Multiple mutations in a specific gene in a small population.

Hereditary diseases have been reported with relatively high frequency in some small populations. Founder effect and genetic drift, associated or not with selective advantage of heterozygotes in case of recessive diseases, are the main explanations. Therefore, if we consider one population and one particular disease, only one deleterious allele should be observed. Determination of mutations has shown that in most cases the situation is more complex; more than one mutation is found among the patients. This finding can be explained by a multiple founder effect, with genetic drift and new mutations.