8 Statistical interpretation of STR profiles
Once it has been established that two DNA profiles are the same, the significance of the match has to be estimated. This requires some knowledge of population genetics and some statistical analysis of the data. This chapter will briefly cover the funda- mental concepts involved with estimating the frequency of an STR profile in a given population.
It is necessary from the outset to define what is meant by a population. In the context of forensic genetics a population can be described as a group of people sharing common ancestry. In forensic terms the classification of a population within a country is usually quite broad and many subgroups that can differ in language, culture and religion are placedtogetherandclassifiedas,forexample,Caucasian,sub-SaharanAfricanandEast Asian.
The Hardy-Weinberg law
Population genetics can be defined as the study of factors affecting the allele and genotype frequencies of different genetic loci in a population. The Hardy-Weinberg Law (HW law), also called the Hardy-Weinberg principle, provides a simple math- ematical representation of the relationship of genotype and allele frequencies within an ideal population [1, 2] and is central to forensic genetics. The HW law states that within a randomly mating population the genotype frequencies at any single genetic locus remain constant. When a population is obeying the HW law it is said to be in Hardy-Weinberg equilibrium (HWE). Importantly, when a population is in HWE, the genotype frequencies can be predicted from the allele frequencies. This relationship can be represented in a Punnett square (Figure 8.1). The polymorphic STR loci used in forensic genetics have multiple alleles; how- ever, the genotype frequency of a homozygote can be calculated using p2 and that of
Figure 8.1 A Punnet square showing the relationship between alleles A and B, along with all the possible resulting genotypes. If allele A occurs at a frequency (p) of 0.6 and allele B occurs at frequency (q) of 0.4, then it is possible to estimate that the population will contain individuals with genotypes AA, AB and BB at frequencies of 0.36, 0.48 and 0.16 respectively. Each homozygote appears only once, hence p2, or q2. The heterozygote will be represented twice, hence 2pq heterozygotes can be calculated using 2pq, removing the need to construct elaborate Punnet squares.