Biology Reference
In-Depth Information
Table 11.3
Formulae used to calculate paternity indices in
cases where no mother is available for testing. The alleles are
represented by
A
,
B
and
C
where
A
=
B
=
C
[21]
G
C
G
TM
PI
1
p
A
AA
AA
1
2
p
A
AB
1
2
p
A
AB
AA
1
2
p
B
BB
p
A
+
p
B
4
p
A
p
B
AB
1
4
p
A
AC
Prior probability
=
0.1
2 920 823
×
0.1
(2 920 823
×
0.1)
+
(1
−
0.1)
=
Probability of paternity
=
0.999996919
When this figure is used to report the results of a test it is often quoted as a
percentage, which is more accessible to non-scientists. In this case the probability of
paternity would be quoted as 99.9997%.
The value that is attributed to the prior odds of paternity is, of course, subjective.
In civil cases, the value of 0.5 is commonly used, although there is little scientific
merit to this value. In criminal cases, probabilities of paternity are often not presented
because it is the duty of the jury/judge to assess the prior odds of paternity. If results
are presented as a probability of paternity, a range of values calculated using different
prior odds is often quoted (Table 11.5).
With low paternity indices the impact of prior odds can be significant. However,
with the possibility of analysing a large number of STR loci, the PIs are typically
in the millions and the posterior probability of paternity is therefore extremely high,
even when the prior odds are very low. In the paternity test presented above, even
with the prior odds as low as 0.001, the probability of paternity is still 99.9966%.
Calculations of paternity indices can also incorporate correction factors to allow for
mutations and silent alleles [21] and also deficiencies in allele frequency databases;
