Genetic Principle

• Deterioration of varieties: Genetic purity (Trueness to type) of a variety can deteriorate due to several factor 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 environment, under different soil and fertility conditions, or different climate conditions, or under different photoperiods, or at different elevation for several consecutive generations, the developmental variation may arise sometimes as 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 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 are kept on same threshing floor, resulting in considerable varietal mixture. To avoid this sort 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 the cases, it is difficult to identify or detect minor mutation.

d) Natural crossing: In sexually propagated crops, 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 following three reasons

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

The breeding system of species

• Isolation systems
• Varietal mass
• Pollinating agent