Abstract: Objective: To study the relative effects of genes and environment on craniofacial height and upper airway width during the mixed dentitions. Methods: The sample used in this study consisted of lateral cephalograms of one hundred and eighty-three female twins. The genetic and environmental effects on craniofacial height and upper airway width were analyzed by statistical methods in female twins from six to twelve years old. Results: Significant genetic effects were present on the vertical measurement (H1, H2 and H3), and environmental effects were present on the Angle measurement (Cm-Sn-Ls and Li-B`-Pos) and upper airway width (d1, d2 and d3) in female twins. Conclusion: The effect of genetic on the craniofacial depth and upper airway width is significant, while environmental impact can also be founded.

Key words: Genetic and environmental effect, Twins study, Identical twin , Fraternal twin, Craniofacial region