Four coat colour morphs are recognised in Hirta Soay sheep, dark wild, light wild, dark self and light self, where ‘wild’ (more correctly ‘wild-type’) refers to the mouflon pattern with pale belly and rump, and ‘self’ means having the same colour all over the body (see Chapter 2).
The morph frequencies are the same in males and females (data not shown) and the two components, colour and pattern, vary independently, i.e. the ratio of wild: self is the same within dark and light morphs and the ratio of dark: light is the same within wild and self morphs (Table A2.1) (Χ2 = 0.0137, 1 df, ns).
An inheritance model for colour (dark/light) was proposed by Doney et al. (1974) and supported by preliminary data from test matings in the Soay flock brought from St Kilda to the Animal Breeding Research Organisation in Edinburgh in 1963 (Ryder et al. 1974). These authors proposed one autosomal locus, known as the Brown or B locus, with two alleles, at which the dark allele is dominant to the light allele, and a second locus, the Agouti or A locus, at which wild is dominant to self. Data from the Hirta Soays support this model, as shown by the analysis below (Helyar 2000), in which we have adopted the international nomenclature for sheep colour loci alleles (Sponenberg 1997).