Camarillo White - W4

Multiple dominant white mutations have occurred in horses across different breeds. They are termed dominant white because one copy of these mutations results in an all white phenotype where two copies are mostly embryonic lethal.

The mutations causing dominant white in horses all occur in the KIT gene, a gene with a vital role in pigment cell biology. One of these mutations has been preserved through breeding and formed the basis for creation of a new breed with an all-white phenotype. Camarillo White Horses are the result of one such KIT gene mutation called W4, as it was the fourth dominant white mutation to be reported. A Southern California rancher named Adolfo Camarillo purchased Sultan, a white stallion born around 1912, and bred him to Morgan Horse mares to create white horses that became famous performers in the Pasadena Tournament of Roses parade and other local events. The Camarillo family kept the white horses until 1987 when they were dispersed at a public auction. In 1992 the Camarillo White Horse Association was formed to preserve the white horses descended from Sultan.

The Camarillo White mutation is caused by a single nucleotide change in KIT (c.2151C>G), this change in the DNA results in a change in the protein sequence and is thought to make the protein non-functional. KIT has crucial function for the development of blood, gonadal, and pigmentary tissues. Mutations that affect normal functioning of KIT gene products often result in lack of pigment cells (melanocytes) in the skin and hair follicles which leads to white patterning in horses known as dominant white. It is inherited in a dominant fashion in that horses with one copy of W4 will have the all white phenotype. However to date no horses with two copies have been identified it is thus likely that homozygosity is embryonic lethal.

Testing for the Camarillo White (W4) variant can help owners make breeding decisions. Mating of a carrier of Camarillo White (CW/N) to a non-Camarillo White (N/N) horse will result in a 50% chance of producing Camarillo White offspring.