Interaction ofH-2 and nonH-2 linked genes in the regulation of antibody response to a threshold dose of sheep erythrocytes

The antibody response to a threshold dose (¹⁰) of SE was studied in the High responder line (H) and the Low responder line (L) of mice obtained by bidirectional selective breeding for the character quantitative agglutinin response to an optimal dose of SE, and in interline hybrids: F₁, F₁ and both backcrosses. Whereas the interline difference in agglutinin responses at the optimal dose is due to the additive effect of about ten independently segregating loci, one of which isH-2 linked, the responsiveness to the threshold dose is determined by the effect of two loci. The direction of the dominance effect also varies with the antigen dose: high responsiveness is partially dominant at the optimal dose while at the threshold dose nonresponder character is partially dominant. The role of theH-2 linked locus was investigated. It has been demonstrated that on an identical background (equivalent to that of F₁ hybrids) this locus is responsible for 12% of the interline difference at the optimal antigen dose, and for 61% at the threshold antigen dose. For the two antigen doses, the quantitative effect of theH-2 locus is in agreement with the estimate of the number of loci obtained by variance analysis. The intervention of a second gene, non-H-2 linked, in the regulation of responsiveness to 10⁶ SE is demonstrated by appropriate assortative matings. The interaction between the two genes is discussed.