Mid-Parent Heterosis

Cross two inbred lines and the offspring frequently outperform both parents, sometimes dramatically. Mid-parent heterosis measures that advantage against the average of the two parents, and it is the phenomenon the entire hybrid seed industry rests on.

Maize is the extreme case: grain yield heterosis of 100% or more over the mid-parent is routine, which is why essentially all commercial maize has been hybrid since the 1930s. Rice, sorghum, sunflower and many vegetables show useful but smaller effects, typically 15–50%. Self-pollinating crops with little inbreeding depression to begin with — wheat, barley, most beans — show correspondingly little heterosis, which is precisely why hybrid wheat has repeatedly failed to become commercial: the gain does not cover the cost of producing hybrid seed.

Two explanations compete and both are partly right. Dominance says heterosis is the masking of deleterious recessives — each inbred carries its own set, and the hybrid covers each parent's weaknesses with the other's functional allele. Overdominance says the heterozygote is genuinely superior at some loci in its own right. A century of work has not fully settled the balance, but the practical implication is identical either way.

The critical practical fact is that heterosis depends on heterozygosity, and heterozygosity halves with every generation of selfing. Save seed from an F₁ hybrid and the F₂ loses roughly half the advantage, along with all uniformity. That is not a commercial restriction imposed by seed companies; it is the biology, and it is also the reason breeders talk about better-parent heterosis as well as mid-parent — only exceeding the best parent, not merely their average, makes a hybrid worth buying.