## 1 Introduction  
Tooth agenesis is a very common developmental dental anomaly of human dentition, and the most common dental anomaly among different ethnic groups, with third molars being the most common missing tooth. Congenital missing teeth is usually determined clinically, by the absence of tooth eruption in the oral cavity, and by radiographic examination where the dental bud cannot be detected in the radiograph. It is often detected in orthodontic patients and is found most frequently in the maxillary anterior region [1, 2]. Tooth agenesis, based on the number of missing teeth, commonly termed hypodontia, is characterized by the congenital missing of one to five teeth, excluding the third molars. Oligodontia refers to congenital missing of six or more teeth, excluding third molars, and anodontia represents total missing of teeth in the dental arches which is extremely rare [3]. Anodontia or Oligodontia are usually associated with other systemic abnormalities such as ectodermal dysplasia or Down syndrome. However, Oligodontia may also appear as an isolated condition, referred to as non-syndromic oligedonia. In addition, a strong correlation has been reported between the congenital absence of deciduous and that of their successor teeth [4, 5]. When the deciduous tooth is missing, its permanent counterpart will also be absent. Additionally, agenesis can be linked to Sella Turcica Bridging (STB), or calcification of the interclinoid of Sella Turcica. This has been reported to be associated with some dental anomalies like canine impaction and transposition [6]. In addition, restriction of maxillary growth (maximally hypoplasia) found in cleft lip and palate patients has been reported to be associated with dental agenesis [7]. The most frequently missing permanent teeth according to rank are the maxillary lateral incisors, second premolars and third molars, while missing maxillary central incisors, canines and first molars are very rare [5]. Hypodontia can be classified as non-syndromic or sporadic when the absence of a tooth is an isolated anomaly, or syndromic when it is a part of a complex of multiple congenital anomalies [8]. Etiological factors related to tooth agenesis can be environmental or genetic resulting in failure of tooth development. Heredity is the main factor for missing teeth due to an autosomal dominant gene with incomplete penetrance [9; 10]. Several genes have been discovered to affect tooth development and cause tooth agenesis by mutations in the MSX1, PAX9, AX1N2, TGFA, IRF6, WNTA10, and FGFR1 genes [11; 12]. It has been proposed that dental agenesis may also be an expression of an evolutionary process (phylogenetic tendency), in which the jaws of humans together with the quantity of dental units are being altered in size and number, although no correlation was found between the size of the jaws and the absence of teeth [13]. It can also be a result of systemic factors such as endocrine disturbances, a part of a syndrome or due to environmental factors such as radiation or facial trauma [14]. According to [PERSON]'s Field Theory of tooth development in mammals, teeth were divided into \"stable\" and \"unstable\" groups where the missing tooth will always be the most distal tooth of any given group [15]. Therefore, the third molar is the most common missing in the molar group, the second premolar in the premolar group and the lateral incisor in the incisors group. While teeth such as central incisors, canines and first molars are rarely missing. In patients with congenitally missing teeth, deformities in tooth size and shape (peg-shaped or small laterals) are more frequent. Peg-shaped lateral incisors can be considered as an incomplete expression of the gene responsible for agenesis of this tooth [13]. The high prevalence of tooth agenesis and peg-shaped lateral incisors suggests a common genetic etiology [2; 16]. Hypodontia has additional localized influences such as its reported association with other dental anomalies such as palatially displaced maxillary canines [2; 17].