Population genetics of C4 with the use of complementary DNA probes.

The C4 system is unusually variant both in the number of expressed genes and in the variety of those expressed. It is also closely linked to other two complement loci (C2 and Bf), which are structurally distinct but not linked to the two complement loci (C3 and C5) which are structurally similar. The C2, Bf, C4 segment lies within the MHC system, and their closeness to one another, and to the surrounding recognition loci (HLA-A, -B, -C, -DR) makes it difficult to attribute benefit or handicap, in relation to any disease, to any single locus by simple comparisons. The variation at the protein level, expressed by specificity, or charge, or size, or haemolytic activity, can now be supplemented by variation at restriction sites, or in the length of DNA between sites. The latter may be either causally or coincidentally related to the variant proteins. These data provide information on the evolutionary relations of the variants, both within the between species, and on the determinants related to inadequate or inappropriate defence against disease. They also provide evidence relevant to the sufficiency of the classical explanations of mutation, recombination and misalignment with recombination, and selection as explanations for what exists now.