10-11-2011, 08:13 PM
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No one is doubting that she is black. Just that she cannot possibly be homozygous and have a chestnut foal. Black and chestnut are controlled on the same locus, the E locus, and black is dominant (E), while chestnut is recessive (e). To be homozygous for black, your horse has two black genes on the locus, so she would be EE. So she can only ever pass on an E gene, which is black, and dominant. Even if the foal got a red gene from it's sire, it would still be Ee, which would display as black. So since her foal is chestnut, it proves that she is not homozygous. There is no need to even test - a chestnut has two ee genes - one from each parent. Therefore, your mare is clearly Ee - heterozygous black carrying chestnut.