Glossary Plant Breeding

probability that the two alleles at a locus is homozygous by descent. F varies from
zero for random mating population to unity for a completely inbred system. It is a
very useful parameter to describe the property of any other population relative to
Mendelian population:
AA Aa aa
p 2 + 2pq + q 2 = 1.0 (Mendelian population)
[p 2 + F (p)(q)] + [2pq - F (2pq)] + [q 2 + F (p)(q)] = 1.0 (Inbred population)
Inbreeding Depression. A reduction in biological fitness and vigour following
inbreeding (in individuals, which are normally cross-pollinated). The degree of ID is a
function of number of deleterious recessive genes kept concealed in the
heterozygotes. These harmful alleles are exposed in homozygotes upon inbreeding
through segregation and recombination. Thus ID may vary among the inbred lines
derived from the same population. Numerically, it is expressed: ID = [(F 1 -F 2 )/F 1 ] x
100 (in percentage).
Inbreeding Load. The extent to which inbreeding, which ordinarily increases genetic
load, impairs an equilibrium population under random mating.
Inbreeding Superiority. A condition wherein mean F 2 performance is greater than the
average performance of the respective F 1 hybrid. Under stress conditions, even a
small difference in physiology of individuals can make a big difference. Contrary to
F 1 population, F 2 population has better buffering capacity owing to genetic differences
among individuals which may result in superior performance under stress conditions.
Inbreeding Tolerance. The ability of a genotype or a population or a species (belonging
to outcrossing group) to endure with the stress imposed by inbreeding. Self-pollinated
species are highly tolerant to inbreeding. Among outcrossing species, cucurbits and
sunflowers are most tolerant, whereas alfalfa is highly susceptible. Genotypic
variations for inbreeding tolerance have been noticed in maize.
Inbreeding. Mating between relatives; mating of individuals more closely related than
individuals mating at random. The degree of inbreeding is a function of genetic
correlation between the mating partners. The closest form of inbreeding is selffertilization;
however, in self-incompatible species, sib mating is the closest possible
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