The aim of this study was to estimate the magnitude of genetic factors involved in the pathogenesis of Class III malocclusion in Japanese patients. We investigated 311 index cases (112 male, 199 female ; mean age 23.7 years) with Class III malocclusion. A questionnaire was used to determine the occurrence of Class III malocclusion among relatives. There were no significant differences in 1) the occurrence of this disease in index cases and the occurrence rates between the affected fathers and

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... thers, or between the affected grandfathers and grandmothers (either maternal or paternal), 2) the occurrence of this disease in index cases and the occurrence of Class III malocclusion in 1 or both of the paternal or maternal grandparents, 3) the risk of developing Class III malocclusion in the risk of a case developing Class III malocclusion according to the occurrence of the condition among parents and grandparents. The sibling recurrence risk (K5) obtained was 55.7% (95% confidence interval, 52.0-59.3), and the sibling recurrence risk ratio (As) was 18.8. The high sibling recurrence risk ratio supports a strong genetic background for Class III malocclusion in Japanese, and provides a basis for designing future genetic linkage studies.