Abstract: Objective To introduce an universal algorithm for kinship index between a baby and a random person with biologic mother reference. Methods Based on the formulas of paternity index in trios（PIT）, common factors shared in these formulas were deduced following reconstructions of these formulas with the common factors. Universal algorithms for other common kinship indices, such as grandparental index （GI）, half sibling index （HSI）, avuncular index （AI） and first cousin index （CI1st）, were investigated according to avuncular index rule and the coefficient of relationship（r）. Results The common factor shared in the formulas for PIT calculation was 1 plus reciprocal of the frequency of the allele with identity by state between the alleged father and the detected baby. Two general formulas for PIT, GI, AI, HSI and CI1st with biologic mother reference were successfully established with the common factor and r value. Conclusion The calculation was simplified with the universal algorithms for common kinship indices between random person and the baby with biologic mother reference and the batch arithmetic operation with the universal algorithms can be easily realized with programming.

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