Dissection of a continuous distribution: red cell galactokinase activity in blacks.

A significant difference between blacks and whites in the distribution of red cell galactokinase (GALK) has been found by Tedesco et al. [2]. From the shapes of the distributions, it was inferred that whites are essentially all homozygous for one allele (GALKA), but blacks are polymorphic. A second allele (GALKP), for lower GALK activity, is presented at high frequency in blacks but rare or absent in whites. This paper presents a method which, assuming the genetic model presented, estimates the genotype composition of the black sample. We make some reasonable biochemical assumptions and fit a mixture of three normal distributions to the black data to obtain an estimate of p, the frequency of GALKA in blacks. The fit of the model to the data is excellent and the best estimate of p is .217 +/- .025. Since admixture of white genes in blacks from the United States is known to be about 20%, the value of p implies that virtually all GALKA alleles were introduced by admixture, and that the ancestral black population was monomorphic for GALKP. If whites are indeed monomorphic for GALKA, they differ from unmixed blacks by a full gene substitution at the locus for GALK.