Epidemiology and genetics of hypodontia and microdontia: A study of twin families

Objective: To identify genetic and environmental factors contributing to hypodontia and microdontia by using Korean twin family data. Materials and Methods: A total of 1267 individuals (525 men and 742 women; 180 monozygotic twins [MZ] and 43 dizygotic twins [DZ] from 282 families) underwent an oral examination as part of the Healthy Twin Study in Korea. Dental anomalies classified as hypodontia or microdontia were diagnosed using radiographs and clinical examinations. In order to estimate genetic contributions to dental anomalies, we estimated the pairwise concordance rate (PCR), recurrence risk ratio (RRR), and heritability (h2). Results: The prevalence of hypodontia and microdontia was 3.55% and 3.00%, respectively. MZ had the highest PCR and RRR (13.0-15.3). The PCR and RRR values for both anomalies were much higher for DZ (5.0-11.9) than for siblings (1.4-2.6), despite the fact that DZ pairs and sibling pairs share 50% genetic identity. Further genetic analysis revealed both an additive genetic effect (0.38 when hypodontia and microdontia were pooled) and a strong "twin effect" (0.52 when hypodontia and microdontia were pooled). Conclusions: This twin-based study revealed that the formation of dental anomalies is affected by both genetic and environmental factors, and that the impact of these factors varies according to the specific dental anomaly.