The present study deals with the method of calculating the probability of being MZ, when many Mendelian characters are concordant in regard to same-sexed twin pairs. This method is based on the Bayes' theorem. For the calculation of the probability, the ratio of D by M (D/M) is needed, where M or D means the conditional probability of MZ or samesexed DZ respectively. The general formula of DIM was presented by means of gene frequency among general population, by considering the parents' genotype according as whether both are known or one of them is known, or both are unknown. The procedures are applicable to all kinds of Mendelian characters, provided the mode of inheritance is confirmed. The value of D/M was given by using gene frequencies among Japanese general population. The tables of these value were shown in about 30 Mendelian characters, for example, ABO, MN, Rh blood groups and so on, for practical use. By using these tables, the probability of being MZ can be calculated. It was concluded that the bjective criteria for zygosity diagnosis was given by the method in this study.
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