Plaster study models and intraoral and extraoral radiographs of 165 twin pairs (99 mono and 66 dizygotic) were used to investigate: 1. the frequency of hypodontia of permanent teeth in twins, 2. the concordance-discordance rate of hypodontia in monozygotic and dizygotic twins, 3. the empirical risk of hypodontia occurring in twins. Out of 165 sets of twins, 14 (9 mono and 5 dizygotic) or 8.5% (5.5% monozygotic and 3.0% dizygotic) had hypodontia. This was within the frequency range of the general population (from 2.3% to 9.6%). Second maxillary incisors were the teeth most frequently absent or hypoplastic (peg-shaped). This was found in 5 monozygotic and 3 dizygotic sets of twins. It was followed by agenesis of the lower second premolars (in 5 pairs). Hypodontia of the remaining teeth was a rare finding. One member of a monozygotic pair showed agenesis of the left maxillary canine and the other agenesis of the right (a mirror-image similarity). The simultaneous appearance of missing teeth (hypodontia) and supernumerary teeth was found in two pairs of twins (1 monozygotic and 1 dizygotic). Monozygotic twins showed a significantly higher concordance rate for hypodontia than did the dizygotic. Out of 9 monozygotic pairs, 8 (88.9%) were concordant and only one pair discordant for the trait. On the other hand, all 5 dizygotic pairs were discordant for hypodontia. This suggests a high genetic component in the trait. Calculating the risk of hypodontia appearing in twins on the basis of the 14 pairs (both monozygotic and dizygotic) considered here showed that if one member of a pair of twins is affected by hypodontia, the risk of the other twin having the same abnormality increases to 57%. However, if one of a pair of monozygotic twins has the trait, the risk that the other will also have it is increased to 89%. Should however the pair be dizygotic, the risk of the other being affected is reduced to zero.
使用165对双胞胎（99对单卵双胞胎和66对双卵双胞胎）的石膏研究模型以及口内和口外X光片来调查：1. 双胞胎恒牙先天性缺牙的发生率；2. 单卵双胞胎和双卵双胞胎先天性缺牙的一致率与不一致率；3. 双胞胎发生先天性缺牙的经验性风险。在165对双胞胎中，有14对（9对单卵双胞胎和5对双卵双胞胎）或8.5%（单卵双胞胎为5.5%，双卵双胞胎为3.0%）有先天性缺牙。这处于一般人群的发生率范围内（从2.3%到9.6%）。上颌侧切牙是最常缺失或发育不全（呈钉状）的牙齿。在5对单卵双胞胎和3对双卵双胞胎中发现了这种情况。其次是下颌第二前磨牙缺失（5对）。其余牙齿的先天性缺牙情况较为罕见。一对单卵双胞胎中的一个表现为左上颌尖牙缺失，另一个表现为右上颌尖牙缺失（镜像相似）。在两对双胞胎（1对单卵双胞胎和1对双卵双胞胎）中发现了同时出现缺牙（先天性缺牙）和多生牙的情况。单卵双胞胎先天性缺牙的一致率显著高于双卵双胞胎。在9对单卵双胞胎中，8对（88.9%）该性状一致，只有一对不一致。另一方面，所有5对双卵双胞胎先天性缺牙情况均不一致。这表明该性状具有较高的遗传成分。根据这里所考虑的14对（包括单卵双胞胎和双卵双胞胎）双胞胎计算先天性缺牙出现的风险表明，如果一对双胞胎中的一个患有先天性缺牙，另一个双胞胎患相同异常的风险会增加到57%。然而，如果一对单卵双胞胎中的一个具有该性状，另一个也具有该性状的风险会增加到89%。但如果这对双胞胎是双卵双胞胎，另一个受影响的风险则降至零。
