DNA analysis in disputed parentage: the occurrence of two apparently false exclusions of paternity, both at short tandem repeat (STR) loci, in the one child.

In cases of disputed paternity investigated by DNA analysis, it is generally accepted that at least two independent exclusions be observed before concluding that a putative father is not the biological father of the child. We report here a case in which two apparent exclusions of paternity were obtained (at the loci FESFPS and TPOX), from a total of nineteen loci examined (short tandem repeats, AMFLPs, DQA1, Polymarkers). The combined paternity index of the other seventeen loci, and of two multilocus probes (33.15 and 33.6) would exceed twenty-eight trillion to one. In the absence of any alternative putative father (including close relatives of the man tested), we concluded that the most likely explanation for these results was that the putative father was indeed the biological father, but that two independent mutations had occurred in either the ovum and/or spermatozoon. We have altered our internal laboratory criterion for exclusion of paternity to require exclusions in three independent loci.