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,
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
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
An Ee horse "carries" red, "e",
"hidden" by the "E" (black)
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
for those traits is
(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)
(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
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