## Paternity calculation with placental mixtures

Sometimes a paternity test is needed based on a mixture of mother and child. That is, the embryo died or was aborted, and the only DNA available is the mother's type, the alleged father's, and a type from the abortus that is a combination of the mother and the fetus. In such a situation it's easy enough to work out the proper likelihood ratios from first principles. Before doing that ...

### Shortcut

It turns out that it comes down to a simple rule that works for calculating any pattern of the placental mixture situation. Just relate it to a standard paternity pattern as follows:

### Example

Let's check the rule for the pattern
 Mother PR Mixture PQR Alleged father QS

The paternity index PI=X/Y where

X = Pr(such types | paternity by Alleged father), and
Y = Pr(such types | man unrelated)

as usual. For neatness let M=Pr(mother's type) and F=Pr(Alleged father's type). Then

X = M•F•Pr(Mixture | parents as alleged) = M•F•1/2;

Y = M•F•Pr(Mixture | Mother & random sperm) = M•F•Pr(Q).

PI = 1/2Pr(Q).

Note that this is the same formula as that for the normal paternity pattern
 Mother PR Child PQ Alleged father QS