Molecular cytogenetics of cattle: a genomic approach to disease resistance and productivity.

Classical cytogenetics has been merged with somatic cell and molecular genetics to facilitate mapping of the bovine genome. A physical map is presently being generated by the use of hybrid somatic cells and in situ hybridization to assign genes to chromosomes. A complementary genetic map is being generated by analysis of recombination of polymorphic loci, many of them identified with the same cloned DNA probes used for physical mapping. The eventual utilization of this map for selective breeding of disease resistance and productivity is dependent on the saturation of the map with polymorphic markers at a density that will assure linkage of genes influencing desirable phenotypes with at least one polymorphic marker. The identification of regions of chromosomal conservation among cattle, mice, and humans is being exploited to facilitate the saturation of the bovine map with useful markers.