and muzzle, and white markings on the legs often end in white hooves. Furthermore, other genes may produce similar conditions: white sclera are associated with broad white face markings, striped hooves with the Silver dapple gene, and freckled skin with the Champagne gene. A DNA test can now identify the Lp gene, though a combination of pedigree knowledge and coat characteristics also help.
While both heterozygous and homozygous Lp horses possess the aforementioned characteristics, heterozygotes and homozygotes differ significantly in the presence of true spots. True leopard spots are produced only by the Lp gene, and directly reflect the underlying coat color (bay, black, gray, cremello, red dun, and so on). Since these spots match the coat color, they are not visible unless the surrounding pigment is removed. As a rule, heterozygous leopards have larger, more abundant spots, while homozygotes have smaller, scarcer spots.