The extension coat color locus and the loci for blood group O and tyrosine aminotransferase are on pig chromosome 6.

A linkage map of pig chromosome 6 was constructed using a wild pig/Large White intercross pedigree. The map comprises 23 polymorphic loci, and the sex-average map length is approximately 170 cM. The study adds three new genes to the chromosome 6 map: the extension (E) coat color locus, and the blood group O (EAO) and tyrosine aminotransferase (TAT) loci. Segregation at the E locus determined two coat color phenotypes among the F2 animals: wild-type color (El-) and black-spotting (Ep/Ep). The E locus showed close genetic linkage to the most distal marker (S0035) on the short arm of chromosome 6. Comparative coat color genetics as well as comparative mapping strongly suggest that E in pigs encodes the melanocyte-stimulating hormone receptor, as previously shown for the corresponding coat color loci in mouse and cattle. TAT was also mapped to the distal part of 6p, whereas EAO was the most distal marker on 6q. A clear tendency for a higher recombination rate in both terminal regions was observed. A model for the evolution of pig chromosome 6, based on comparative mapping data, is presented.