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> Oh Thanks Heidi for the equine color genetics piece. You know if
> you ask black breeders what happens if you breed their black to your
> grey, they kind of hem and haw. I never did figure out what happens if
> you breed the Homozygous grey to the Homozygous Black. I take it, it
> would be black? Battle of the titan genes? Toss of the dice?
The grey gene is not in "competition" with the black pigment gene or the
agouti gene--it acts regardless of what the base body color is.  A
homozygous grey will have ALL of its offspring turn grey, regardless of
their base body color.  If bred to a "homozygous black" the foal will have
a black-pigment-based coat (black or bay), but will nonetheless turn grey.

And the term "homozygous black" isn't very workable.  A horse can be
homozygous for black pigment (which is a dominant trait) but in order to
actually BE black, must also be homozygous for the recessive of the agouti
gene that causes segregation of black pigment to the points.  Otherwise,
if there is a dominant version of agouti there, the "black" will be bay. 
So when you hear people advertising that horses are "homozygous blacks"
they are usually referring to a horse that is EEaa.  (There is a gene test
for E so it is possible to actually determine if a horse is EE without
having to have a bazillion offspring and do probabilities.)  But--if you
breed an EEaa horse to anything that carries a large A gene (can be
chestnut, bay OR grey), you can still get a bay instead of a black.  The
only guarantee is that you WILL get black pigment.

Keep in mind that all three of these genes are at different loci, and are
inherited independent of each other.

Heidi



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