Calculations on the decrease of genetic variation due to the founder effect

The expected amount of genetic variation in the progeny population (50 or 5000 individuals) of a founder group (2–50 individuals) sampled from a large base population was calculated. The measures of diversity at a locus were the expected allelic entropy, the expected number of alleles and the probability that all the alleles existing in the base population reach the progeny population. The initial number of alleles per locus in the base population was 2 or 5. At equal initial allele frequencies the expected loss of the allelic entropy was larger than that of the number of alleles. At a locus with one main allele and rare alleles the expected allelic entropy decreased less than the expected number of alleles. For a 2-allelic locus (initial gene frequencies 0.99 and 0.01) in the case of 24 founders the expected losses of the allelic entropy and the number of alleles were about 20 % and 30 %, respectively. For a 5-allelic locus (initial frequency of the most common allele 0.99, frequencies of the other alleles 0.0025) the corresponding losses were about 30 % and 70 %. If a minimum gene frequency 0.01 is assumed, about 200 and 500 founders are needed for the conservation of 99 % of the original number of alleles and the allelic entropy, respectively.