## 4 Discussion  
This study is an epidemiological investigation of hypodontia found in consecutively treated subjects in a university post-graduate orthodontic clinic. Therefore, the prevalence rate of this anomaly discovered here within does not necessarily directly reflect that of the general population. Spaced dentition resulting from hypodontia especially in the maxillary anterior region is one of the major complaints for which individuals seek orthodontic treatment. Furthermore, it has been reported that females comprise the majority of orthodontic patients [28; 29]. This was reflected by our sample as well (i.e., 59% female participation). Independent records analysis of participating individuals in this study revealed a prevalence of 11% for congenitally missing teeth. This was found to have a higher female incidence but was not statistically different from that found in the male individuals. The quantity of missing teeth found in each individual with this anomaly varied from between one to eleven (in two individuals). The present study indicates that the more commonly missing teeth are the maxillary lateral incisors (54%), followed by the mandibular second premolars (42%). This is in agreement with [PERSON] [30; 31], but is not with [PERSON] who reported that the mandibular second premolar were more commonly missing [32]. The next most frequently missing teeth in our study were found to be the mandibular second premolars (42%) followed by the maxillary second premolars, similar to reports on missing teeth in the Turkish [22], Brazilian [33], Qatari [34] and Arab orthodontic patients [21]. It is worthy to note that previous studies on the incidence of missing permanent teeth reported varied conclusions based on ethnic differences. For example, characteristic to Asian ethnicity, the mandibular second premolars and mandibular incisors are the most frequently missing teeth [24; 35; 36], while in a Caucasian population the mandibular second premolars and the maxillary lateral incisors are reported as most likely to be missing [37]. Another example of ethnic difference is in the Druze population in Jordan known for their endogamy and consanguineous marriages, led to their genetic isolation, where the most missing teeth are the maxillary lateral incisors and canines [38]. Most of previous reports demonstrated that hypodontia was more prevalent in females than in males, similar to our findings [21; 22]. However, the distribution of missing teeth according to sex was not found to have a male/female prevalence, a finding reported in an Egyptian population [39]. Our study indicates a significant difference between missing teeth in the maxilla and the mandible, with higher prevalence of missing teeth in the maxilla (305-59%) compared with the mandible (210-41%), which is in agreement with [PERSON] et al. [26], [PERSON] [38], [PERSON] et al. [40], and [PERSON] et al. [41]. A possible explanation for the higher prevalence of missing teeth and other dental anomalies in the maxilla is in the differences in jaw ontogenesis where the maxillary growth and development is different from that of the mandible [41]. Interestingly, most of the dental anomalies, including missing teeth and supernumerary teeth were found in the maxillary anterior region while the majority of dental anomalies (missing and supernumerary teeth) were detected in the mandibular posterior region [41]. However, [PERSON] et al. reported a higher prevalence of missing teeth in the mandible [36]. Our study showed in unilateral cases of maxillary lateral incisor agenesis, more missing teeth on the right side compared with the left side, and the contralateral tooth present a reduction in size (peg shape teeth), similar to findings reported by [PERSON] et al. [1], [PERSON] et al. [40]. Unilateral missing maxillary lateral incisors and the reduction in size in the contralateral side present different expressions of the same gene responsible for missing teeth.