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Arabian
Foundation
Horses
Color Inheritance
in Arabians (Or how you can cross a black and a gray, each carrying a chestnut
gene, and get a bay foal) - in Layman's Terms
The gray mare with a chestnut base coat has the "agouti" gene which
is not expressed in chestnut horses, because its color modifying effect is
to restrict black to the points thus making a horse who would otherwise have
been black into a bay. The color genes are not all different versions of a
single gene locus; there are many different gene sites that effect color.
The "base" coat color is determined by a single gene locus that
has two pairs of genes (one inherited from each parent). The two options for
these base color genes are red (chestnut) and black. The "red" version
is recessive to black so that if a horse has one red and one black version
(heterozygous) it will have a black base coat.
But nature didn't leave it so simple. There are then many modifier genes that
will alter the base coat. The "agouti" gene locus is one of these.
Like all other genes, each horse will have two pairs. This gene does not effect
red coat pigment so it is not expressed in red horses. It comes in two versions
that are basically like "on" and "off". "On"
means if a horse has a black base coat the black coloration will be restricted
to the legs, muzzle, mane, and tail (a bay). "Off" means that if
a horse has a black base coat, it will not be restricted to the body points.
Since the "on" version is dominant to the "off" version,
only one version of the pairs needs to be the "On" kind to make
a black horse into a bay. Since a chestnut horse has no black coat to be affected
by this gene you cannot tell by looking at it whether or not it carries any
copies of the gene that will cause a black coat to become a bay one. Obviously,
black horses have two copies of the "off" version of agouti while
bays have one or two copies of the "on" version.
Gray is determined by another gene site. The "graying" version is
dominant to its "non-gray effect" version so that only one copy
of this gene is needed for the gene to be fully expressed. One copy will cause
the horse to turn gray no matter what other color genes it also has. All horses
have gene versions at all the color gene sites, but not all of the gene versions
cause an effect. For example, horses that are not gray merely have two copies
(homozygous) of the non-graying version of that gene. There are many other
color gene sites that can have subtle or dramatic effects on a horse's color.
Many of these genes are not present in their "active" version in
Arabians (ie: no dilutions like cremello or buckskin, or tobianos, or overos,
etc.)
For more complete information on equine color genetics (or to clarify what
I have attempted to translate into layman's terms):
http://www.vgl.ucdavis.edu/~lvmillon/
Contributed
by Dawn Jones-Low
Faerie Court Farm
Ferrisburgh, VT
