Because it is protected, the recessive trait will continue to reappear whenever two heterozygotes mate. Heterosis, heterozygote advantage, maintains variation because it maintains both allelels. This condition, which selects for the heterozygote genotype, maintains variation because it favors individuals with genotypes that display the highest degree of variation. In addition, neither allele can ever be lost under this condition. Then those two populations hybridize and their offspring flourish as a result.
Hybrid vigor occurs when two inbred populations are crossed.
The inbred populations have reduced the heterozygous genotype and are most likely made of homozygous genotypes. By crossing two homozygous populations, the variation of genotypes increases as heterozzygous genotypes are produced that contain two different alleles for one locus. So, a less favored genotype has a greater chance of producing offspring. As the less favored produces more offspring, it becomes the more frequent genotype. Then, the genotype that was originally more frequent becomes the less frequent and begins to reintroduce its genotype into the population until it becomes the more frequent.