Glossary Plant Breeding

Centric Fusion. A reciprocal translocation in which the large arms of two acrocentric
chromosomes unite to form a metacentric chromosome. It necessarily reduces the
chromosome number by one through eliminating the small translocation product,
which is mostly heterochromatic. However, the number of large chromosome arms
remains unchanged. Also see Robertsonian translocation.
Centromere. A localized region in each chromosome to which the spindle fibres appear
to be attached and which seem to determine movement of chromosomes during cell
division. (Also called kinetochore by cell biologists).
Centripetal Selection. Also called stabilizing selection. Selection acting against extreme
phenotypes, that is, selection favouring an intermediate “stable” value (phenotype).
This happens more often in natural populations. Genotypes having intermediate value
for most survival and reproductive traits are favoured owing to greater biological
fitness. If the population has a number of different intermediate genotypes, it will
cause preservation of genetic variability.
Certified Seed. Seed produced from the foundation or certified seed under the regulation
of a legally constituted agency. This class of seed is used for commercial crop
production. It is produced on a large scale in a way so as to meet the standard set by
the certifying agency.
Chaos Theory. A small change to a complex system can result in a large and unpredicted
outcome. For example, the introduction of semi-dwarfing gene in the hexaploid wheat
has resulted in short stature, increased photo-receptivity and harvest index, high yield,
and the like.
Chargaff’s Rule. A rule that is followed in all DNA molecules. Total amount of purine
bases (A+G) isalways equal to the total amount of pyrimidine bases (C+T). Further,
the amount of A always equals the amount of T, and C always equals G. However,
(A+T) /(C+G) ratio varies across species; but within the species, it remains relatively
constant. This rule seemed to be one of the milestones towards deciphering double
helical model of DNA by Watson and Crick (1953).
Character. A specific property of an individual resulting from the interaction of a
gene(s) with the environment. A gene cannot cause a character to develop unless
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