Genetic Principle

• Deterioration of varieties: Genetic purity (Trueness to type) of a variety can deteriorate due to several factors during production cycles. The important factors of apparent and real deterioration of varieties) are as follows:

a) Developmental variation: When the seed crops are grown in difficult environments, under different soil and fertility conditions, or different climate conditions, or different photoperiods, or at different elevations for several consecutive generations, the developmental variation may arise sometimes as a differential growth response. To minimize the opportunity for such shifts to occur in varieties it is advisable to grow them in their areas of adaptation and growing seasons.

b) Mechanical mixtures: This is the most important source of variety deterioration during seed production. Mechanical mixtures may often take place at the time of sowing if more than one variety is sown with the same seed drill; through volunteer plants of the same crop in the seed field; or through different varieties grown in adjacent fields. Often the seed produce of all the varieties is kept on the same threshing floor, resulting in considerable varietal mixture. To avoid this sort of mechanical contamination, it would be necessary to rogue the seed fields and practice the utmost care during the seed production, harvesting, threshing, and further handling

c) Mutations: This is not a serious factor of varietal deterioration. In the majority of cases, it is difficult to identify or detect minor mutations.

d) Natural crossing: In sexually propagated crops, the natural crossing is another most important source of varietal deterioration due to introgression to genes from unrelated stocks which can only be solved by prevention

Natural crossing occurs due to the following three reasons

• Natural crossing with undesirable types.
• Natural crossing with diseased plants.
• Natural crossing with off-type plants.
• Natural crossing occurs due to the following factors

The breeding system of species

• Isolation systems
• Varietal mass
• Pollinating agent