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Principles of Forensic DNA for Officers of the Court

Estimating STR Profile Frequencies - The Product Rule

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National Institute of Justice (NIJ) (see reuse policy).

The product rule is used to estimate the chance of finding a given STR profile within a population. This is done by multiplying the frequency of each of the genotypes (combination of alleles) found at all loci in the STR profile.

In the case of Y-chromosome STRs and mitochondrial DNA (mtDNA) the product rule can not be used. For more information on Y-STRs and mtDNA, refer to Module 8.

The product rule requires using formulas defined by "Hardy-Weinberg Equilibrium," which predicts the probability of observing a particular homozygote (same allele inherited from each parent) profile or a heterozygote (different allele inherited from each parent) profile in a population.

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