Stongly suggest that if you are actually interested in the
subject you go to http://www.equinecolor.com/color.html , 'else
you get some really wrong ideas about horse color and spotting on
horses.
Actually it's not quite that way. There are two colors in horses,
chestnut and black. Any other colors are different genes, called
dilution genes or graying genes, etc. The bay horse is not heterozygous
necessarily; it just has dilution genes that make it bay instead of
black. There are many genes that control coat color, and some are
simple, such as cremello x chestnut = palomino, chestunt x chestnut =
chestnut.
Hope this helps some. It is really quite interesting, and not
so very hard to understand, just lots of "what ifs" and unknowns
involved.
Louise (Biology teacher :))
Melissa Alexander <mcalex@xxxxxxxxxx>
wrote:
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!
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