The "B" locus. Base colors.

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All sheep are genetically either black factored (BB), or brown factored (Bb). Black is dominant over brown. A sheep can be black, but carry brown recessively (BB/Bb). These black sheep can throw brown lambs.  Sometimes a pattern covers up the black or brown base color, such as white. The white will mask the base color.

These sheep are black factored (BB)

These sheep are brown factored (Bb)

Black can sunbleach, giving the tips a brown or orange color. The sheep may look dark brown on the outside, but when the fleece is parted, it will still look black near the roots.