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In this section I'll explain how simple dominants and recessives work. In the introduction, I mentioned that a horse needs to possess only one copy of a dominant gene in order to show that gene's trait. I used tobiano as an example of this: A horse with TT and a horse with Tt look alike -- both are tobianos -- while a horse with tt does not show tobiano spotting, and therefore cannot pass it on to its offspring.

TT horses are homozygous for tobiano. When they are bred, they always pass a tobiano gene on to their offspring, since T is the only gene they have at that locus. When tt horses are bred, they always pass on a t gene. This is why a tobiano must have at least one tobiano parent. (This is true for all the dominant genes: Black, grey, roan, dun, overo, sabino, splashed white, and dominant white. All must have at least one parent of that color.)

On the other hand, heterozygous tobianos (Tt) have a 50% chance of passing on their tobiano gene to their offspring. Two Tt horses, when crossed, have a 75% chance of producing a tobiano foal.

This phenomenon is illustrated in the chart below. This is called a Punnett square. The sire's genes are in the red boxes at the top; the dam's genes are in the blue boxes at left. The sire and dam are both heterozygous for the tobiano gene (Tt). The foals in the other boxes represent the possible combinations of the four genes that the sire and dam are passing on. It's kind of like multiplication tables: T times t equals Tt, for example.

With this mating, there is a 25% chance that the foal produced will be heterozygous for tobiano. The foal in the green box represents that 25% chance. He inherited a T gene from his sire and a T gene from his dam. As an adult, he will produce 100% tobianos no matter what color mare he is bred to.

The foals in the yellow boxes represent the 50% chance of a foal that is heterozygous for tobiano, like its parents.

The foal in the pink box represents the 25% chance that the foal produced from this cross will be solid-colored due to inheriting a t gene from each parent. This is a bummer if you're trying to breed pintos, but it happens.

Conversely, let's assume that one of the parents -- the dam, let's say -- is homozygous for tobiano, like the foal in the green box above. In that case, it doesn't matter what color the sire is, because all of that mare's foals will be tobiano. Let's say the sire solid-colored.

Since the mare can only pass on a T gene, and the stallion can only pass on a t gene, all possible foals will be heterozygous Tt, represented by the four tobiano foals in the purple boxes.

As I said, this method works for all dominant genes. Here's a table showing a black stallion crossed with a chestnut mare. Black is E- and chestnut is ee. We'll assume this particular stallion is Ee.

Therefore, an Ee sire and ee dam have a 50% chance of producing a black foal and a 50% chance of producing a chestnut foal.

Palomino works a bit differently. Palomino requires a heterozygous arrangement of alleles at the C locus. The homozygous dominant CC will be chestnut, the homozygous recessive cc will be cremello, and the heterozygous Cc will be palomino. Since palominos have to be heterozygous, the color cannot breed true -- in other words, crossing a palomino with a palomino will only give you a 50% chance of a palomino foal, with a 25% chance of a chestnut and 25% chance of a cremello.

That makes it tough to breed palominos. However, there is a corollary. Chestnuts (CC) crossed with cremellos (cc) always produce palominos. Since the chestnut CC parent can only give C genes and the cremello cc parent can only give c genes, there's only one possible combination: Cc, or palomino. This is illustrated below:

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