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Equine Color Inheritance
When we realized we had a black foal on our hands with Tux, we had him DNA tested for coat color to be 100% sure he was a black. The test proved he had the black gene "aa". Tux is "Eeaa" which is considered heterozygous black. Homozygous would be "EEaa". So I did some research to find out what this meant for breeding color for Tux.
The main difference between heterozygous and a homozygous black is that a heterozygous black stallion can produce a chestnut foal where a homozygous black cannot. This chart which is from www.vetgen.com reflects the odds of the various colors based on the color genetics of the mare and the stallion. The odds for producing a black foal from a heterozygous black stallion vs. a homozygous black stallion are actually not that different. Blue highlights in this chart show where the odds of producing a black foal are the same. Yellow highlights show where the odds vary some, So if you have your mare tested for coat color (see www.vetgen.com for how to do this) which will give you her coat color gene makeup, this chart will give you a reflection of the odds on the color your foal will be.
The basic coat color of a horse is determined by the following genes:
White (W), Gray (G), and E and A. The first two genes, W and G are dominant (need only one copy to produce the white or gray color). While these horses may contain genetic variations at the E and A locus, these variations cannot be determined by inspection of the coat or skin.
In the absence of one or two copies of the W or G genes, the coat colors of Bay, Chestnut/Sorrel and Black are controlled by variations at the E and A locus as seen in the following table.
All other colors are variations of the above core colors, and are controlled by other genes that have not yet been located and for which no test exists.
Once your horse has been tested and you know its genotype, use this table for breeding strategies and breeding stock selection. The outer ring of the color buttons shows the visible coat color. The inner rings show the hidden color genes carried by the horse. Entries in the table show the probabilities of producing each of the various coat colors for a possible foal. For example, if your results show that your horse is a Genotype eeAa, your horse is a chestnut horse carrying the hidden genes for black. If you were to mate this horse with a Genotype EEaa, the resulting foal would, on the average, have a 50/50 chance of being black or bay.
These probabilities represent expectations averaged over the long run and are subject to the law of probabilities. While a test for the W and G genes does not yet exist it should be noted that: In the presence of the dominant allele W, a horse from birth will typically lack pigment in skin and hair. The skin is pink, the eyes brown (sometimes blue), and the hair white. Sometimes such a horse is called albino. The W allele is only rarely encountered. All non-white horses are ww. The homozygous condition (WW) is lethal.
A young horse with a G allele will be born with any color but gray and will gradually become white or white with red or black flecks as an aged animal. Earliest indications of change to gray can be seen by careful scrutiny of the head of a young foal since often the first evidence of the gray hairs will be seen around the eyes. In intermediate stages of the graying process, the horse will have a mixture of white and dark hairs, a most confusing stage for trying to identify color. In the later age, the hair color tends to become more silver.
See www.vetgen.com for information on Cream and Silver Color Dilution