Archived Posts

Date: Tue, 6 Jul 2002
by Nicole Kinsey


*Zaon is black because of pure luck. ;)

Seriously, the "base color" of the horse: black, chestnut, or bay, is the result of the interaction of two color loci.

'E' is the extension locus. It controls the black/chestnut color (and is the "true" base color, but it's usually easier to speak in terms that include bay). 'E' is black and 'e' is chestnut. Well, technically, 'e' is red and 'E' is not-red, but that's immaterial to this. ;)

'A' is the agouti locus (poorly named in horses, it's based on other species). It controls the placement of the black coloration. 'A' is bay and restricts black to the points, 'a' is not-bay and allows the entire horse to be black. 'A' has no effect on a chestnut horse since there is no black pigment to affect.

So, the following are the possible genetic notations of the base colors:

chestnut - eeAA, eeAa, eeaa (always ee--)
bay - EEAA, EeAA, EEAa, EeAa (always E-A-)
black - EEaa, Eeaa (always E-a-)

Right here you can see that there are three ways to make chestnut, four ways to make bay, but only two ways to make black.

Grey is then a masking gene placed "on top" of the base coat. It's notation is 'G' for grey and 'g' for not-grey. A homozygous grey (GG) will always have grey offspring, while a heterozygous grey (Gg) will have a 50% chance of each foal being grey, and a non-grey (gg) will never pass a grey gene to it's foals.

Because of the way recessives work, they can "hide" for many generations. That's how generations of grey can suddenly produce a non-grey foal -- despite the repeated crosses to grey, the parents were still heterozygous grey. Each heterozygous grey x heterozygous grey breeding has a 25% chance of a non-grey offspring. So, that's how *Zaon is not-grey. :)

As for the black part, again, it's really luck. At least one of his parents was black or bay based under the grey and gave him a copy of 'E'. Now, as *Zaon is heterozygous black, we also know that his other parent gave him a copy of 'e'.

Additionally, both parents gave him a copy of 'a' (not-bay). This is really the hard part in breeding blacks. 'A' is really common in most breeds (including Arabians). And chestnuts and greys will both hide that they have it. *Zaon got lucky and both parents had a copy of 'a' and both gave it to him.

So, there's *Zaon: Eeaagg
And his parents had to be E?a?Gg and e?a?Gg

As for it not being common in the Spanish gene pool, I think it's just that 'E' is not common in the Spanish gene pool (it's not common in Crabbet either, hardly a coincidence). Look how rare bay is in Spanish -- or at least it was rare until some bays were used enough to start spreading it a bit more. :) And, as black will always be rarer than bay (due to that pesky 'A'), it's no surprise that black is even rarer than bay in Spanish lines. :)

Did that help? Or was the alphabet soup too much? :) As always with genetic notation, the upper case letter represents the dominant of that pairing and the lower case letter represents the recessive. Oh, and the 'E' and 'A' are the most common letters used for these two loci, but don't be surprised if you see other letters used by other people.

The best book currently out on color genetics is Philip Sponenberg's "Equine Color Genetics" and even that text is slightly out of date. Genetic knowledge is growing faster than books can be published. Any book other than Sponenberg's latest has to be taken with a grain of salt, there are a lot of very outdated books out there. :)

Oh, and I should note that "accidental blacks" are my favorite kind. I'm not a fan of color breeding unless it can be done secondary to quality. And that appears to not be the case in many cases. But when a black just appears as a total surprise, that's likely to be a much better individual. :)

~Nicole