Also called the heterozygote advantage, Overdominance occurs when the most fit individuals in a population have allelic heterozygosity in a single locus. In invasive ecology the heterozygotic advantage is often linked to heterosis. Heterosis is an increase in fitness as the result of hybridization (which can either be from interspecific or intraspecific breeding.) Perhaps the most famous case of overdominance is that of sickle cell anemia. In that case individuals who are have a ss combination on locus have an allele which causes their blood cells to be S shaped, a potentially life threating condition. However, when a person has the Ss combination they have a mix of s shaped hemoglobin and normal hemoglobin which results in an increased resistance to malaria, which can increase fitness in many countries where the life threatening disease is prevalent.
Hedrick, Philip W. "What Is the Evidence for Heterozygote Advantage Selection?" Trends in Ecology & Evolution 27.12 (2012): 698-704. Web.
Miko, I. "Genetic dominance: Genotype-phenotype relationships." Nature Education 1.1 (2008).