Types of tooth cusp occurrence asymmetry in human monozygotic and dizygotic twins

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Abstract
Four types of cusp occurrence asymmetry were defined and studied in 90 monozygotic and 90 dizygotic twin pairs. Presence or absence of the hypoconulid (fifth cusp) and seventh cusps of the permanent mandibular first molar and cusp number on the mandibular first and second premolars was reported. Bilateral asymmetry, which produces three of the asymmetry types, occurred equally frequently in MZ and DZ twins. Type I asymmetry (bilateral asymmetry in one individual of a twin pair) was observed much more frequently than Types II and III which involve bilateral asymmetry in both twins of a pair. Although mirror imagery (Type III asymmetry) was observed only in MZ twins, its appearance in a DZ pair is expected in a larger sample. Type IV asymmetry (bilateral appearance of a cusp in one twin associated with bilateral absence of the cusp in the co‐twin) was observed significantly more often in DZ twin pairs than in MZ twin pairs. The similarity in concordance for MZ and DZ twins observed in this study may be due to the present‐absent method of description which was employed. An examination of the data using a scale to rate the degree of expression of a cusp might reveal differences in concordance between MZ and DZ twins.