Biology Reference
In-Depth Information
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 fun-
damental concepts involved with estimating the frequency of an STR profile in a
given population.
Population genetics
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 placed together and classified as, for example Caucasian,
sub-Saharan African and East 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 - 3] 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; however,
the genotype frequency of a homozygote can be calculated using
p
2
and that of
