Gray Horse Genetics
By Meredith Kahn, M.S.
“White” horses are a special type of horse. Beyond their obvious beauty, these horses actually have a unique genetic make-up. Technically termed “gray,” even at their whitest, these horses are born any color, and naturally progress whiter in color with age. White hairs gradually mix with the original color, and horses are usually completely white by 6-8 years of age, although this can vary greatly depending on the horse. A rose gray, for example, is born brown and progresses to nearly, completely white with the exception of brown specs throughout their coat. Dappled gray coats usually occur between the original pigment and white stages, with white dapples that progressively grow in size. A gray horse’s mane and tail also change in color, often times beginning as a dark shade, such as dark gray or black, and ending as a brilliant white.
Genetically, the gray gene is known to cause progressive depigmentation of the hair. The first signs of white hair are usually found on the head, specifically around the eyes. Gray is a dominant gene, meaning that if a horse inherits it, they will display that specific trait. If a horse has the gray gene, they will exhibit gradual depigmentation with age. There are two different genetic circumstances in which a horse will be gray. The table below, adapted from the website for the Veterinary Genetics Laboratory at UC Davis, shows and describes the possible genetic combinations for a gray horse. G/G, or homozygous, ‘the same,’ dominant means that the horse has two copies of the gray gene and will turn gray with 100% certainty. In addition, this horse will pass on the trait to all of its potential offspring, who will also be gray. G/N, or heterozygous, ‘different,’ means that, as the gray gene is dominant over non-gray, the horse will be gray 100%, however any offspring have a possibility of not inheriting the gene. This of course also depends on the color genetics of the other horse. Research has shown that horses with one copy of the gray gene can retain some of the original pigment, while G/G horses tend to progress to almost completely white.
The next time you see a ‘white’ horse, think about how they may have looked when they were born. Furthermore, these horses are so unique that they may exhibit different color characteristics at various ages throughout their early to mid life. A horse that is born brown may be completely unrecognizable at age five and different, yet again, at ages eight and 15. While all horses are unique in personality and beautiful in their own way, gray horses are exceptional in this fascinating way.
|G/G||Two copies of the gray gene. Horse will turn gray and all offspring will be gray.|
|G/N||One copy of the gray gene. Horse will turn gray and approximately 50% of offspring will be gray.|
|N/N||No copies of the gray gene. Horses will not turn gray. |
Pictured above is Meredith’s gray horse Gatsby.
Veterinary Genetics Laboratory, UC Davis Veterinary Medicine
Pielberg G.R., Golovko A., Sundstrom E. et al. A cis-acting regulatory mutation causes premature hair graying and susceptibility to melanoma in the horse. Nature Genetics, 40 (8):1004-1009 (2008).