WHAT MAKES BAY & CHESTNUT (A)
What Makes Bay & Chestnut (A)
Possible Color Genotypes/Black & Black-to-Grey
Possible Color Genotypes/Bay & Bay-to-Grey
Possible Color Genotypes/Chestnut & Chestnut-to-Grey
BAY
The DNA test for the presence of the A allele is called the AGOUTI Test. This test has been in use for a relatively short time. [Before this test, some horses that were tested and considered homozygous for black, produced chestnut--this is now explained to be caused by a rare allele at a different locus and named the "Black Forest" gene.] The presence of the dominant A allele will restrict the black hair produced by the dominant E allele to the points (mane/tail, ears, knees and hocks, lower leg areas), making the horse a bay.
Therefore, a bay must have at least one dominant A allele and one dominant E allele. (Most horses, including Arabians, have TWO dominant A alleles.) A black-bay horse will have some reddish/brownish hair around the muzzle even though its body might look totally black.
CHESTNUT
If no dominant E allele is present, the horse will be chestnut. [Without additional information either by parentage or DNA testing, it is impossible to know for certain whether a chestnut horse possesses dominant A allele(s) or recessive a allele(s).] Chestnut horses have manes/tails that are flaxen (lighter), darker (but not black), or the same color as its body hair.
-------------------------------------------------------------------------------------------------------------------
Special Note: The recessive a allele is rare. Since a black horse must have two recessive a alleles, this is the reason that black is a rare color and the bay and chestnut colors are so common.
-------------------------------------------------------------------------------------------------------------------
What Makes Bay & Chestnut (A)
Possible Color Genotypes/Black & Black-to-Grey
Possible Color Genotypes/Bay & Bay-to-Grey