The OLA major histocompatibility complex of sheep. Study of six new factors and evidence of a third locus of the complex: OLA-C.

6 new factors and 2 specificities 8L (A8-like) and 6L (B6-like) are described in sheep, in addition to the previously described 11 factors. 3 new factors are the products of three alleles (C14, C15, C17) at a third OLA-C locus of the OLA major histocompatibility complex, closely linked to the OLA-A and B loci. 2 other factors are the products of 2 alleles (A13 and B12) at OLA-A and B loci. The last new factor (16) is also the product of an OLA gene, the locus of which is not yet defined. An additional 8L factor is defined as the product of an OLA-A or B new allele. On the whole, 18 factors now known in sheep are the products of genes at 5 loci. 16 factors are distributed in 3 allelic series corresponding to the 3 OLA-A, B and C loci (the last 2 factors depend each on one of the two loci OL-X and OL-Z). 28 haplotypes are described in the studied 'Préalpe' flock. The new factors identify 6 haplotypes which were previously undetermined, and they subdivide some previously described haplotypes; nevertheless, OLA genetic structure appears to have changed little in the flock for 10 years. Genes at the three OLA loci are in linkage disequilibrium.