In parentage testing using DNA markers, the formulae for calculating the probability of exclusion generally overstate the power of a test battery by considering its ability to exclude a random man. It is known that in many cases, in particular immigration applications, the false father is more likely to be a relative, e.g. brother, of the true father than an unrelated man. This work presents formulae that take this consideration into account. A practical example using Hong Kong data is provided to illustrate the effect of the modification. Also discussed is how the expected efficacy of a test battery will be affected when possible mutations and null alleles or genetic inconsistencies are taken into consideration.

DNA profiling is currently the most widely used method for parentage verification, although many forms of it have limitations of some sort. In this paper, a general formula is derived to depict a simple relationship between the probability that a random man and the probably that a male relative of the child, other than the child's father, is excluded from paternity, when the phenotype of the child's mother is unavailable. With this, the possible limitations of a finite set of STR loci in excluding close relatives of the child from paternity are illustrated. Genetically, among the commonly encountered biologic relationships, to exclude a full sibling of the child from paternity if they pose themselves as father and child remains the most difficult.

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The allele distributions at the 12 short tandem repeat (STR) loci D3S1358, HUMvWA, HUMFIBRA/FGA, HUMTHO1, HUMTPOX, HUMCSF1P0, D5S818, D13S317, D7S820, D8S1179, D21S11 and D18S51 have been determined for 284 unrelated Chinese in Hong Kong. The combined probability of identity for the 12 STR loci was about 4.1 x 10(-14) and the overall probability of excluding paternity 0.999978. None of the 12 loci were found to deviate from Hardy-Weinberg expectations according to the results of the exact test. There was also little evidence for association of alleles between loci. The results demonstrate that the loci are useful for forensic human identification and parentage testing for the Chinese population in Hong Kong.

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