Several environmental factors like virus infections, toxins and radio- or chemotherapy may cause missing of permanent teeth. However, most of the cases are caused by genetic factors. The heritability of congenitally missing teeth has been shown in many studies. The genetic factors may be dominant or recessive and it is obvious that in many cases multiple genetic (and environmental) factors are acting together. The importance of genetic factors is shown by appearance of multiple cases among relatives (familial clustering) and higher concordance in identical than in non-identical twins.

Dominant inheritance of congenitally missing teeth has been shown both in hypodontia and oligodontia. However in both cases the amount and identity of missing teeth may vary between relatives. In hypodontia, the variability may extend to no teeth actually missing (“reduced penetrance”). The variability is probably caused by other genetic and environmental factors, and in some cases the etiology is analogous to multifactorial traits.

An example of recessive inheritance is given by recessive incisor hypodontia (RIH). In this condition described by us, a recessive gene causes congenital missing of several incisors, including lower permanent incisors and often decidusous incisors, too) the inheritance is recessive.