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

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.

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).

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.

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.