Expected genetic drift and observed gene variation in a small isolated human population.

Data on the gene frequencies of four blood groups and six polymorphic erythrocyte enzymes were collected in a small population of the Northern Apennines (municipality of Zeri, Tuscany, Italy), whose civic records are available from 1866 on. Starting from the assumption that the mean gene frequencies of large populations of the neighbouring regions may represent a good estimate of the frequencies of the 'mother population' of the isolate, an observed value of the standardized ('Wahlund') variance of gene frequencies was computed and found to be 0.0131. The expected value of the same variance, due to random drift, has been computed on the basis of an evaluation of the immigration to the isolate, drawn from the marriage register of the municipality: this value was 0.0103.