Genetics : Color and Pattern in Soay Sheep

Genetics of Color and Pattern in Soay Sheep

(after Appendix 2 in Clutton-Brock & Pemberton, 2004)

Pattern and Color are determined by two different genes, or loci, in sheep as well as in mammals generally. A number of alleles exist at each locus in other sheep and mammalian genera. However, so far anyway, Soay sheep seem to have only two alleles at each of the two loci. This simplifies the basic genetic description of Color and Pattern enormously.

Pattern refers to the distribution of colored fiber on the sheep's body as well as along the length of its fleece and hair fibers. This character, or trait, is determined by the genotype (which two alleles the sheep carries) at the Agouti or A locus.

Color refers to whether the wool is fundamentally light or dark, which is a manifestation of the type of eumelanin found in the fleece fibers. In light sheep, the eumelanin bleaches readily, in dark sheep it bleaches much more slowly. This trait is determined by the genotype at the Brown locus.

Pattern determined by the Agouti locus
Allele Symbol		Description
Formal	Simple
A⁺	A	Wild allele at agouti locus, dominant, banded (eumelanin/phaeomelanin) fibers, "mouflon" pattern (phaeomelanin in belly/rump)
A^a	a	Recessive allele, commonly known as "nonagouti", solid fibers, self coloration
Color determined by the Brown locus
B⁺	B	Dark allele, dominant, dark (unbleached) eumelanin in fiber
B^b	b	Light allele, recessive, light (bleached) eumelanin in fiber; tan, blond animals; lambs difficult to type

Genotypes and corresponding phenotypes

Genotypes	B⁺/B⁺ B/B	B⁺/B^b B/b	B^b/B^b b/b
A⁺/A⁺ A/A	Dark Wild	Dark Wild	Light Wild (tan)
A⁺/A^a A/a	Dark Wild	Dark Wild	Light Wild (tan)
A^a/A^a a/a	Dark Self (black)	Dark Self (black)	Light Self (chocolate)

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Genetics of Color and Pattern in Soay Sheep