DNA analysis in a paternity case involving a triploid fetus

A parentage testing laboratory was asked to perform testing in a case of sexual assault that resulted in the conception of a child. Samples submitted to the laboratory included blood from the mother, the alleged father, and the fetus.

DNA typing was used to determine if the suspect in this sexual assault was the father of the expected child. DNA extracted from these samples was subjected to both restriction fragment length polymorphism and polymerase chain reaction/short-tandem repeat analysis at a total of 13 genetic loci. Examination of DNA profiles for selected markers suggested that the fetus was triploid. Triploidy was confirmed through the use of fluorescent in situ hybridization of chromosomes, employing three chromosome-specific alpha satellite probes and fetal trophoblast nuclei. Statistical interpretation of the test results required identifying a method for calculation that would consider two transmitted paternal genes. Attempts to modify the standard method of calculating a paternity index were unsuccessful, because it was not possible to distinguish between dispermy and diandry as the mechanism of conception. Therefore, the likelihood ratio was calculated as the reciprocal of the random men not excluded value or the proportion of the population that possesses all of the paternal markers observed in the triploid fetus.

Calculation of a likelihood ratio employing the exclusionary power of a collection of DNA markers appears to be the only method suitable for assigning weight to the significance of DNA matches between an alleged father and a child who is triploid.