Horse coat color is a fairly well understood science in the modern age, controlled mainly by the expression of two genes: the red/black factor and the Agouti gene. The color gray is a bit different, as gray horses are born colored and experience de-pigmentation as they age, moving more progressively toward gray.
In the case of leading sire Tapit, a gray, he carries a chestnut (red factor) allele as well as the dominant gray gene – the Agouti does not come into play in this case. According to Gainesway Farm, the odds of him producing four gray horses out of non-gray mares is very small, only 6.25 percent.
As a gray stallion w/chestnut allele, Tapit's odds of siring 4 gray Derby contenders out of 4 non-gray mares: 6.25% pic.twitter.com/c8xj6mhMsf
— Gainesway (@Gainesway) April 17, 2016
There is an estimated foal crop of 21,275 Thoroughbreds across the United States in 2013, according to the Jockey Club's Fact Book. Of those, only 20 will make it to the first Saturday in May, just 0.094 percent of horses born in 2013.
The odds that four of those horses in the Kentucky Derby would be grays by Tapit, out of non-gray mares, are fairly astronomical: approximately 0.00588 percent. And yet, here we are.
Tapit's four gray sons with enough points to qualify for this year's Kentucky Derby are Creator, Lani, Mohaymen, and Cupid.
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