Biology Reference
In-Depth Information
population to population but are typically around 0.01 (1%). Any allele occurring
with a frequency of less than 0.01 will be adjusted to this figure. An alternative
approach is to use a minimal allele count, for example five alleles being the smallest
number of alleles that is considered: the allele frequency is simply calculated using
the formula 5/2
N
,where
N
is the number of individuals in the database [31].
Simple correction for sampling bias
Allele frequency databases are relatively small when compared with the popula-
tions from which they are drawn and therefore there remain sampling uncertainties.
A simple method for addressing such uncertainties, which are inherent in allele
frequency databases, is suggested by Balding [28]. The allelic information in the
evidential material is incorporated into the database to adjust for the potential under-
representation of alleles.
When there are matching DNA profiles there must be two DNA profiles: one from
the crime scene and one from the reference sample. The alleles from these profiles are
added to the allelic frequency database. By adding both profiles we are making the
assumption that the material found at the crime scene did not come from the suspect.
If we look at the profile in Table 8.1, at the vWA locus is a heterozygous locus with
alleles 14 and 17; these have frequencies of 0.0850 and 0.2500, respectively. By
multiplying the allele frequency with the total number of alleles in the database, we
can calculate that the numbers of observed alleles in the database are 34/400 for
allele 14 and 100/400 for allele 17. We now have two profiles to add to the database;
we have seen a total of four new alleles: 14, 17 in the crime scene sample and also
14, 17 in the suspect's sample. These can be added to the database and the frequency
recalculated. The database now has 36 observations of allele 14 out of a total of 404
observed alleles, which leads to an allele frequency of 0.090. Similarly, for allele 17
we now have 102/404, which gives us an allele frequency of 0.2525. This procedure
is repeated for each heterozygous locus.
In Table 8.1 the FGA locus is homozygous, and in the original database we have
71/400 observations but now need to add four more observations (21, 21 and 21, 21) to
both the frequency of allele 21 and the total number of alleles, so the new frequency is
75/404
=
0.1856. The profile is recalculated using the correction method in Table 8.2.
The Balding correction for size bias has the greatest impact when the database
is made from a small number of alleles or when the allele is rare. If the allele is
common and the database is large, the effect is negligible.
The above methods both compensate for the limitations of allele frequency databases
that are caused by sampling effects. Other more complex methods, such as calculating
the 95% confidence interval, can be employed but are not widely used [31, 32].
Subpopulations
In addition to correcting for sampling effect, it may also be necessary to allow for
the presence of subpopulations when calculating profile frequencies. Even within
