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At present there is a DNA test for the “a” allele. It is possible to determine with this test, whether, for example a bay horse can produce a black foal. Interactions of the genes and alleles of Extension and Agouti are summarized below.

Table 1.

Agouti Extension Color AA, Aa EE, Ee Bay aa EE, Ee Black AA, Aa ee Chestnut (often) aa ee Chestnut (rare)

However there are additional alleles in these loci. An allele of Agouti designated “A^t” is responsible for the seal brown color and has an intermediate phenotypic effect between “A” and “a.” There is a hierarchy of domination (A > A^t > a), and a DNA test developed in 2009 helps breeders to distinguish the seal brown color from dark bay. Interactions of this allele with others in Agouti and Extension are summarized in the following table.

Table 2.

Agouti Extension Color AA^t EE, Ee Bay A^tA^t, A^ta EE, Ee Seal Brown

DNA tests show that the “A^t” allele is encountered much more frequently than it was thought previously. In the United States, Dr. Michal Prochazka (the translator of this text) detected this allele and developed the DNA test, but as of writing the information has not been published yet. Based on my research, “A^t” has been found in several breeds, including: the Thoroughbred, Quarter Horse, Paint Horse, Arabian, Morgan, American Miniature Horse, and in some British ponies. This allele was also found in six Przewalski horses.