Heterosis or Hybrid Vigour

Heterosis, or hybrid vigour, is the superior performance that offspring can show when genetically different crop plants are crossed. In plant breeding, it usually describes the advantage of the first filial generation (F1) over the parental average or the better parent for a specified trait, such as grain yield, growth or fertility.

How hybrid vigour is expressed and measured

Heterosis is measured for a particular trait under comparable growing conditions. A hybrid need not outperform its parents for every characteristic.

• Mid-parent heterosis measures the hybrid's performance relative to the average performance of its two parents.
• Better-parent heterosis, also called heterobeltiosis, measures superiority over the better-performing parent.
• Standard heterosis compares the hybrid with a suitable commercial variety or hybrid, helping breeders judge its practical value.
• Depending on the parental combination, heterosis may appear as greater yield, faster growth, increased biomass or improved reproductive performance.

How breeders produce and exploit heterosis

Breeders select genetically different parents and test their crosses to identify combinations that perform well. Genetic differences alone do not guarantee a useful hybrid.

• One explanation is dominance, in which favourable dominant alleles from one parent mask harmful recessive alleles from the other.
• Overdominance describes situations in which a heterozygous combination at a locus performs better than either homozygous combination; interactions among genes can also contribute.
• Commercial hybrid production requires controlled pollination so that seed results from the intended parental cross rather than unwanted self-pollination or cross-pollination.
• Male sterility can facilitate hybrid seed production by preventing the seed parent from producing functional pollen.

Agricultural importance and limitations

Heterosis is an important basis of hybrid breeding in crops such as maize, rice and sorghum. Its agricultural value depends on whether the hybrid's advantages justify seed-production costs and suit farmers' growing conditions.

• When F1 plants reproduce, segregation and recombination in subsequent generations can reduce uniformity and break up favourable genetic combinations.
• Farmers therefore generally need fresh F1 seed to reproduce the original hybrid's performance reliably; this does not mean that hybrid seeds are inherently sterile.
• A hybrid is not necessarily a genetically modified crop: conventional crossing can produce heterosis without introducing genes through genetic engineering.