Apolipoprotein A-I (apo A-I) is the major protein component of the high-density lipoprotein (HDL) found in all primates. Using radioimmunoassay, we measured plasma apo A-I levels in 97 individuals from 23 pedigrees ascertained through cases of hypertension or early coronary artery disease (CAD). Using complex segregation analysis, we found that a genetic model with both a single locus with a major effect and polygenic loci gave the best explanation for the distribution of apo A-I levels in these pedigrees. There was no evidence for a major locus effect on HDL cholesterol in these pedigrees. This is the first study to show evidence of a major effect of a single genetic locus on the quantitative variation of plasma apo A-I in a sample of pedigrees enriched for individuals at risk for CAD.
