Glossary Plant Breeding

Mutation Pressure. The continued recurrent mutation tending to increase the frequency
of the mutated allele in the gene pool of a population.
Mutation. In broadest sense, any heritable variation in a gene or in chromosome structure
and/or number. However, the use of the term has now been restricted to include only
gene mutation, which refers to a process by which new allele(s) is produced through
heritable structural changes in the gene(s). It may be the process or even the result of
the process. It may occur in nature spontaneously, but the frequency is very low
(about one per million). The rate of mutation can be increased in breeders’
populations through chemical or physical agents called mutagens. The molecular
basis of mutation relates to mispairing between nucleotides of the DNA molecule
irrespective of whether the mutation is natural or artificial. In populations of a species,
it the ultimate source of all variation. If an allele A mutates to a (ignoring back
mutation) with the mutation rate for a given number of generations n, then
frequency of the allele A after n th generation, p n =p 0 e -n (whereas, eisbaseofthe
natural logarithms). Thus the frequency of the A allele decreases with time but the
rate will be very slow. Therefore, the process of mutation cannot drive the process of
evolution unless supplemented with recombination or migration. The statement that a
mutation, if not favoured, is lost from a population is not true absolutely. The
probability that it could be lost is (2N-1)/2N (N = size of the population); if not, then
the probability that it is fixed is 1/2N. Thus a mutation can become established in a
population even though it is not favoured by natural selection simply by a process of
random genetic drift.
Mutation Event. The actual occurrence of a mutation in time and space.
Mutation Frequency. The proportion of mutants in a population.
Mutation Rate. The number of mutation events per gene copy in a population per unit of
time (for instance, per cell generation).
Mutational Dissection. The study of the components of a biological function through a
study of mutations affecting that function.
Mutator Gene. Any gene that increases mutation rate of other genes.
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