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 The
second gene we'll study is the "Agouti" gene:
A, At, or a
It was named the "agouti" gene because it was thought to be
similar to the location and action of a color gene
of a Central and South American rodent of that name.
It's pronounced uh - GOO - tee . Yes,
there are three possibilities this time. It's OK.
Only black
pigment is affected by Agouti.
 So,
a horse with no black pigment will be the same color, no
matter which agouti genes are present. It will still be
a chestnut, or red, based color.
The Agouti genes are:
-
A ,
(bay) which is dominant over the other two varieties, and causes the black pigment to be
restricted to the horse's points (mane, tail, legs), leaving
the rest of the body red, thus causing
bay
when an E gene is present.
-
 At,
(brown) which
is dominant over a but recessive
to A, causing
brown (or, "seal brown") when an
E but no A gene is present.
-
 a, which is recessive
to both of its alleles, and which does not restrict the
black pigment in any
way, but allows it to be fully expressed all over the horse's body, thus giving
us a solid black horse when an
E gene is present.
Even though there are three varieties
of agouti gene, there are still only two genes at
each location.
In this case, the two can be any two of the
three (including two identical, of course.)
Picturing the genes
Now we need a drawing that highlights
a second pair of genes;
a different pair than black and red.
So, let's
pick a second pair to represent "Agouti":
In the drawing above,
the red and black (e and E) genes are shown in red and black colors,
with a magenta circle around that pair. Two of the agouti genes are shown
in various degrees of black, and have a blue circle around the pair.
(This combination would represent a bay horse.)
The agouti genes represented
above are
-
clear/white, for
a (solid), which
allows the black to show all over
-
part black, part
clear/white, for A
(bay), which allows the black to show only on the mane, tail and legs
-
At
(brown), not shown, which allows the black to cover most
of the body
 Why
the set of genes
in the above drawing (Ee, Aa) (or Ee, AAt) would be found in a horse that is
bay:
-
The black gene is dominant
over the red, so the horse DOES HAVE BLACK
PIGMENT
-
The bay gene is dominant
over the solid (solid black), so the black pigment is
RESTRICTED to the mane, tail and legs of the horse
-
If this horse had a
brown (At)
gene instead of the (a), as shown above, it would still be a bay,
because
brown (At)
is recessive to bay (A)
There are
DNA
tests available for
all three variations of the the Agouti
allele
To continue, and see more black, red and
agouti combinations and the colors they produce,
click
Dark Colors
Pages like this will have actual
photographs showing the various color, when this web site has found
a photographer, and is completed. For this reason a few pages
have a little extra "White space". Thank you for your
patience.
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