Zygosity diagnosis of twins by questionnaire.

Subjects were 189 twin pairs, consisting of 165 MZ and 24 same-sexed DZ who entered the junior high school affiliated to Tokyo University (sample T), and 93 twin pairs, consisting of 71 MZ and 22 same-sexed DZ who were registered at Kinki University (sample K). The zygosity among them was previously identified by many genetic markers. This study aimed at the zygosity diagnosis by questionnaire. Questionnaire included threequestions. The first question was the similality of twins: "How are you alike?". The second was the frequency of being mistaken: "How often are you mistaken". And the third was to ask the person: "By whom are you mistaken?". According to the degree, 1-3 points, 1-3 points, and 1-4 points were given for each question. And the sum of the points of each pair of twins was calculated. Zygosity was determined by the sum of points, distributed from 6 to 20. Namely, if the sum is 6-13, we consider the twin pairs as MZ, and if the sum is 14-20, as DZ. It was revealed that more than 90% of twins were diagnosed correctly by use of this cutting point. And this result was in accordance with that obtained by use of discriminant function analysis. It was concluded that zygosity diagnosis by questionnaire was convenient and useful, in particular, for epidemiological research.