Why Do Some Gray Horses Turn White? Scientists Reveal the Genetic Secret Behind Their Color Change"

Scientists from the Texas A&M College of Veterinary Medicine and Biomedical Sciences (VMBS) and Uppsala University in Sweden now understand why some gray horses turn completely white as they age, while others remain an eye-catching “dappled” gray color. 

As published in the journal Nature Communications, the deciding factor is the number of copies of a small DNA sequence within the gray coat gene carried by each horse; while “slow-graying” horses have a gene variant with two copies of the duplication, “fast-graying” horses — those that will eventually become white — have a gene variant with three copies.