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What you need to know about genetics to understand how the dun gene works.

"Equine Colors 101"

The Two Basic Colors: Red and Black

Research to date has shown that there are two forms of pigment (melanin) in horse hair: red and black.

Black: unless it is blocked, the black pigment will Extend all over the horse's body.  The gene that allows this is called the Extension gene, and is written as "E".

Red: the black pigment not allowed to Extend anywhere at all, leaving only red pigment. (In its purest, undiluted form, the horse will be chestnut).  The absence, or blockage, of black pigment is caused by a recessive gene written as "e".

Genes are believed to exist in pairs. Two genes for every trait, one from each parent.  That's why there are two sexes in the animal world.

A recessive gene does not express itself unless there are two of them.  Think of it as shy.  If its exact double is there, it will express itself.  Otherwise, if the dominant gene is there with it instead (there will always be two genes), it "hides".

In genetic shorthand:

EE stands for two black-extending genes; the horse will be black-based. (Black, bay, buckskin, some duns, smoky black, perlino, smoky cream, classic champagne, amber champagne, etc.)

ee stands for two black-blocking genes; the horse will be red-based, (Chestnut, sorrel, red dun, palomino, cremello, gold champagne, "dunalino", etc.)

Ee (never written eE) stands for one of each; the recessive gene will hide; the horse will be black based. (See examples above.)

Genes That Change Black

Obviously, red (ee) changes black; it blocks it completely.  But it can also be blocked partially, as in a bay, dun, etc.

Bay (also Brown): the "interfering" gene

Called "Agouti" by geneticists, the gene that makes a black horse lose its black EXCEPT ON ITS POINTS is written as "A" (for agouti.)  When that gene is not active, it is written as "a", and the horse is allowed to show its black color all over (as long as black, or "E", is there). Also exists as At, which is the form of agouti that causes the seal brown color when black is present.

So, in that shorthand again,

EE-AA, EE-Aa, Ee-AA or Ee-Aa genes would produce a bay horse; black on the points only (points = mane, tail, & legs.)

EE-aa or Ee-aa genes would produce a solid black horse. No bay (A).

EE-AtAt, EE-Ata, Ee-AtAt or EeAta genes would produce a (seal-) brown horse.

Since bay (A) is also dominant over brown (At)... EE-AAt and Ee-AAt genes would produce a bay horse.

The A and a genes have no proven effect on red.  For example, ee-AA or ee-Aa genes would not produce a bay, because there is no black to show up on the points only.  The horse would be red-based: chestnut/sorrel, etc. (see above examples).

Dominant vs. Recessive Genes

The genes represented by lower case letters (recessive) will "hide" if they are paired with their upper-case (dominant) partner.  "Hide" simply means "not express themselves". 

An Ee horse "carries" red, "e", "hidden" by the "E" (black) dominant gene.

And Aa horse "carries" non-bay, "a", "hidden" by the "A" (agouti) dominant gene.

One more quick genetics lesson: heredity.

If you have a bay horse, it might be EE or Ee, and it might be AA, AAt or Aa.  You can't tell just by looking.

BUT...since a horse gets one gene of each kind from each parent, if that horse had a red parent and a solid black parent, then you know its genotype for those traits is

  • Ee

    • (one red gene from the red parent because a red horse has no black genes to give; a black gene from the black parent because the horse is bay and that's the only parent that HAD a black gene to give) 


  • Aa

    • (one non-bay gene from the solid black parent because a non-bay black has no bay genes to give; a bay gene from the red parent because that's the only parent that could have one, because you can't see bay on a red horse.)


    • If a bay horse ever produced a red  you'd know it was Ee

    • If a bay horse ever produced a solid black you'd know it was Aa

    • If a bay horse ever produced a brown you'd know it was not AA

To go on to the dun, cream, silver, pearl and champagne "dilution genes", click here




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