root of the frequency of the homozygotes for the particular allele or half the summed frequencies of the heterozygotes, which, in the case of the five-allele system, will be of four types for each allele.

Population Substructure

It is intuitively obvious that relatives have genes in common. Thus, the chance that DNA typing will yield a match with a suspect when the evidence sample in fact came from a brother or other relative is considerable, especially if only a few loci are tested.

The U.S. population is a conglomerate of many different population groups, which might be viewed as extended families derived from all parts of the world. The major ethnic groupings—white, black, Hispanic, Asian, etc.—are each composites of many different subpopulations, which might have quite different frequencies of the alleles used in forensic DNA typing. Allele frequencies estimated from sampling of an overall ethnic group represent weighted averages. Some of the component subpopulations might have allele frequencies quite different from the mean values of the whole population. Further discussion of this problem and recommendations for handling it are given in Chapter 3.

CHARACTERISTICS OF AN OPTIMAL FORENSIC DNA TYPING SYSTEM

The methods of DNA typing continue to evolve as new ways to detect individual variation are developed. Sequencing of DNA might ultimately be the optimal method of personal identification, but that is still far from practical. It is important that the flexibility to adopt new methods be retained as standardization of DNA technology is developed (see Chapter 4) and databanks are created (see Chapter 5).

Any method of forensic DNA typing, like methods for medical DNA and other testing, should be rapid, accurate, and inexpensive. In addition, to achieve maximal discrimination among individuals, forensic DNA typing requires the use of markers with a high level of variability or polymorphism. Ideally, the high degree of variability would be found in all the world's populations. The markers and the probes used to detect them should have a unique sequence, so that each probe hybridizes with only one part of the genome. Single-locus probes should be used. The loci of the markers should be independent, e.g., on separate chromosomes. The markers should, furthermore, come from noncoding and therefore presumably nonfunctional parts of the genome, to avoid claims, spurious or otherwise, of association of particular markers with particular behavioral traits or diseases.

The automation of DNA typing might help to reduce its time and expense. An advantage of speed and low cost is that one can test more parts



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