Pretense of parentage by siblings in immigration: Polesky's paradox reconsidered.

Background: Older and younger siblings occasionally attempt to impersonate parent and child to expedite immigration under US family-based visa policies. The rate with which full siblings escape detection by current relationship tests is unknown.
Study Design and Methods: Retrospective study of full-sibling immigrant pairs was undertaken to determine the proportion that show insufficient genetic evidence to exclude parentage. Sibship and parentage indices (SI and PI) were compared/case in unexcluded sibling cases and true parent-child cases. Alleles shared per short-tandem-repeat locus were compared in sibling and parent-child pairs. The proportion of successful parentage fraud by siblings was estimated from the parentage exclusion rate among immigrants and the proportion of sibships without genetic inconsistencies (GIs).
Results: When 11 to 25 independent loci were tested per two-sibling case to verify or refute parentage, tests failed to demonstrate any GI in 9% and PI was greater than SI in seven of 10 of these cases. Another 29% of full-sibling pairs demonstrated insufficient evidence (fewer than two GIs) to exclude parentage. Thus, 0.4% of sibling pairs could falsely claim a parent-child relationship and show no GIs. Another 1.4% could make that false claim and not present sufficient evidence to be excluded.
Conclusion: At present, with no evidence of parentage exclusion in a full-sibling pair, the relative magnitudes of PI and SI are misleading relationship indicators because too few loci are examined and rates of sharing one and two alleles/locus vary greatly in parentage and sibling pairs. Only evidence of exclusion ascertains false parentage claims by siblings. Nevertheless, the expected rate of successful fraud is quite low.
(© 2013 American Association of Blood Banks.)