[Genetics and the dento-facial complex].
Recently mutations in single genes have been found to cause dentofacial deviations like hypodontia, cleft lip and palate, as well as some craniofacial malformation syndromes like Aperts/Crouzon syndromes, cleidocranial dysostosis... These findings may lead to an overestimation and oversimplification of the role of genes in our daily life. The "one gene, one disease" principle has been applied too often, while most complex human characteristics are determined by a large number of genes, each leading to an increased liability for a certain condition. Whether this condition is expressed or not depends upon the environment the individual is exposed to as well as upon the position of a certain gene in the genetic network. The question is not whether a condition is genetically or environmentally determined, but to what extent environment and genes play a role and how they interact with each other. As early as 1870 Galton with his classical twin studies initiated the Nature-Nurture debate, whose immediate result was a great belief in the primacy of "Nature". Later, after Darwin proposed his theory of evolution (1871), the pendulum swung too far in the direction of "Nurture". At present, the dominance of the Nature camp has returned because of exceptional genetic scientific progress. Again new insights are expected that will clarify the mutual interaction of genes and between genes and the environment. After deciphering DNA, scientists need to carry out more research to explain how it is translated into the human phenotype, in health and in disease. Therefore, analysing the phenotypic variance with detailed clinical data--especially in twins--will contribute equally as much as DNA analysis to elucidating the functioning of our genes. More than their concordance, the discordance of monozygotic twins will be extremely instructive for the task of determining the genotype-phenotype relationship as well as for explaining the role of epigenetic phenomena.