Directionality of dental trait frequency between human second deciduous and first permanent molars.

Dental traits were scored for second deciduous molars (dm2) and first permanent molars (M1) on casts of the dental arches of children aged 7-11 years. The children were from four different ethnic groups. The overall frequency of traits differed between the groups but the relative frequency of expression of traits in the two teeth showed a similar pattern. Traits such as continuous oblique ridge, Carabelli cusp, Y pattern and 7th cusp, that appear in the early stages of development, were more frequent in dm2 than in M1. Wrinkling, occlusal tubercles and marginal ridge cusps were more frequent in M1; these appear later. Thus the relative frequency of traits in these teeth reflects their ontogenetic history.