Plasma IgA concentration was determined on 94 individuals of an eastern Kentucky family (IGAN1) 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 IGAN1 than in pedigree HGAR29, indicating that it is a minor, rather than a major, etiologic factor in IgA nephropathy.
