Abstract: Objective To introduce a method of calculating the paternity index(PI) for autosomal codominant markers when the mutation is encountered.Methods Assuming mutation arises with a mutational probability before an allele segregates and transmits to child with 1/2 chance,the probability of a random man giving an allele to child is the allele frequency in population.Considering only one mutation event in one case,the mutant allele can be determined by comparing the allele between parents and child.Furthermore,the probability of child's genotype can be calculated to identify the father between a tested man and a random male.Subsequently,the PI can be calculated.Results Formula of PI was deduced for trios,duos and missing child cases,including maternal mutation.Conclusion The method is easy to understand and useful in paternity testing.

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