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Barbara, say there's one gene that controls coat color -- simple
inheritance, I believe that's called. (In practice, I don't *think* horses
have just one gene involved in color inheritance, but the definitions of
homozygous and heterozygous hold up.) 

In simple inheritance, an individual horse inherits one part of that gene
from his father and one part from his mother. If the horse inherited the
same color gene from both parents, that horse would be homozygous for the
color. If each parent contributed a different color gene, that horse would
be heterozygous for whichever color was dominant.

Each individual horse, then has two parts to this color gene. He can pass on
only one to a particular offspring. Which one he passes is determined by a
genetic roll of the dice.

Say you have a homozygous black stallion, written BB -- a dominant black
gene was contributed by each of his parents -- and you have a heterozygous
black horse, who also has the recessive bay gene, written Bb. If BB mates
with Bb, statistically you'd get offspring who were BB, Bb, BB, and Bb. All
of them would be black, but 1/2 would be homozygous and half would have been
heterozygous.

If you had two heterozygous horses, both black with a recessive bay gene (Bb
and Bb), then the offspring would be (statistically) BB, Bb, Bb, bb. Three
blacks, one bay. Two of the blacks carrying recessive bay. One black
homozygous, and the bay homozygous.

In practice, I don't think it's this simple. There are a lot of colors and
combinations. I don't, personally, know if it's simple inheritance or if
there's more than one gene involved, nor do I know what colors are dominant
or recessive. This is purely an academic explanation of homozygous and
heterozygous!

Melissa 


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