THE MH GENE CAUSING DOUBLE-MUSCLING IN CATTLE MAPS TO BOVINE CHROMOSOME-2

While the hereditary nature of the ''double-muscling'' phenotype (a ge neralized muscular hypertrophy documented in several cattle breeds) is well established, its precise segregation mode has remained controver sial. Both monogenic models (autosomal dominant or recessive) and olig ogenic models have been proposed. Using a panel of 213 bovine microsat ellite markers, and an experimental pedigree obtained by backcrossing ''double-muscled (Belgian Blue) x conventional (Friesian)'' F-1 dams t o double-muscled sire, we have mapped a locus on bovine Chromosome (Ch r) 2 that accounts for all the phenotypic variance in the backcross ge neration. This locus, referred to as mh (muscular hypertrophy), has be en positioned with respect to a map composed of seven Chr 2-specific m icrosatellites, at 2 cM from the closest marker. This result confirms the validity in the Belgian Blue population of the monogenic model inv olving an autosomal mh locus, characterized by a wild-type ''+'' and a recessive ''mh'' allele, causing the double-muscling phenotype in the homozygous condition. The linkage relationship between the mh locus a nd the Chr 2 markers was confirmed in three informative pedigrees coll ected from the general Belgian Blue Cattle population, reinforcing the notion of genetic homogeneity of the double-muscling trait in this br eed. This work paves the way towards marker-assisted selection for or against the double-muscling trait, and towards positional cloning of t he corresponding gene.