Hypodontia in twins discordant or concordant for cleft lip and/or palate

The aims of this study were to determine the frequency of hypodontia of permanent teeth in twins discordant or concordant for clefts in each type of cleft, to determine the concordance of hypodontia in mono- and dizygotic twins, and to compare the findings with some earlier data. Thirty-nine pairs of twins between 7 and 23 yr of age were investigated. Six of 13 monozygotic and 24/26 dizygotic pairs were discordant for clefts. Orthopantomograms of the dentition, treatment records, and anamnestic data were studied. Twenty-three pairs of twins (59%) had at least one twin with hypodontia: 9/13 (69%) monozygotic and 14/26 (54%) dizygotic. The prevalence of hypodontia was 37% for monozygotic and 32.7% for dizygotic twins, 16.7% in the noncleft and cleft lip groups, 41.1% in the cleft palate group, and 64.3% in the cleft lip and palate group. All these were above the values observed earlier in the noncleft twins and in the Finnish normal and cleft group populations. Four of 13 pairs of monozygotic twins had no hypodontia, 8/9 pairs were discordant, and only 1/9 was concordant for hypodontia. The corresponding figures for the 26 dizygotic pairs were: 12/26, 11/14, and 3/14 pairs. Maxillary second premolars were the most frequently absent teeth, followed in order of frequency by the maxillary lateral incisors and the mandibular second premolars. Hence, for this sample of twins, the genetic component seems to be weak.