Linkage of quantitative variation of a serum protein (Sp) to the major histocompatibility complex of the rat.

An antiserum made in rabbits against a rat serum protein (provisionally designated Sp) has been used to establish the existence of a partially inbred line of animals that have low levels of this protein in their serum. This line of animals, the WRD line, was derived from a cross between a wild rat, WRF 6, and a rat of the F344 inbred strain. Offspring from this mating were selected for homozygosity at one wild major histocompatibility complex (MHC) haplotype and inbred for nine generations by brother-sister matings. When tested by immunodiffusion or rocket immunoelectrophoresis with a rabbit anti-rat Sp serum, the sera of these animals showed low levels of the protein when compared with other wild or inbred lines. Animals from the WRD line were backcrossed to the WF inbred strain and normal levels of the Sp protein were transmitted as an autosomal dominant trait. The gene controlling the serum levels of the protein is tightly linked to the MHC (RT1 complex) of the rat.