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Genetics 000
Dark Colors

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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.


To follow the educational, logical progression of this web site, click "Next", below.

Back Pigment Agouti

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