The probability of exclusion based on the HLA locus

It has been estimated that in 1978 aproximately 543,900 illegitimate births occurred in the United States. A large number of these cases appear before courts of law each year to adjudicate questions of paternity. It has also been shown that by using 57 blood-group and enzyme marker systems more than 95% of falsely accused males in these paternity suits may be excluded. The probability with which a marker locus will exclude a falsely accused male depends on the degree of polymorphism at that locus. Since the closely linked HLA loci that reside on human chromosome 6 constitute the most polymorphic marker system known, these loci are of great utility for excluding paternity among falsely accused males. For evaluating the usefulness of the HLA system one needs to compute the average probability of exclusion for each HLA locus. In other words, we need to compute the average probability with which a falsely accused male will be exonerated on the basis of HLA testing of a mother-child-putative farther trio. Several authors have given expressions for computing this probability. Unfortunately, these formulas are incorrect because these authors consider each HLA type as an allele dominant over the blank allele but fail to consider the fact that expressed alleles are condominant to each other. This paper describes a simple method for evaluating the probability of exclusion for HLA loci.