Townsend, Grant ClementSampson, Wayne JohnRibeiro, Daniela Cisoto2013-09-022013-09-022012http://hdl.handle.net/2440/79678Studies in rodents and humans have suggested that masculinisation of females and feminisation of males can occur between fetuses in utero due to hormonal diffusion. Opposite-sex twin pairs provide a useful model to study the effects of prenatal hormone diffusion on tooth size. This study aims to determine whether primary and permanent tooth sizes are altered in females and/or males from opposite-sex dizygotic (DZOS) twins compared with same-sex monozygotic (MZ) or dizygotic (DZSS) twins; to assess the sexual dimorphism between males and females from all zygosities; and to quantify the amount and magnitude of dental asymmetries, both directional and fluctuating asymmetries, in the different twin groups. Serial dental models of the primary, mixed and permanent dentitions of 122 males and 135 females, aged from 4 to 16 years of age from DZOS, MZ and DZSS twin pairs, were used. Mesiodistal (MD) and buccolingual (BL) crown dimensions, crown heights (CH) and intercuspal (IC) dimensions of all primary teeth and the permanent central incisors, lower lateral incisors, canines, second premolars, first and second molars were measured to an accuracy of 0.1mm using a 2D image analysis system. Dental crowns of DZOS females were consistently larger by approximately 1-3% in MD and BL dimensions, by 2.7-4.7% in CH dimensions and by 0.5-0.8% in IC dimensions of permanent teeth compared with other female groups. Although the differences were smaller than in the permanent dentition, the primary dentition also showed larger dental crown size in DZOS females by 0.5-2% in MD and BL dimensions, 3% in CH and by 1% in IC dimensions. No systematic trend for altered tooth size was found in either dentition or for any of the crown dimensions between the male twin groups. There was no evidence of systematic directional asymmetry in either dentition in males or females from all the different zygosity groups, whereas fluctuating asymmetry was evident in all groups with the magnitude being smaller for MD and BL dimensions compared with CH and IC dimensions. There was no evidence that the magnitude of fluctuating asymmetry differed significantly between the different zygosity groups. This analysis shows a trend for primary and permanent dental crown size to be larger in DZOS females than in the other female twins. A multivariate analysis of variance confirmed that there were statistically significant differences between the twin zygosity groups, with the CH dimensions showing the greatest effect. No effects were found in DZOS males. The effects on DZOS females may be related to circulating male hormones from the co-twin in utero, supporting the view that variation in tooth size reflects an interplay between genetic, epigenetic and environmental influences during development, with hormonal effects playing a small but significant role.tooth size; primary dentition; permanent dentition; twins; sex hormones; geneticsThesis20130710162529