Black Vs Brown

Black/brown – the B locus

Every sheep carries two color genes at the B locus that determine the basic color of the sheep, and every sheep is either black or brown. Even sheep that look white to the eye have either a black or brown pigment, which can be determined by looking at nose leather, hooves, tongue, or any spots in the fleece or hair. If there is any black at all on a white sheep (or on a colored sheep, for that matter), that sheep is phenotypically black. If the sheep has no black at all, but has a brown spot or brown coloring in the above areas, then the sheep likely has a moorit brown phenotype. As far as the genotype at the B locus, there are only two possible alleles: B+, which designates a black color base, and Bb, which designates a moorit brown color base. The possibilities for the B-locus pair can then be any of the following:

• B+B+ where two copies of black result in a sheep whose cells produce black coloring and will pass a copy of the black gene (B+) to every one of its lambs.
• B+Bb where there is only one copy of black and one copy of brown. Black is dominant to brown, so the sheep will phenotypically be black. Yet it will pass black coloring (B+) to only half of its lambs. The other half of its lambs will inherit brown coloring (Bb).
• BbBb where two copies of brown result in a phenotypically moorit brown sheep whose cells produce eumelanin that cannot be protected, so it oxidizes to brown. Every one of this sheep’s lambs will inherit brown coloring from this parent. If any black coloring appears on the sheep, it cannot be BbBb – it must carry at least one copy of B+.