Genetic discussion of Agouti (bay) gene horses: The
Agouti gene restricts black hair to the points, that is, the agouti
gene makes a black horse into a bay horse. Now we know that the
Agouti gene also affects the shades of red horses too. First, let's
review the basics of the black and red based horse.
series (E) - causes black or red, whereas the most recessive
form of the extension series (e/e) = a red based horse. Any horse
carrying an (E) will be black based.
Non-red (black based) parents may have a red foal, because red
is recessive to black.
Red bred to red always gives red.
HOW AGOUTI GENES AFFECT BLACK HORSES:
(A) Classic bay. A black horse with the agouti modification that
restricts black hair on a black based horse to its points. This
is the most dominant of the Agouti series, which explains why classic
red bays are more common than the other variants.
(A+) Wild bay. I believe this is the most rare of the Agouti gene
variants. This is a black based horse with agouti modification to
the greatest extent. Visually similar to classic bay, except the
black points are further restricted, in that the black is only up
to the fetlocks, or may be absent altogether, along with an overall
lighter body color, and some degree of washing out, bleaching or
streaking of the mane. Although counterintuitive, A+ is recessive
to A. A+ is dominant over At.
(At) Brown. Black horse with agouti modification to the least extent.
Recessive to both A and A+. When in homozygous form, (At/At) the
horse will have a further bleached (cocoa color) to its hair coat.
When I learned this, I first thought that sounded counterintuitive...
however upon reflection, to the contrary: This is a doubling of
the lightening or restricting effect.
(a) Self. No agouti gene at that loci.
(A/A) (A/A+) (A/At) (A/a) Visually: a classic bay.
(A+/A+) (A+/At) (A+/a) Visually: a wild bay.
(At/At) Visually: usually will be a horse that is not as "true
black" color in is body coat as the easier-to-recognize At/a
horse; the coat will look more evenly "brown" or solid
cocoa colored, (think very dark chestnut, or sun-faded) with black
(At/a) Visually: a "brown" or a "seal brown".
A black bodied horse, with varying amounts of red hair on soft spots
(muzzle, around eyes, in flanks or under elbows).
(a/a) Visually: no agouti to see here, so let's move along...
illustration on left is linked to its source website:
Morgan Colors- The Base Colors: Chestnut, Black, Bay and Brown Morgan
Horses; author, Laura Behning, Covington, GA, email@example.com
HOW AGOUTI GENES MAY AFFECT RED HORSES:
Depending upon where you live, or what discipline you work in, the
words red, chestnut and sorrel may or may not be interchangeable.
To keep things interesting, now let's add in some more equine genetic
terms to explain how the Agouti gene affects the red horse, so now
there are even more ways to use these terms. While we have fun with
this, keep in mind I am a cowgirl, and that both sorrels and chestnuts
are red based horses, as opposed to black based horses, and we'll
see what we come up with, here.
Note: All shades of red may or may not have a flaxen (blonde) mane
and/or tail and/or fetlocks.
(A) A sorrel horse with a plain A (classic bay) should have, simply
put, no effect on red hair. A is dominant over the other agouti
gene variants, so again, being the most dominant of the Agouti series,
likely explains why classic bright red sorrels are more common than
other shades of red.
Buffalo Gal, one of my "primitive
sorrel" Curly Horses
I do not know, (yet) if or how the A+ Agouti gene affects red hair.
I have my suspicions however... for many years I have raised a rare
strain of hypoallergenic Curly Horse that include what I refer to
as "primitive" chestnuts and sorrels... this bloodline
traces back to horses owned by Native Americans in the 1700 &
1800's. Perhaps I was not too far off. What I call "primitive
sorrels" have a bit duller or lighter red coats that are more
prone to sun-fade, with faded feathering on their fetlocks and silvery-grey-flaxen
manes & tails; while the chestnuts have mottled brown on the
lower legs and faded feathering, with grey manes & tails. (¿Quién
Sabe?) A+ is recessive to A and dominant over At.
(At) Visually, a dark sorrel or medium chestnut (depending upon
your particular venacular). At is recessive to A+ and dominant over
(a) Self. No agouti gene at that loci.
So now let's look at how the shades of red are affected
by the various combinations of Agouti gene pairs (and for the sake
of clearer representation, let's drop the /slashes/ between the
gene pairs when illustrating these gene combinations):
(ee AA) (ee AAt) (ee Aa) - Visually: a sorrel (which would explain
why the "plain" red sorrel is the most common red based
(ee AtAt) (ee Ata) - Visually, dark sorrels to light & medium
(ee aa) - Liver chestnut - and if you think about it, this makes
sense, because underneath everything, this combination has no agouti
to restrict any pigments - and - the more recessive the gene combinations,
the more rare they are.