brown horses
Brown horses can be various shades from light brown to almost completely black. Their bodies may be shaded black and brown or be mostly black (which is often called seal brown). Their soft parts, such as the muzzle and eyebrows, and around the flanks, quarters and girth, are red or golden brown (sometimes called “mealy”). Seal browns are often mistaken for black horses, but the coloring on the soft parts identifies them as brown. They are also genetically distinct from black horses, as discussed below.
the genetics of brown horsesBrown horses must have at least one E+ allele at the extension locus (i.e. they are of genotype E+E+, E+e or E+ea at this locus). This allele causes the production of the black eumelanin pigment that occurs in black, brown and bay horses, and the colors derived from them (e.g. buckskin).
The A or agouti locus controls the distribution of black pigment in horses with at least one E+ allele: whether it occurs evenly throughout the body, as in true black horses, or only in certain parts, as in brown and bay horses. Brown horses are of genotypes AtAt or AtAa at this locus.
The recessive allele Aa has no effect on eumelanin (black pigment) production. Horses of genotype AaAa therefore have a base color of black. The allele At is dominant over Aa, but recessive to the other alleles of the agouti series. Molecular genetic tests have confirmed that seal browns are not blacks and do not have the AaAa genotype. This is contrary to a previous theory that seal brown horses were in fact black horses with the Pangarre (or mealy) trait (which is thought to be caused by a seperate gene, though one wonders about this).
The At allele obviously causes less restriction of black pigment than the bay alleles of the agouti locus. However we know that brown is genetically distinct from bay since breeding brown horses together only ever results in brown or black foals, and not bay ones.
A summary of the effects of the A series on black is shown below. Some genotypes are shown using an underscore, e.g. AA_. This represents where an allele can either be the same as the allele shown (i.e. AA in the example) or any other allele recessive to it (i.e. either At or Aa in the example, but not A+ which is dominant to AA).
The agouti locus reduces eumelanin (black pigment) production in brown horses, probably due to the gene only being operative (“switched-on”) in certain parts. Different alleles of the agouti locus are responsible for different forms of phaeomelanin (red/ brown pigment).
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