Spotting due to the Spotting locus S
The spotting locus, designated as S, is responsible for white spotting that can exist anywhere on the sheep, and in young lambs, can make agouti identification difficult as it covers areas of identifiable display. Spotting due to this locus is always white, and often appears first at the midline of the lamb/sheep (top of head, tail, at the anus, or at the bottom of the legs). There are currently only two possible alleles identified at this locus: S+, which produces no spotting, and Ss, which is the designation for spotting. Again, there are only three possible genotypes for this locus:
- S+S+ – no spotting, the black/brown locus and the agouti locus both express normally as expected with no interference.
- S+Ss – one copy of spotting produces white spotting on the top of head and tip of tail. In some breeds with more extensive spotting, even one copy can produce white at the front of the neck under the chin, around the anus, or as a blaze down the face from the top of the head to nearly the nose.
- SsSs – two copies of spotting can produce white spots anywhere on the lamb – both symmetrical and asymmetrical spots have been reported as a result of this genotype. More work is underway to better identify the specifics of this allele.