Genetics

The Genetics of Graying in Horses

Like in human genetics, equine genetics are the responsible party for variations of different coat colors and patterns. For the species, Equus callabus, the foals are born normally colored, like bay or chestnut, and if they have a certain gene, they can slowly lose their pigmentation over time. No horse is born gray. Some might argue that stress causes premature graying like it does in other species. However, graying in horses is very much a nature over nuture occurrence. The graying is completely caused by genetics rather than environment, meaning it has a heritability score of one. It usually takes around six to eight years for a horse to fully “gray out”, though there are horses that are completely gray by the time they are two to three years old, and they keep the natural dark skin pigment they were born with. Graying is possible in any equine breed but is most predominantly found in the Lipizzan breed, originally from Europe and one of the oldest horse breeds to date. This phenotype is the result of a protein in a gene being repeated and thereby increasing its expressivity. The genetics behind this mutation were initially discovered in 2008. 

Phenotypes are always associated with genotypes and genes, be it one gene or many genes. In this particular case though, a single gene is the deterministic factor and it is referred to as STX17. Syntaxin-17’s (STX17) expressivity is largely increased due to unnecessary triplication, as well as the neighboring gene NR4A3. It was originally thought that the gene was duplicated but further studies suggested that STX17 was a triplication. The repeated section of STX17 is said to have a length of 4,600 base pairs. When these genes are overexpressed, it causes the melanocytes within the hair follicles to hyperproliferate, which results in the loss of pigmentation of the hair. This also results in an increase of potential to develop dermal melanomas. The STX17 gene is said to be found on chromosome 25. Horses have 64 total chromosomes and chromosome 25 is autosomal, meaning that it is not a sex chromosome. This trait is Mendelian and autosomal dominant. The allelic code for that depicts the presence of the graying trait is referred to as G. If the graying trait is not present in a horse, they are given the allelic code that represents the absence of graying, N. Horses that have the N/N genotype have a zero percent chance of graying out and passing any kind of graying trait or phenotype onto their offspring unless mated to a horse with at least one copy of the graying gene. Horses that are heterozygous N/G however, will gray out and will have a fifty percent chance to pass on the graying trait to their offspring when mated to a horse with a genotype of N/N, a one hundred percent chance of passing it on when mated to a horse with a G/G genotype, and have a typical Mendelian ration of 3:1 when the heterozygous N/G are crossed with each other. Horses with genotype G/G are guaranteed to gray out and pass it on to their offspring. Horses with G/G also tend to end up being more white than gray because they have two copies of the gray allele while horses with N/G tend to have a more muted gray color. Gray horses and white horses more so, are very desirable and are fortunately relatively easy to breed due to the determining allele being dominant.

While equines, Equus callabus, have almost triple the number of chromosomes compared to humans, the genomes are still very similar and function relatively the same. Traits are expressed from the genotype which is received from the DNA of the gametes of the parents, which ultimately results in the expression of a phenotype. Haploid gametes, defined by N, each contain only one lone chromatid, better known as the egg and the sperm. An embryo develops from the joining of the two gametes, eventually further developing into a diploid zygote, defined as 2N. If a horse receives even a single G allele, the horse will end up graying out and losing its pigmentation. Like all mammals, horses are diploid organisms and the alleles are on the same part of two different chromosomes. Haploid organisms cannot obtain multiple alleles as diploid organisms can. The particular allelic combination N/N will be the only variation that does not result in the horse graying out as it ages and matures. All other genotypes will result in the coding for proteins in which the outcome will be the graying phenotype.