A major gene model for the familial aggregation of plasma IgA concentration.

Plasma IgA concentration was determined on 94 individuals of an eastern Kentucky family (IGANI) with some members having clinical and biopsy-proven IgA nephropathy, and on 197 individuals of a large Louisiana family (HGAR29) with no clinical history of IgA nephropathy but on whom 30 polymorphic markers had previously been typed. Pedigree segregation analysis was used to fit a major gene model, and a moderately large lod score for linkage to the ABO locus (1.50 at 0% recombination) suggested the existence of a recessive allele for high plasma IgA concentration. This allele is only slightly more prevalent in pedigree IGANI than in pedigree HGAR29, indicating that it is a minor, rather than a major, etiologic factor in IgA nephropathy.