Biology Reference
In-Depth Information
Table 8.1
The profile frequency is estimated using the principles of the Hardy-Weinberg law
and an allele frequency database that was constructed using 400 alleles. Because the loci do not
display any linkage and appear to be inherited independently of each other, the product rule can
be used, multiplying each genotype frequency together to calculate the overall profile frequency
Locus
Allele
Allele frequency
HWE
Genotype frequency
D3S1358
15
0.2825
2pq
0.1257
17
0.2225
vWA
14
0.0850
2pq
0.0425
17
0.2500
D16S539
11
0.2975
2pq
0.1041
13
0.1750
D2S1338
24
0.1000
2pq
0.0240
25
0.1200
D8S1179
11
0.0625
2pq
0.0434
13
0.3475
D21S11
30
0.2625
2pq
0.0551
31.2
0.1050
D18S51
14
0.1675
2pq
0.0477
15
0.1425
D19S433
14
0.3275
2pq
0.0164
15.2
0.0250
THO1
9
0.1375
2pq
0.0963
9.3
0.3500
p
2
FGA
21
0.1775
0.0315
21
0.1775
7.579
×
10
−
14
Profile frequency
the alleles in a large population and the frequencies are only estimates, and prone to
inaccuracies due to the limited size of the database. For common alleles the impact
is small, but with rare alleles, which can easily be under-represented in a frequency
database, the impact of limited sampling can have a large effect. It should be noted
that the deficiencies in the frequency databases can also lead to over-representation of
allele frequencies but, as a general principle, when we are estimating the significance
of forensic evidence the emphasis is not to over-state the strength of the evidence.
Different approaches have been taken to overcome the limitations of allele frequency
databases. These include the allele frequency ceiling principle, the Balding size bias
correction [28], allowing for the effects of subpopulations [29] and using a maximum
profile frequency [30].
Allele ceiling principle
Very rare alleles may not appear at all in the frequency database. If a rare allele
not previously represented on the frequency database is detected in a crime scene
sample then the frequency of the allele would be zero - which cannot be the case!
A mechanism must be put in place to deal with this situation. One approach is to set
a minimal allele frequency. The minimum frequency values that are used vary from
