Equine Coat Color Genetics
Lesson Two
A Simple Approach
All horses fall into two (2) base
pigment color categories, either black or red. Some authorities include a 3rd
horse color at this point, which is bay.
However, since bay is a horse of which the base color is black, this
course will include bay, along with brown, a similar color result, under the
black base pigment color.
For each of these colors we will
discuss:
▪ How the genes that create it
work
▪ How to breed for the color
▪ How to recognize the coat color in foals
The Black Horse
The truly black horse is relatively
rare. To qualify visually, the horse must be completely black. It must have a black
muzzle and be black in the flank and around the muzzle and eyes.
However,
not all genetically black horses
The
color of some black horses fades in the summer. These horses appear to have a
rusty hue to their coats during the deep heat of summer, the result of sweat or
UV light on the hair coat. This visible
color variation has very little to do with a black horse's genetics, as far as
is known at this time.
How
the Gene Works
The
gene that creates the black pigment in a horse is called the
"extension" gene. The
abbreviation for is is universally accepted to be
E.
The extension gene (E)
works under the rules of simple dominance. Therefore it only takes one E
gene for a horse to be black.
Historically, horsemen thought that
the horse heterozygous for the black gene, the Ee
individual, had less depth to the color of their coat. It was also thought that
black horses that bleached-out in the summer were heterozygous blacks. This last
statement has never been proven.
If the black gene is present the
horse will be a black-based color. A
horse without the E gene (ee) will be not-black as
its base color. In horses, not-black (e)
is red.
How
to Breed for the Black Base Color
The
best way to produce black horses is to breed two homozygous black horses. Two
homozygous blacks (EE) will produce homozygous black foals
100% of the time. (See chart below.)
A
homozygous black (EE) bred to a heterozygous black (Ee)
will also produce 100% black foals because the homozygous black will pass a
black (E)
gene 100% of the time. (See chart below.)
This
is because it only takes one E gene to create the black-pigment-producing horse. 50% of
the foals will be homozygous for black and 50% of the foals will be
heterozygous for black, and all will produce black pigment where other
modifying genes allow for it.
However,
the best two heterozygous black horses can do is to produce 75%
black foals. The remaining 25% will always be chestnuts or sorrels.
What
Black Foals Look Like
Black
foals are usually not born black, but rather have a blue-gray hue to their
coat. They shed to black as weanlings or
yearlings.
The above picture is of a solid black miniature colt born
at Lucky C Acres Miniature horses. http://www.luckycacres.com
He is a good example of what a black horse will look like
when born.
When
a black foal will be born that looks truly black, like an adult horse, there
are often other genetic factors (like the gray gene) at work, and as an adult
the color will have changed.
The Chestnut or Sorrel Horse (Red)
What is often referred to as a Sorrel
What is often
referred to as a Chestnut
The
horse with a reddish body and a reddish mane and tail, historically, has been
referred to as chestnut in Thoroughbreds and Arabians, and sorrel in stock and
draft horses. Genetically, they are the same (ee)
and in many instances it is very difficult to tell them apart by looking at
them.
Today
most breed associations distinguish between these two colors. Chestnut is
described as a darker red color. Sorrel is described as a lighter or brighter
red. Chestnuts or sorrels, whichever term you prefer, come in many shades, from
a very light sorrel to deep, rich brown-red.
In
this course, since all of these shades are created by the homozygous recessive
"e",
we will use the words chestnut and sorrel interchangeably, and call the base
color "red".
How the
Gene Works
The
chestnut or sorrel horse is created when the E gene is in the homozygous recessive form – ee
Because all red horses are Homozygous
for the recessive (ee), one truism in breeding horses is
red x red = red.
Chestnuts
can also be produced from the mating of two heterozygous blacks (Ee) about 25% of
the time.
Take
a few minutes and, using the
How to
Breed for the Color
If
chestnut/sorrel is your favorite color, you're in luck: it is the easiest to produce. Red parents
only produce red foals.
Chestnut/sorrel
(red) is a valuable color when trying to breed other colors based on the red
color.
Palominos,
strawberry roans, red roans and red duns are all built upon the basic red color.
It
is possible to test for the presence of the recessive e gene. While this has little value for breeders of red
horses, it does help breeders of black horses to determine whether their horses
are homozygous or heterozygous for the dominant E gene. This is referred to as a red
factor test. http://www.vgl.ucdavis.edu/horse/redfactr.htm
What
Red Foals Look Like
Foals
with the ee genotype are born some shade of red although
they may lighten or darken as they shed off their baby coats.
Red foals are usually born with very
light legs. This leads many folks to think that these foals have lots of white
on their legs. Most of the time, this isn’t so!
Although it is difficult to tell
until they shed off, the hoof does provide some insight. The hoof of newborn
red horses is grayish in color where the leg is going to shed off red. Where
the leg is going to shed off white the hoof will be cream or white. This does
create a change in the color of hoof, but may be difficult to see.
The Bay Horse
A
A
A
Bay
horses are the result of the Agouti gene on a black base horse. They have reddish bodies with black manes, tails,
and lower legs. The tips of the ears are black, as are their eyelashes. The
hoof of a bay horse is basically black. White leg markings change the black to
white in the area where the white color touches the coronet band.
Bay
horses range in color from dark blood bays to golden bays. This variation
occurs with all color categories and is referred to as shade.
There
are basically three recognized shades: dark, medium, and light. The genetics
behind these shade variations is unknown as of this writing.
How
the Bay (agouti) Gene Works
The
A
gene is a very "important" gene in coat color genetics, in terms of
its predominant distribution in the horse population. This gene limits its
expression to horses with a black base color. Its action is to lighten the body
color leaving the legs, mane and tips of the ears black, thus creating the
bay. Another way to describe it is that
it restricts the black color to the points of the horse (much like the coloring
of a Siamese cat).
The
predominance of the A gene is the reason for the small number of true black
horses. Although the black horse does not have the bay (agouti) gene, many red
horses do. Crossing chestnut/sorrel horses with the A gene on black horses does produce bays.
In
this example, a red mare carrying one A
gene has been bred to a homozygous black. This cross produces a 50% chance of a
bay and a 50% chance of a black.
(Note that the mare boxes include the
gene possibilities for both the E and A. When
first working with Equine genetics and the
How
to Breed for the Bay Color
The best odds of producing bay horses
come from the mating of two bay parents. And, of course, bays crossed on blacks
allow the A gene the opportunity to
act on the black base coat.
Bay is the base color for buckskins, Perlinos, bay roans and buckskin duns. Crossing any of
these colors provides the opportunity for a bay foal to occur, but it is not
the way to increase the odds that you will produce a bay.
What
the Foals look Like
Regardless
of the shade, bay foals are born with the tell-tale sign of black tips on their
ears. Most of them have black manes and tails. Their legs, however, may be
light, shedding to black later.
This bay colt is about 2 weeks of age. Notice the
characteristic grayish color to the lower legs.
This colt has one hind white sock.
Summary of bay: Bay horses are black-based horses which have
at least one (1) copy of the normal (A) agouti
gene. The bay agouti gene limits the
expression of the black coloration (E) to the “points”, mane/tail and lower
legs, of a horse. This leaves the body
color a reddish color. A bay horse does not need to carry a red gene to cause
the reddish body color. Agouti only affects the black coloration, so you can
have a chestnut/sorrel horse which carries agouti, but you cannot tell that by
its appearance.
The Other AGOUTI Gene: At
NOTE: in order to accommodate all
browsers, this gene will be noted as (At) for the remainder of this
course. However, the more correct way is
with the "t" as superscript (raised).
Brown (aka Seal Brown)
Brown (Seal Brown)
Horse -- black plus At
With
the ability to test for the agouti (bay or A) gene came the revelation that seal brown horses were not solid black horses with another gene
(usually believed to be the Pangare/Mealy gene) added
to cause the lighter shading. It is now known that Seal Brown is due to the At allele at the
agouti locus. This allele has been sequenced, named "brown", and
there is now a test for it.
http://www.petdnaservicesaz.com/equine-testing/
With
this fact comes a complication to the simple
The brown (At)
gene is recessive to regular bay (A), but dominant over non-agouti (a) (solid
black).
Other Modifiers
Additional
genes, called “Modifiers” work upon these two colors to produce the different
colors that appears on the body of horses.
As with any palette of colors, there
are factors which affect the hue, tone and shade of the true color. These modify,
but do not change, the genetic color makeup of horses, but they alter the
visual appearance of the colors.
They are:
Shade: This describes variations within a
basic color group resulting in light to dark body color variations. On a
chestnut/sorrel this modifier can result in colors from light and nearly yellow
to dark and almost purple, brown or nearly black. (Sometimes a "red"
horse can have a near-white, or "flaxen", mane & tail. The genetics behind this are unknown as of
this writing.)
Light chestnut/sorrel miniature horse
Dark Chestnut (Liver) stock type horse
Keep
in mind that both of the above horses are genetically red (ee).
If
you were to test these horses they would test identically for the red gene (ee).
Sooty: This refers to the presence of black
hairs among the otherwise-lighter body hairs. Most often this is noticeable
across the back, shoulder and croup, with the lower body appearing lighter.
Mealy (Pangare): This
modification causes pale red or yellowish areas on the lower belly, in the
flank area, behind the elbows, inside the legs, on the lower legs, on the
muzzle and over the eyes. This modification can occur on any base color and the
effect varies from minimal, very subtle and easily missed, to extensive,
causing dramatic paleness to the body. In the American Southwest, it is often
referred to as “muley", after a mule's
coloration.
This modification occurs on
chestnut/sorrel and bay horses.
On otherwise-black horses, this
appearance is actually caused by the At, or brown
gene, which was in the past usually called “seal brown”, after the coloration
of one species of seals.
This is a Belgian (breed) sorrel /chestnut horse showing
mealy/pangare
This is a dark bay showing the Mealy/Pangare
The Ultimate Modifier – Gray
The “color” gray is a modifier, rather
than a color. All gray horses are born a ‘base/birth’ color, and they carry
those ‘base/birth’ genetics throughout their lives, and those genetics will
have an impact on what you get when breeding.
If
a horse carries the gray gene (G), then regardless of the color the horse is
born, the horse will turn gray. Gray is a progressive color
changer, in that as graying horses get older, most will eventually end up
white; the amount of time it takes to reach the white stage can vary from horse
to horse with some horses being white by five or six years of age and others do
not reach the “white” stage until they are in their teenage years. Some go on to develop "flea bites"
after turning white: specks of color that may or may not match their birth
color. This is called a
"flea-bitten gray".
The
photos below are of the same horse taken when the horse was a nursing foal, a
yearling and as a two year old. As you can see, this "gray horse" was
born chestnut/sorrel and started turning gray as a weanling/yearling. Its body
hair has lightened considerably with age, and will continue to lighten until
the horse is almost all white.
Photos are courtesy of “Cedar Ridge Ranch” www.grullablue.com
Gray
horses will continue to change color until they reach the “white” stage. Some
of the most famous gray horses are the Lipizzaners found at the Spanish Riding School of
Vienna. Lipizzaners
are born dark and turn white with age, with the occasional one staying dark,
indicating it did not receive the gray gene from either parent.
So,
to summarize lesson 2:
▪
All
horses come in one of two “base” colors; Black or Red.
▪
When
the black gene is present it will mask the presence of a red gene
▪
Bay agouti (A) limits black to the “points" of a horse
▪
Brown agouti (At) dilutes black on the muzzle, flanks, "armpits", and
some other areas of an otherwise black horse.
▪
Agouti
(A) or (At) only affects the black color
▪
Other
modifiers may be present which can affect the Shade, Smuttiness or Mealiness of the color of the horse. The latter two are often referred to as
"countershading".
▪
Gray,
while called a color, is actually a modifier that changes all colors from birth
or base color to gray.
▪
The
Punnett Square can be used to determine the possible
colors from a mating and their probability.
ADDITIONAL
http://www.horsecolors.us/darks/darks.htm