<P> Heterozygote advantage is a major underlying mechanism for heterosis, or "hybrid vigor", which is the improved or increased function of any biological quality in a hybrid offspring . Previous research, comparing measures of dominance, overdominance and epistasis (mostly in plants), found that the majority of cases of heterozygote advantage were due to complementation (or dominance), the masking of deleterious recessive alleles by wild - type alleles, as discussed in the articles Heterosis and Complementation (genetics), but there were also findings of overdominance, especially in rice . More recent research, however, has established that there is also an epigenetic contribution to heterozygote advantage, primarily as determined in plants, though also reported in mice . </P> <P> When two populations of any sexual organism are separated and kept isolated from each other, the frequencies of deleterious mutations in the two populations will differ over time, by genetic drift . It is highly unlikely, however, that the same deleterious mutations will be common in both populations after a long period of separation . Since loss - of - function mutations tend to be recessive (given that dominant mutations of this type generally prevent the organism from reproducing and thereby passing the gene on to the next generation), the result of any cross between the two populations will be fitter than the parent . </P> <P> This article deals with the specific case of fitness overdominance, where the fitness advantage of the cross is caused by being heterozygous at one specific locus alone . </P> <P> Cases of heterozygote advantage have been demonstrated in several organisms, including humans . The first experimental confirmation of heterozygote advantage was with Drosophila melanogaster, a fruit fly that has been a model organism for genetic research . In a classic study, Kalmus demonstrated how polymorphism can persist in a population through heterozygote advantage . </P>

Which of the following is an example of both heterozygous advantage and balanced polymorphism