Glossary Plant Breeding

Line. A group of individuals/plants with similar appearance and breeding behaviour. The
group is pure-breeding (or nearly so) and may be diploid or polyploid, and can be
distinguished from any other such group of the same species by some unique
phenotype and genotype. It is a more narrowly defined group than a strain or a variety.
Linkage Disequilibrium. A state of population in which there is a difference between
repulsion and coupling phase gametes, that is, (Ab) x (aB) – (AB) x (ab) 0.
However, under the condition of random mating, the difference (± d) is reduced to
one-half each generation so that after 4-5 generations, only 10% or even less of the
disequilibrium value remains in the population.
Linkage Drag. The transfer of undesirable genes along with useful gene(s) because of
their close association on the whole (or a segment of) chromosome. The problem of
linkage drag is often encountered in wide/distant hybrids, which largely restricts its
application in crop improvement.
Linkage Equilibrium. A state of population in which the difference between repulsion
and coupling phase gametes is zero, that is, (Ab) x (aB) – (AB) x (ab) = 0. Consider a
population in which frequencies of both A & B are 0.6 each and of both a & b are 0.4
each, then (0.6 x 0.4) (0.4 x 0.6) – (0.6 x 0.6) (0.4 x 0.4) = 0 (that is, population is in
equilibrium). This state of population will conserve the existing proportion of
genotype frequency generation after generation of random mating in absence of other
disturbing forces.
Linkage Group. A group of genes known to be linked. As linkage occurs in two
conformations (either in coupling or in repulsion phase), the number of linkage group
always equals the haploid number of chromosomes of the given species.
Linkage Map. A map showing position of genes on chromosomes as determined by
recombination relationships.
Linkage Value. Recombination fraction expressing the proportion of crossovers versus
parental types in a progeny; also called the recombination value. It varies from zero to
one half. The reason for this upper limit is that at a time only two out of four (½ =
50%) chromatids are involved in a crossing over.
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