Abstract: Objective To calculate the exclusion power of STR loci in motherless parentage testing and to discuss how to draw a conclusion if there are inconsistent loci. Methods Based on the law of inheritance and allele frequency, the powers of exclusion of STR loci in motherless parentage testing (PEM) were calculated. Based on the mean PEM and mutation rate of 13 CODIS loci, The probabilities of inconsistence under paternity and non-paternity were calculated respectively according to binomial theorem. Results The PEM of locus having co-dominate alleles could be calculated as:?According to the formula, the average PEM of 13 CODIS was 0.411. Based on the mean PEM and mutation rate, the likelihood ratio of true father to random man (paternity index) was got using binomial theorem. Conclusion The conclusion in motherless parentage testing could be drawn based on the likelihood ratio (paternity index) derived from mean PEM and mutation ratio.

Key words: paternity testing, short tandem repeats, paternity index