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Suspect and Evidence from the Same Subpopulation

It might be that the crime took place in a very small, isolated village, and the source of the evidence and suspect are both known to be from that village. In that case, we use the modified Equation 0.2b.

Consider first D2S44, in which p1 = 0.083 and p2 = 0.024. Suppose that the village is very small and that we wish to be very conservative, so we take image. The probability from Equation 0.2b is

image

Continuing in the same way through the other four loci, using Equation 0.2a for D1S7, and multiplying the results gives about 1/(600 million).

A PCR-Based System

We shall not give a specific example for a PCR-based system. The reason is that the situation is simpler, since there is usually no matching and binning. The detailed procedures are specific for each system and will not be repeated here. The techniques in general (e.g., for STRs) are the same as for VNTRs. They involve positions of bands in gels and photographs of the bands. The methods often use chemical stains rather than radioactive probes; that saves time. The allele frequency is determined directly from the database, and the calculations of match probabilities and likelihood ratios are exactly the same as those just illustrated.



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