White horses have unpigmented skin and a white hair coat. Many white horses have dark eyes, though some have blue eyes. In contrast to gray horses which are born with pigmented skin they keep for life and pigmented hair that lightens to white with age, truly white horses are born with white hair and mostly pink, unpigmented skin. Some white horses are born with partial pigmentation in their skin and hair, which may or may not be retained as they mature, but when a white horse lightens, both skin and hair lose pigmentation. In contrast, grays retain skin pigment and only the hair becomes white.
White colorings, whether white markings, white patterns or dominant white are collectively known as depigmentation phenotypes, and are all caused by areas of skin that lack pigment cells (melanocytes). Depigmentation phenotypes have various genetic causes, and those that have been studied usually map to the EDNRB and KIT genes. However, much about the genetics behind various all-white depigmentation phenotypes are still unknown.
Dominant white is best known for producing pink-skinned all-white horses with brown eyes, though some dominant white horses have residual pigment along the topline. Dominant white is, as the name implies, a genetically dominant color. At least one parent must be dominant white and it does not "skip" generations because it is not recessive. Nonetheless, new variations or mutations producing dominant white do occur spontaneously from time to time. Dominant white is rare, but has occurred in many breeds. It has been studied in Thoroughbreds, Arabian horses, the American White horse and the Camarillo White horse. There are 11 identified variants of dominant white, each corresponding to a spontaneously-white foundation animal and a mutation on the KIT gene. No horse has been identified as homozygous dominant white, and researchers have suggested that at least some forms of dominant white results in nonviable embryos in the homozygous state. While homologous mutations in mice are often linked to anemia and sterility, no such effects have been observed in dominant white horses. Dominant white horses typically have white noses that can be subject to sunburn.
Sabino-white horses are pink-skinned with all-white or nearly-white coats and dark eyes. They are homozygous for the dominant SB1 allele at the Sabino 1 locus, which has been mapped to KIT. Without a DNA test, Sabino-white horses are indistinguishable from dominant white horses. The Sabino1 allele, and the associated spotting pattern, is found in Miniature horses, American Quarter Horses, American Paint Horses, Tennessee Walkers, Missouri Fox Trotters, Mustangs, Shetland Ponies, and Aztecas. Sabino 1 has not been found in the Arabian horse, Clydesdale, Thoroughbred, Standardbred horse, or Shire horse. The Sabino 1 allele is not linked to any health defects, though sabino-whites may need some protection from sunburn. Horses with only one copy of the Sabino1 gene usually have dramatic spotting, including two or more white legs, often with white running up the front of the leg, extensive white on the face, spotting on the midsection, and jagged or roaned margins to the pattern.
The leopard complex, related to the Leopard (LP) gene, characterizes the Appaloosa and Knabstrupper breeds with their spotted coats. Leopard is genetically quite distinct from all other white and white-spotting patterns. The fewspot leopard pattern, however, can resemble white. Two factors influence the eventual appearance of a leopard complex coat: whether one copy (heterozygous LP/lp) or two copies (homozygous LP/LP) Leopard alleles are present, and the degree of dense white patterning present at birth. If a foal is homozygous for the LP allele and has extensive dense white patterning, they will appear nearly white at birth, and may continue to lighten with age. In other parts of the world, these horses are called "white born." "White born" foals are less common among Appaloosa horses than Knabstruppers or Norikers, as the extensive dense white patterning is favored for producing dramatic full leopards. Homozygous leopards have the LP/LP genotype, and may be varnish roan, fewspot leopard, or snowcap patterned. Homozygous leopards are substantially more prone to congenital stationary night blindness. Congenital stationary night blindness is present at birth and is characterized by impaired vision in dark conditions.
Lethal white syndrome is a genetic disorder linked to the Frame overo (O) gene and most closely studied in the American Paint Horse. Affected foals are carried to term and at birth appear normal, though they have pink-skinned all-white or nearly-white coats and blue eyes. However, the colon of these foals cannot function due to the absence of nerve cells, and the condition cannot be treated. Foals with Lethal White Syndrome invariably die of colic within 72 hours, and are usually humanely euthanized. Carriers of the gene, who are healthy and normal, can be identified by a DNA test. While carriers often exhibit the "frame overo" pattern, this is not a dispositive trait and testing is necessary, as the pattern can appear in a minimal form as normal white markings or be masked by other white spotting genes.
True white horses have unpigmented pink skin and unpigmented white hair, though eye color varies. The lack of pigment in the skin and hair is caused by the absence of pigment-producing cells called melanocytes. Some coat colors are characterized by light or white-like coats and even pinkish skin, however these white-like coats are not lacking melanocytes. Instead, white-like coat colors result from various changes in the ways melanocytes produce pigment.