The Japanese geneticist Motoo Kimura and American geneticist James F. Crow (1964) introduced the infinite alleles model, an attempt to determine for a finite diploid population what proportion of loci would be homozygous.
This was, in part, motivated by assertions by other geneticists that more than 50 percent of Drosophila loci were heterozygous, a claim they initially doubted.
They determined that in the neutral case, the probability that an individual would be homozygous, F, was: where u is the mutation rate, and Ne is the effective population size.
However, this result only holds for the neutral case, and is not necessarily true for the case when some alleles are subject to selection, i.e. more or less fit than others, for example when the fittest genotype is a heterozygote (a situation often referred to as overdominance or heterosis).
Crow and Kimura showed that at equilibrium conditions, for a given strength of selection (s), that there would be an upper limit to the number of fitter alleles (polymorphisms) that a population could harbor for a particular locus.