AbstractA major locus that determines levels of apolipoprotein B (apoB) was revealed by likelihood analysis on 331 members of 36 pedigrees. The major locus explained 43.2 of the observed variance, with the remainder attributed to random environmental factors. Estimated mean apoB levels (mg/dl) were 110.5 ± 2.5, 141.9 ± 4.4, and 208.1 ± 11.5 for low homozygotes, heterozygotes, and high homozygotes, respectively. The corresponding genotypic frequencies were 0.718, 0.259, and 0.023. the apoB locus explained 13 and 14 of the variance in total and low‐density‐lipoprotein cholesterol levels, respectively. Persons with elevated apoB had normal to high levels of total serum cholesterol and triglyceride and low to normal levels of high‐density lipoprotein and apolipoprotein A‐I. Sixteen members of three of the pedigrees were heterozygous for familial hypercholesterolemia (FH). Their apoB levels were estimated as 35.72 ± 7.16 mg/dl above the apoB genotypic means, assuming that the two loci act independently. Therefore, two major loci, the FH locus and the apoB locus, affect two levels, apoB and LDL
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