Heritability of cephalometric and occlusal variables as assessed from siblings with overt malocclusions.

It has been thoroughly documented that measurements of the craniofacial complexes have moderate to high heritabilities--that they are primarily a consequence of "nature" rather than "nurture." In contrast, recent studies, which used twin and sibship analyses, have shown that malocclusions per se (i.e., measures of tooth position and dental interrelationships) have low heritabilities; malocclusions sensu stricto are primarily acquired (environmentally induced), not inherited. All such studies have, however, focused on persons not treated orthodontically. This can introduce selection bias since cases with moderate to severe malocclusions would have received treatment and, thus, be excluded from study. The present analysis addresses this issue of ascertainment bias by examining just that portion of the adolescent population receiving comprehensive orthodontics. Initial treatment records of 104 pairs of siblings were studied, all of whom subsequently received full-banded treatment. In this selected series of overt malocclusions, heritability estimates for craniometric variables were significantly lower than in a comparable series of adolescents with naturally occurring good occlusions, whereas heritability estimates for occlusal variations (e.g., rotations, crossbites, displacements) were significantly higher. This vindicates the clinical perception that siblings often present with similar malocclusions. We propose that the substantive measures of intersib similarity for occlusal traits reflect similar responses to environmental factors common to both siblings. That is, given genetically influenced facial types and growth patterns, siblings are likely to respond to environmental factors (e.g., reduced masticatory stress, chronic mouth-breathing) in similar fashions. Malocclusions appear to be acquired, but the fundamental genetic control of craniofacial form often diverts siblings into comparable physiologic responses leading to development of similar malocclusions.