All the allotypes presented in Table 1 are expressed on immunoglobulin constant regions. Since the genes encoding the constant region of the heavy chains are closely linked within the IgG gene locus they are inherited together as haplotypes with a very low frequency of cross-overs. However, cross-over events have occurred during evolution, resulting in present populations expressing characteristic haplotypes, hence the usefulness of the allotype system in population studies. Thus in northern Europe (The Netherlands) the haplotype Glm'; G2m"; G3mh is present with a frequency of 0.45 and virtually all individuals are homozygous for the A2m' allele, however, amongst Nigerian Africans the G im~; G2m"; G3mh haplotype occurs with a frequency of 0.678 (the Glmf; G2m"; G3m1' haplotype not being encountered) and the A2m2 allele with a frequency of 0.826.

The structural difference between alleles is frequently restricted to a single base change within a codon resulting in a conservative amino acid inter change; examples are lysine or arginine at residue 214 in IgGl proteins correlating with the expression of Glm(z) and Glm(f) allotypes respectively and leucine or valine within k light chains with the expression of the Kml,2 or Km3 allotopes. Similar minor structural differences are observed between allotypic variants of Ig constant region in the mouse, rat and rabbit.

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