Abstract: Objective To introduce a new method for calculating the paternity index(PI). Methods Assuming that each allele from parents has undergone a transition before it segregates and transmits to child. The transition probability is 1 when parent allele is the same as child′s, the transition probability is 0 when parent allele is different from the child′s. Every allele has a transmission probability with 0.5. Base on these theories, it is easy to gain the probability that child inherits an allele from the alleged father or mother. Thus, the X value(numerator) and Y value(denominator) of PI formula can be calculated, as unknown man provide an allele for child with the allele frequency. Results A general formula that calculated the PI for trios, duos and missing child cases was deduced. Conclusion The new method is practical in all kinds of forensic paternity case.

Key words: forensic genetics, parent-child relations, paternity index