Heterosis

Heterosis, also known as hybrid vigor, is the phenomenon whereby the offspring of genetically diverse parents exhibit superior biological performance for traits such as growth rate, yield, vigor, or fertility compared with their parents or with the average of the parents. It is commonly exploited in plant and animal breeding to improve performance.

The genetic basis of heterosis is complex and involves increased heterozygosity at many loci. Two classic explanations are the dominance hypothesis, which posits that hybrids mask deleterious recessive alleles from one parent by dominant alleles from the other; and the overdominance hypothesis, which posits that certain heterozygous genotypes at specific loci confer higher fitness than either homozygote. Pseudo-overdominance, caused by tightly linked deleterious and favorable alleles, can mimic true overdominance and contribute to observed heterosis.

Heterosis is typically measured by comparing hybrids to mid-parent or best-parent values. Relative heterosis or heterosis percentage expresses the degree of improvement of the hybrid over the parents for a given trait. The effect can vary by trait, environment, and genetic background.

Heterosis is widely observed in plants, with maize being a classic example, and is also detected in other crops such as rice and wheat. It occurs in animals as well, including cattle and sheep. In agriculture, heterosis is exploited through hybrid breeding and seed production to achieve higher yields, improved disease resistance, and greater stress tolerance.

Limitations and considerations include its context-dependence, potential decline in subsequent generations, and the ongoing scientific debate about the balance of dominance and overdominance effects. Advances in genomics and breeding are enhancing the practical use of heterosis in modern agriculture.