Assessing exclusionary power of a paternity test involving a pair of alleged grandparents

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Abstract

BACKGROUND

The power of a genetic test battery to exclude a pair of individuals as grandparents is an important consideration for parentage testing laboratories. However, a reliable method to calculate such a statistic with short-tandem repeat (STR) genetic markers has not been presented.

STUDY DESIGN AND METHODS

Two formulae describing the random grandparents not excluded (RGPNE) statistic at a single genetic locus were derived: RGPNE = a(4 − 6a + 4a2– a3) when the paternal obligate allele (POA) is defined and RGPNE = 2[(a + b)(2 − a − b)][1 − (a + b)(2 − a − b)] + [(a + b)(2 − a − b)] when the POA is ambiguous. A minimum number of genetic markers required to yield cumulative RGPNE values of not greater than 0.01 was calculated with weighted average allele frequencies of the CODIS STR loci. RGPNE data for actual grandparentage cases are also presented to empirically examine the exclusionary power of routine casework.

RESULTS

A comparison of RGPNE and random man not excluded (RMNE) values demonstrates the increased difficulty involved in excluding two individuals as grandparents compared to excluding a single alleged parent. A minimum of 12 STR markers is necessary to achieve RGPNE values of not greater than 0.01 when the mother is tested; more than 25 markers are required without the mother. Cumulative RGPNE values for each of 22 nonexclusionary grandparentage cases were not more than 0.01 but were significantly weaker when calculated without data from the mother.

CONCLUSION

Calculation of the RGPNE provides a simple means to help minimize the potential of false inclusions in grandparentage analyses. This study also underscores the importance of testing the mother when examining the parents of an unavailable alleged father (AF).

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