The increased vigour displayed by the offspring from a cross between genetically different parents. Hybrids from crosses between different crop varieties (F₁ hybrids) are often stronger and produce better yields than the original varieties. Mules, the offspring of mares crossed with donkeys, have greater strength and resistance to disease and a longer lifespan than either parent. One explanation invokes the phenomenon of overdominance. Another, called dominance complementation, proposes that slightly superior alleles from either inbred parent will complement each other and result in a superior phenotype compared with either parent.