ular place in his or her DNA—one representing each of the genes that are present on the two chromosomes of a given pair. The forms of a given gene are referred to as alleles. A person who received the same allele from the mother and the father is said to be "homozygous" for that allele; a person who received different alleles from the mother and the father is said to be "heterozygous." Many RFLP systems are based on change in a single nucleotide. They are said to be "diallelic," because there are only two common alternative forms. And there are only three genotypes: two kinds of homozygous genotypes and a heterozygous genotype. Another form of RFLP is generated by the presence of variable number tandem repeats (VNTRs). VNTRs are sequences, sometimes as small as two different nucleotides (such as C and A), that are repeated in the DNA. When such a structure is subjected to cutting with restriction enzymes, fragments of varied length are obtained.
It was variation of the VNTR type to which Alex Jeffreys in the United Kingdom first applied the designation "DNA fingerprinting." He used probes that recognized not one locus, but multiple loci, and "DNA fingerprinting" has come to refer particularly to multilocus, multiallele systems. A locus is a specific site of a gene on a chromosome. In the United States, in particular, single-locus probes are preferred, because their results are easier to interpret. "DNA typing" is the preferred term, because "DNA fingerprinting'' is associated with multilocus systems. Discriminating power for personal identification is achieved by using several—usually at least four—single-locus, multiallelic systems.
The entire procedure for analyzing DNA with the RFLP method is diagrammed in Figure 2.
After the bands (alleles) are visualized, those in the evidence sample and the suspect sample are compared. If the bands match in the two samples, for all three or four enzyme-probe combinations, the question is: What is the probability that such a match would have occurred between the suspect and a person drawn at random from the same population as the suspect?
Answering that question requires calculation of the frequency in the population of each of the gene variants (alleles) that have been found, and the calculation requires a databank where one can find the frequency of each allele in the population. On the basis of some assumptions, so-called Hardy-Weinberg ratios can be calculated. For a two-allele system, the ratios are indicated by the expressions p2 and q2 for the frequency of the two homozygotes and 2pq for heterozygotes, p and q being the frequencies of the two alleles and p + q being equal to 1. Suppose that a person is heterozygous at a locus where the frequencies of the two alleles in the population are 0.3 and 0.7. The frequency of that heterozygous genotype in the population would be 2 × 0.3 × 0.7 = 0.42. Suppose, further, that at three