Originally Posted by Wheatermay
Poseidn! You know ur color and genetics, and you didnt reply! LOL I think you helped me confirm that buck is a light red bay with countershading (I thought he was a dark buckskin or dun in a post long ago). Buck's mother was a chestnut (which means she obviously wouldnt have a bay gene (I think), and so that leaves the sir (the black and white paint) to be the one carrying the bay gene. And if that is so, why wasnt it passed to either of foals (the original gelding in question, and his full brother, who is a chestnut and red ONLY- no points AT ALL!!! Paint)
It is entirely possible for Buck's dam to carry agouti (the bay gene). Agouti restricts black to certain points, which is the difference between a black and a bay. However, chestnut horses are red based, not black based, so there is no black to restrict. They can carry agouti, but there is no way to know without genetic testing.
It is impossible for a black horse to have agouti because the black is not restricted. Bay is black + agouti A (the At mutation of agouti is brown). Agouti is a dominant gene, meaning if a horse carries it, it WILL show it.
I started answering before I finished reading this entirely. Based on what you said about his brother, their dam is heterozygous for agouti, meaning she will pass it on 50% of the time. And also that their sire is heterozygous for black if he has a red foal because black is dominant. A homozygous black horse will always produce a black-based foal. Sorrel/chestnut horses are homozygous for red, which is why crossing a chestnut with a chestnut will always result in chestnut.