Glossary Plant Breeding

Heterosis (Shull 1908). A phenomenon in which an F 1 derived by crossing genetically
dissimilar parents may fall outside the range of their parents with respect to a
character (s). Improved biological fitness of such an F 1 hybrid is often called hybrid
vigour. In strict sense, it describes a different genetic make-up of the F 1 compared to
its parents. Thus heterosis is the process and hybrid vigour is the consequence of that
process. However, for all practical purposes, these two terms are used synonymously.
Historically, it was attributed to heterozygosity (dominance and/ or overdominance,
that is, intra-locus interactions at individual loci) in F 1 hybrids. However, the high
proportion of homozygous loci (79% of the 23 loci studied) of the 4 most widely
grown single cross hybrids of maize (in USA) casts doubt on the single locus
heterozygosity and associated overdominance in promoting high performance of such
hybrids. More recently, multi-locus epistasis has been shown to be the major
mechanism of heterosis in a leading hybrid variety of rice (Shanyou 63) in China.
Thus epistasis (especially multi-locus epistasis) plays a major role in the genetic basis
of heterosis.
Heterostyly. A mechanism to promote outbreeding in certain angiosperms. It implies the
presence in a species of two (distyly) or three (tristyly) different types of individuals,
distinguished by the relative positions of stigma and anthers in the flowers. Also see
heteromorphic incompatibility.
Heterothallic Fungus. A fungus species in which two different mating types must unite
to complete the sexual life cycle. “Heterothallic” literally means, “different bodied”.
Mating types are physically identical but physiologically different.
Heterozygosity. A measure of the genetic variation in a population. It refers to the
frequency of heterozygotes (that carry dissimilar alleles of a gene) with respect to a
given locus.
Heterozygotes. Organisms carrying dissimilar alleles (Aa) of a gene pair. They are
sometimes called hybrids.
Heterozygous Balance (Mather 1943). A type of genetic organization adjusted to
heterozygosity. This adjusted state confers high adaptive value upon associated
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