In a Muslim Arab village, relatively isolated because of the preference of consanguineous marriages, we studied the fate of 12 mutations in 5 different genes. The study was based on carriers detected among relatives of affected patients and of carriers discovered in a random sample of 424 adults.
Most of the mutations have been introduced by a carrier(s) originating from another village, but a few have been de novo events. Mutations that are very frequent in the entire village were introduced soon after the foundation of the village. Examples of such mutations are [GBJ2, 35Gdel] and [MEFV, M680I], with a carrier frequency of 7.8% and 6.2%, respectively. Many of the other mutations that are rare were introduced recently into the village and are frequent only among the descendants of the first couple carrying the mutation. For instance all the carriers of [ARSA, Q190H], responsible for metachromatic leukodystrophy, were found among the 218 descendants of a couple who were living in the village 4 generations ago.
Since the village is typical for the region this study allows for some general conclusions to be drawn. In a population with a high degree of inbreeding the diagnosis of a single family with a patient(s) affected with a recessive disorder points to a recent event, while the finding of a rare disease in several families from an inbred population points to an older mutation. Mutations are often “exported” from one population to another by marriage. In the new inbred population this novel mutation will either be lost or will become frequent as the result of a founder effect. These observations are important for genetic counselling in the case of a recent mutation, since only the descendants of the founder couple are at risk, while in the case of older mutations the risk may be for the entire village. In the case of those frequent ancient mutations, the risk for a relative of an affected individual will be similar whether he marries a close relative or any random individual in the village.