C4 Phenotypic Variation Suggests an Unusual Class III Gene Organization

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Abstract
The class III gene markers, C4A, C4B, C2 and BF were determined in 26 HLA [human leukocyte antigen] genotyped families. In 6 of these families the inheritance of the C4A electrophoretic phenotype, C4A 3, A 2 could not be explained assuming the genetic model that the C4A component is controlled by only 1 locus (C4A) and suggests that both C4A 3 and C4A 2 are encoded by the same chromosome. This apparently duplicated C4A locus haplotype is common in Northern Europeans occurring at a frequency of at least 6%. In 4 families the C4A 3, A 2 variant occurred together with HLA-Bw35 and in 2 together with HLA-Bw55. In each family the C4A 3, A 2 variant was found exclusively with the complotype FC3/20 which encodes no phenotypically expressed C4B locus product.