Heteroosis

Heteroosis, also known as heterosis or hybrid vigor, is the phenomenon whereby offspring produced from crosses between genetically distinct individuals exhibit superior biological performance compared with their parents or standard varieties. It is most commonly discussed in plants, where F1 hybrids from inbred lines often show higher yield, faster growth, larger size, or improved stress tolerance, but it is also observed in animals.

The genetic basis of heteroosis is complex and not due to a single mechanism. Three main explanations are typically cited: the dominance hypothesis, which posits that hybrids mask deleterious recessive alleles from either parent; the overdominance hypothesis, which suggests that certain loci confer a heterozygote advantage; and epistasis, where positive interactions among alleles at different loci enhance performance. Environmental conditions can influence the expression and magnitude of heterosis, and its relative contribution may vary by trait and species.

Heterosis is usually measured by comparing a hybrid’s performance to its mid-parent value or to the better-performing parent. In plant breeding, it is exploited by producing F1 hybrids through crosses of distinct inbred lines, a strategy that is central to commercial seed production in crops such as maize, rice, sorghum, and vegetables. The effect is often trait- and environment-specific, with some hybrids showing strong advantages for yield, biomass, or stress tolerance while others do not.

Limitations and considerations include the need for ongoing production of hybrid seed, potential instability of performance across environments, and concerns about genetic diversity. Heteroosis remains a foundational concept in breeding and genetics, guiding approaches to improve vigor and productivity.