Heterosis describes the enhancement of the biological constitution of an offspring after mixing of genetic contributions of genetically distinct parents in a hybird fashion. The effects of the hybridization may be resultant of Mendelian or non-Mendelian inheritance factors and the offspring is said to exhibit heterosis if the traits are enhanced, with regard to fitness. [http:// ]
There is no single explanation for the event of heterosis, as it generally results from the influence of multiple loci. For this reason, multigene analysis models pose to be most applicable in the further understanding of heterosis. 
The resultant hybrid genotypes are acted upon by two influences: processes of selection and epigenetic variation. Through novel mechanisms epigenetic variation interacts with hybrid phenotypes. The result of this influence may be the expression of a given trait in dominance, overdominance, or epistasis along a contiuum of expression. 
Heterosis in PlantsEditWhen defining heterosis in plants the pre-existing value of selected agronomic traits must be considered in relationship to the genomic impacts under the action of selection. [http:// ]
Heterosis in AnimalsEditThe advantage of a purebreed cross between two genetically distinct indivuduals incurs the ability for each purebreed's strength to be utilized within the hybrid offspring.
Purebreed crosses are often used to promote lowly heritable traits, such as reproductive efficiency. This technique is very evident in market, litter-bearing species such as swine where reproductive performance has a positive correlation with profit within the food production system. 
 Birchler J.A. et. Al. Heterosis. The Plant Cell. 2010. 22:7. 2105-2112
 Schnable P.S. et. Al. Progress toward understanding heterosis in crop plants. Annu Rev Plant Biol. 2013. 64: 77-88