True albino--the inability to manufacture melanin--has never been documented in horses.
Several different genes with alternate alleles can produce a white horse.
Among these, only the Graying allele exhibits age-dependent expression.
Gray GeneHorses born with the graying allele of the KIT gene can be born any color.
As they age, the hair follicles progressively lose the ability to manufacture melanin.
The coat takes on a "dappled" pattern that gradually becomes completely white.
The skin does not change color, and continues to produce melanin.
The Graying Gene and MelanomaThe Graying gene has been linked with an unusually high risk of dermal melanoma.
The graying trait in horses has been identified as a
True White HorsesTrue white horses are rare, and are not albino.
They are born white with pink skin, and remain white all of their lives.
The white coat and skin are due to a lack melanin deposition in the skin and hair.
True white has been traced to a dominant loss-of-function in the KIT gene, which encodes one of many receptor tyrosine kinases (RTKs), which serve as receptors for hormones, cytokines, and growth factors.
The exact mechanism of the mutations effect on depigmentation is not fully understood.
(2) Hintz, H. F. and VanVleck, L. D. 1979. Lethal Dominant Roan in Horses. Journal of Heredity, 70:145-146