No Excess of Homozygosity at Loci Used for DNA Fingerprinting

Abstract

Variable number of tandem repeat (VNTR) loci are extremely valuable for the forensic technique known as DNA fingerprinting because of their hypervariability. Nevertheless, the use of these loci in forensics has been controversial. One criticism of DNA fingerprinting is that the VNTR loci used for the "fingerprints" violate the assumption of Hardy-Weinberg equilibrium (H-W), making it difficult to calculate the probability of observing a genotype in the population. If one can assume H-W, the probability of observing the pair of alleles constituting an individual's genotype can be calculated by taking the product of the alleles' frequencies in the population and multiplying by two if the alleles are different. The evidence cited against assuming H-W is homozygote excess, which is presumed to be caused by an undetected mixture of two or more populations with limited interpopulational mating and distinct allele frequencies. For most VNTR loci, measurement error makes it impossible to test these claims by standard methods. The Lifecodes database of three VNTR loci used for forensics was used to show that the claimed excess of homozygotes is not necessarily real because many heterozygotes with similar allele sizes are misclassified as homozygotes. A simple test of H-W that takes such misclassifications into account was developed to test for an overall excess or dearth of heterozygotes in the sample (the complement of homozygote dearth or excess). The application of this test to the Lifecodes database revealed that there was no consistent evidence of violation of H-W for the Caucasian, black, or Hispanic populations.