Phenotypic genetics of complement components.

Both charge and size-dependent electrophoretic techniques have been used to investigate genetic polymorphisms of complement proteins. Of the seventeen complement proteins, ten have been shown to have genetic variants and only one (C9) has been extensively investigated without revealing variants. These investigations give information on the numbers of cistrons and their linkage relations. They demonstrate or confirm the linkage of C2, Factor B and C4 to the MHC. In the cases of both human and mouse C4, it has been shown that the loci are (usually) duplicated. C4 in humans is extremely polymorphic and exhibits a number of strong allelically associated haplotypes. Some of these have only one expressed C4 gene and are associated with disease susceptibility. C8 has at least two cistrons coding for associating subunits. C6 and C7 are linked in several species and sometimes C7 is duplicated. This gene pair is discussed in relation to natural selection and gene conversion.