I wanted to know about coat colour genetics and inheritance in the horse because Kaimanawa Horses are so plainly brown – much more so than historical accounts of wild horse colours that included grey, roan, blue, piebald and skewbald. As it turns out compared to other animals, horse coat colour is a fairly simple genetic problem.
Although selective breeding can produce incredibly diverse coat colours in horses, white, grey, black, red (chestnut) and bay horses are the basic colour types . Which basic type a horse is-is governed by just four genes, each with their own site, called a locus, in the horse’s genome. There is one gene and locus each for white and grey horses, and two others determine the chestnut, bay and black hues.
Four genes can produce five different basic colours because each gene for coat colour has two different forms, called alleles. The allele at each loci can be either dominant or recessive. A recessive allele’s influence on coat colour is weakened or prevented entirely if there is a dominant allele at one of the other three loci. And so different combinations of dominant and recessive alleles at the four loci determine the coat colour of a horse.
Crudely put, dominant alleles rule such that horses are black or chestnut if they are not white or grey. Dominant alleles at loci for white or grey make white and grey horses. A horse is black because it has a dominant allele at the loci for blackness but does not have a dominant red allele at its loci for redness. Chestnut horses do not have a dominant black allele, and so the red colour is uninhibited. Bay horses are an interesting combination of the two. They have a dominant allele at both the loci for redness and blackness, causing much of the black hue to be restricted to their extremities – mane, tail and feet.
After those four loci for the basic colour types, four other subordinate loci on the genome govern the different hues of black, bay and chestnut, such as palameno, buckskin, and dun. And another four loci govern coat colour patterns, like roan, leopard spotting, and tabiano (pinto, pie- or skewbald). But alleles at these eight loci for hue and pattern have no influence if the alleles for black and red are not also present. Different shades and patterns, therefore, are variations of fundamentally black, bay or chestnut horses.
Knowing how horse coat colour is derived and inherited, we can turn back to the question why are Kaimanawa Horses to plain brown? Perhaps they are not afterall? – my next post.
- Thiruvenkadan AK, Kandasamy N, Panneerselvam S. 2008. Coat colour inheritance in horses. Livestock Science 117: 109-129.