Sheep major histocompatibility (OLA) complex: linkage between a scrapie susceptibility/resistance locus and the OLA complex in Ile-de-France sheep progenies.

As seen on their family trees, Ile-de-France sheep with scrapie show genetic susceptibility to this disease, which is transmitted via scrs, an autosomal recessive gene. Scrapie occurred in homozygous recessive sheep, whereas the presence of the dominant resistance allele Scrr sufficed to prevent this disease in heterozygous animals. This hypothesis, previously proposed by Parry (1962), was tested in a study involving 133 crossings of sheep of different genotypes, and verified in the observed progenies. In a contaminated environment, susceptibility to the disease seemed to be transmitted by the scrs gene, whose frequency was increasing and varied in different progenies: the gene was propagated by inbreeding combined with selection; scrapie did not appear in the progeny of a homozygous resistant ram with the Scrr/Scrr genotype. On the family trees, linkage between the Scr locus and the OLA-A and -B loci is clearly visible; it often allowed the transmission of resistance or susceptibility haplotypes to be detected by OLA typing and followed from one generation to the next. The Scrr, OLA-A4, B6 haplotype was not found in any diseased sheep born before 1979, but was propagated by healthy sheep. During and after 1979, several recombinations of this haplotype were observed, giving a recombined haplotype with reciprocal linkage (scrs, OLA-A4, B6), disseminated by one sire in particular. The rate of recombination of Scr and OLA loci is estimated to be between 11 and 16%. The possibility of early selection against the disease without loss of qualities desired by the breeder is discussed.