OBJECTIVETo find the longitudinal genetic effects on mandibular position in mixed dentition.

METHODSThe sample used in this study consisted of lateral cephalograms of eighty-nine pairs of female twins in Beijing. With a mixed longitudinal method, the effective twins were 183 pairs(monozygous 110 pairs and dizygous 73 ones). The genetic and environmental effects on mandibular position were analyzed by statistical methods in female twins from six to twelve years old.

RESULTSStatistical comparisons revealed significant (P<0.05) environmental effects on the angles(ArGo'Me, SArGo', SN-MP, FH-MP, PP-MP), the depths(L1, L2, L3), and the heights(H1, H3) of mandibular, whilst genetic effects on the angle(NSAr) and the heights(H1, H2, H3, H4)(P<0.05); and family inheritance effects can be founded in the angles(ArGo'Me, SN-MP, NSGn, SGn-FH)(P<0.05).

CONCLUSIONThe effect of inheritance on mandibular position is strong, while environmental impact is relatively weak. Stronger genetic influence on heights is shown than that on depths. The environmental influence on depths and heights is different. Using Ba as a reference, the depth of the chin is the most subjective to environment change, then the mandibular angle, and the condyle is the least. Using N and S as references, the environmental influence on heights showed different order from the most to least changeable: The mandibular angle, the condyle and the chin. In later stage of our observation, the mandibular morphology and growth type might be family inherited. For environmental influences plays important roles on mandibular position, these findings can be used in orthodontic treatment planning.