Agriculture Reference
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upon the single locus mutation rate estimate of 1 × 10 -4 this would mean that in an
experiment using 2,000 M 1 plants, there would 10-100 × 10 -3 mutations or 1-10
per treated cell. Of these, some may be easily recognizable, others not; few usable,
most of them not. Admittedly such estimates cannot be very accurate but the old, of
magnitude should be acceptable and, therefore, be taken into account (Roychowd-
hury 2011 ).
SizeofM l Population
The M l population is the plants that are generated from mutagenized seeds or other
propagules. Determination of target population size in M l is the most crucial com-
ponent of mutation breeding. The target should be fixed so as to allow high number
of mutation events, yet the population size should be manageable by the breeder.
It is obvious that the population size will depend on the inheritance pattern of the
gene. If the mutation is monogenic recessive, the probability of recovering a mutant
phenotype will be higher than for a trait controlled by more than one gene. In math-
ematical terms, if 'n' is the number of mutation events in the M l generation after
treatment, and 'P' is the probability of occurrence of at least one mutation, then-
n = log(1
P )
log(1 µ) , where µ is the rate of mutation
With a mutation frequency of 1 × 10 -4 , 'n' equals to 46,520 for a monogenic reces-
sive trait. As there are two alleles, the number may be reduced to 23,260, indicating
that about 25,000 plants are to be grown to obtain a useful mutation in M l genera-
tion. In practice, ten times of the size has to be considered, because the mutation
produced may be useful or undesirable. Mutation breeding is an input intensive
process. It is therefore advisable to select mutagens with high mutation frequency,
so that M l generation size can be reduced.
While planting M l population, it is suitable to divide the whole seed lot in dif-
ferent small sections for ease of screening and analysis of chimera. It is to be re-
membered that germ-line mutations take place only in the initial cells of embryo,
so depending on the nature of the species, products of initial divisions should be
screened. For example, cereals like rice, wheat, barley, oat etc. produce multiple
tillers. Those tillers that generates first (primary tillers) have maximum chance to
carry mutation. In case of tuber crops like potato, the mutation may be present in
any of the stems arising from different discs of a tuber, so each of them has equal
chance to give rise to a mutation. Obviously, here the segregation pattern of muta-
tion will depend on both the number of stems as well as the ploidy level of potato,
which is an autotetraploid crop. Mutation breeding in polyploidy crop is more dif-
ficult than a diploid crop due to therecombination and segregation problem.
Genetically, a mutant plant in M l should be heterozygous, because during treat-
ment, only one allele is affected by one mutation. Probability of occurrence of a
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