Some further data on theH-2 system of antigens

The number of antigenic components detectable in systems such as H-2 in mice depends in part upon technical developments. It is also limited by the number of genetically different individuals available for study and a formula for deducing the number of antigenic components detectable for N unrelated systems is given in the text. If parental types and descendants derived from them by crossing over are included the possibilities are decreased. A new antigenic factor(s) has been brought to light by technical improvements and two more have been revealed by crossing over. Three cross-over combinations $H-2^{g},H-2^{h}$ $\text{and}$ $H-2^{i}$ are now available as homozygotes. A preliminary map gives the order DCVK for genes within the H-2 system. The relative positions of E and K are not yet known but their genes are closely linked. The frequency of recombination between D and K was 1.03% for both sexes and 1.4% for females. Values obtained by other workers are given in the text. It is concluded that crossing over must make a material contribution to the polymorphism of the H-2 system. No convincing example of mutation within the system has been published as yet. One new cross-over system $H-2^{i}$ appears to have certain selective disadvantages whilst statistical evidence strongly indicates that $H-2^{b},H-2^{d}$ $\text{and}$ $H-2^{k}$ are favoured by selection under laboratory conditions.