Dental Arch Asymmetry, Fluctuating Dental Asymmetry, and Dental Crowding: A Comparison of Tooth Position and Tooth Size Between Antimeres Matthew W. Sprowls*, Richard E. Ward, Paul L. Jamison, and James K. Hartsfield, Jr. Heritability in the narrow sense (h 2 ) is the proportion of a trait’s variation that, under ideal, simplified conditions, is attributed to additive genetic variation. This ratio of additive genetic to total (additive genetic plus envi- ronmental) variation does not take into account gene-to-gene interaction (dominance and epistasis) or gene-environment interaction. Heritability es- timates of dental occlusal characteristics (position, rotation, and angulation) that take into account environmental covariance collectively suggest that the predominant source of occlusal variation is environmental. However, the ability of each organism to develop appropriate symmetry relies on complex genetic interactions to buffer differences in right and left symmetrical de- velopment that increase with environmental disturbances during develop- ment. The purpose of this pilot project was to determine the relationship between dental arch asymmetry (right and left tooth position relative to the median palatal raphe) and right and left tooth size asymmetry. Pretreatment dental study casts of 28 patients from the orthodontic residency clinic at the Indiana University School of Dentistry were analyzed in a single blind fash- ion. Measurements were made to determine the amount and direction of right to left asymmetry relative to the median palatal raphe. Transverse and sagittal measurements were made to record asymmetries in canine and molar positions. Furthermore, three sets of antimeric maxillary teeth, the central incisors, the canines, and the first molars, were measured for crown length and width. Fluctuating asymmetry, the difference between two sides of a bilateral trait that does not involve antisymmetry and is not directional, was present in all measurements. A composite measure of total weighted dental (fluctuating) asymmetry (TWDA) in antimeric teeth was calculated by summing the size-adjusted differences in measurements of individual anti- meric pairs. Statistically significant correlations were demonstrated be- tween the extent of fluctuating asymmetry of the teeth and the extent of transverse maxillary dental discrepancies as measured from the canines and first maxillary molars relative to the median palatal raphe. Individuals with above average TWDA values were more likely to manifest dental crowding Private Practice of Orthodontics, Middle town OH, Department of Anthropology, Indiana University Purdue University Indianapolis, Indianapolis, IN. Department of Anthropology, Indiana University, Bloomington, IN. Department of Orthodontics and Oral Facial Genetics, Indiana University School of Dentistry, Department of Medical and Molecular Genetics, Indiana University School of Medicine, Indianapolis, IN. Address correspondence to James K. Hartsfield, Jr., DMD, PhD, Department of Orthodontics and Oral Facial Genetics, Indiana University School of Dentistry, 1121 W. Michigan St., Room DS255, Indianapolis, IN 46202. Phone: 317-278-1148; Fax: 317-278-1438; E-mail: jhartsfi@iupui.edu. © 2008 Elsevier Inc. All rights reserved. 1073-8746/08/1402-0$30.00/0 doi:10.1053/j.sodo.2008.02.006 157 Seminars in Orthodontics, Vol 14, No 2 (June), 2008: pp 157-165