A deletion in the bovine myostatin gene causes the double–muscled phenotype in cattle

While the hereditary nature of the "double-muscling" phenotype (a generalized muscular hypertrophy documented in several cattle breeds) is well established, its precise segregation mode has remained controversial. Both monogenic models (autosomal dominant or recessive) and oligogenic models have been proposed. Using a panel of 213 bovine microsatellite markers, and an experimental pedigree obtained by backing "double-muscled (Belgian Blue) x conventional (Friesian)"1 dams to double-muscle sire, we have mapped a locus on bovine Chromosome (CHr) 2 that accounts for all the phenotypic variance in the backcross generation. This locus, referred to as mh (muscular hypertrophy), has been positioned with respects to a map composed of seven Chr 2-specific microsatellites, at 2 cM from the closet marker. This result confirms the validity in the Belgian Blue population of the monogenic model involving 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 and the Chr 2 markers was confirmed in three informative pedigrees collected from the general Belgian Blue Cattle population, reinforcing the notice of genetic homogeneity of the double-muscling trait in this breed. This work paves the way towards marker-assisted selection for or against the double muscling trait, and towards positional cloning of the corresponding gene.

Chromosomal homologies between individual human chromosomes and the bovine karyotype have been established by using a new approach termed Zoo-FISH. Labeled DNA libraries from flow-sorted human chromosomes were used as probes for fluorescence in situ hybridization on cattle chromosomes. All human DNA libraries, except the Y chromosome library, hybridized to one or more cattle chromosomes, identifying and delineating 50 segments of homology, most of them corresponding to the regions of homology as identified by the previous mapping of individual conserved loci. However, Zoo-FISH refines the comparative maps constructed by molecular gene mapping of individual loci by providing information on the boundaries of conserved regions in the absence of obvious cytogenetic homologies of human and bovine chromosomes. It allows study of karyotypic evolution and opens new avenues for genomic analysis by facilitating the extrapolation of results from the human genome initiative.

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