Abstract: Objective To investigate the statistical method of tri-allelic patterns of STR loci. Methods The DNAs of venous blood and blood stain samples from 8 846 unrelated individuals were extracted using magnetic bead method, and STR genotypes were determined by multiplex fluorescent amplification and capillary electrophoresis and analyzed using GeneMapper ID v3.2 software. The genotype frequency and allele frequency of trizonal were determined by direct counting and formula, respectively, in order to deduce the formula of trizonal in paternity test and individual recognition. Results Of the 8 846 individuals, four tri-alleles and three tri-genotypes were detected. The multiplying of allele frequency and actual rate showed significantly statistical difference. The formula of trizonal in paternity test and individual recognition was successfully deduced. Conclusion The frequency of the two alleles inherited as a whole in the population could be calculated by multiplying the frequencies of each allele in the tri-allelic patterns.

Key words: forensic genetics, statistics, STR, tri-allelic patterns