Our genetic knowledge of a species is reflected in the state of its gene map. Although still primitive, relative to the gene map of the mouse, great strides have been made in recent years in developing the gene map of the rat and nine linkage groups have now been defined. Mapping by conventional backcrossing methods has been supplemented with parasexual methods using somatic cell hybrids. Use of recombinant-inbred strains has contributed significantly to the development of the mouse map and the technique holds promise for the future expansion of the rat map. Of great interest are comparative gene mapping and the relationship of linkage groups of the rat to those of other species. At present the linkage groups of the rat and the mouse reflect a high degree of conservation. This is surprising since chromosome banding patterns of the two species show but 40% homology.
