[Paternity probability in the cases of incest].

To calculate the paternity probability in the cases of incest where the alleged father was either the father or the brother of the plaintiff's mother, some algebraic expressions applicable to a simple codominant diallelic genetic marker system were derived by modifying the formulas of Essen-Möller and Komatsu (the both formulas gave the same result). The paternity probability in the incest case is generally lower than that in usual case, because in the former case an allele present in the mother is sometimes found in both the alleged father and the child (plaintiff), even if the alleged father is not true father. The paternity probability in the incest case, however, becomes higher than that in usual case when an allele is common to both the alleged father and the child but not to the mother. The mean value of paternity probability becomes lower, as the relationship becomes closer between the alleged father and the mother.