Biology Reference
In-Depth Information
Figure 11.1
A STR profile generated from fetal cells recovered from amniotic fluid in early
pregnancy. In this case it was possible to determine that pregnancy had resulted from a rape, and
allowed an informed decision to be made on whether to have the fetus aborted. The profile was
generated using the AmpF
STR
Profiler Plus
STR kit (Applied Biosystems)
(see Chapter 9), which consider two competing and mutually exclusive hypotheses
are used. The hypotheses are:
The tested man is the biological father
The tested man is not the biological father
=
H
p
=
H
d
The symbols
H
p
and
H
d
were introduced in Chapter 9 when comparing the
proposition or hypothesis put forward by the prosecution (
H
p
) compared with the
hypothesis put forward by the defence (
H
d
). Although in many civil cases the terms
prosecution and defence are not appropriate, for continuity and consistency the same
terms are used in paternity testing. The result of a LR in this context is called a
paternity index (PI) and can be assessed using Equation 11.1.
Pr(
G
C
|
G
M
,
G
TM
,
H
p
)
Paternity index
=
(11.1)
Pr(
G
C
|
G
M
,
G
TM
,
H
d
)
To calculate this LR, we compare the probability (Pr) of the child's genotype (
G
C
)
given the mother's (
G
M
) and tested man's genotype (
G
TM
), if the tested man is the
biological father (
H
p
) compared with the probability of the child's genotype given
the mother's genotype, if the tested man is not the biological father (
H
d
).
The numerator and denominator are conditional on the genotypes of the mother,
child and tested man. They can be derived using a Punnet square.
Punnett square
The equations are not difficult to understand, particularly if derived from a Punnett
square and converted to text form. Consider the case where the mother is genotype
(
A,B
) and child is genotype (
B,C
) and the tested man is (
C,D
). If he is the father
then the mother must pass on allele
B
, and the father must pass on allele
C
.Ifheis
not the father then the mother must still pass on allele
B
but some other man must
pass on allele
C
. This is given in the Punnett square on the next page.
