SUPERIOR VARIETIES OF PLANTS

Superior varieties of crop plants are obtained from outside and accilimitised to local environment. Before introducing a planhttp://technologyofbiology.blogspot.com/2010/03/5-induced-mutations-mutations-are-new.htmlt, study of its growth patterns, soil and climatic conditions in the original habitat is carried out. Choice is then made of equivalent soil and climatic conditions. A number of plants having all type of variations in the original habitat are planted in the new habitat. The plants which show good performance in the new habitat are picked up for further propagation.
Hybridization – Hybridization is the obtaining of progeny after crossing two or more types of plants which differ genetically one another in one or more traits. Depending upon the traits involved, the cross is called monohybrid (single trait), dihybrid (double trait) or polyhybrid. Similarly, hybridization may be performed between two plants (single cross) or more than two plants (multicross). Hybridization may be intravarietal, intervariental, interspecific and intergenetic. Hybridization is performed for two reasons-
1. Development of hybrid vigour or heterosis and
2. Bringing together desirable characters present in different races, varieties etc. into the individual or hybrid.

Polyploid breeding – An organism having number of complete chromosome sets higher than diploid number is called polyploid. The polyploid is known as triploid, tetraploid, pentaploid etc. when it contains 3, 4 and 5 sets of chromosomes respectively. Polyploidy occurs in mature due to failure of chromosomes to separate at the time of anaphase either due to nondisjunction or nonformation of spindle, failure of meiosis during sporogenesis or gametogenesis fertilization of an egg with more than one sperm. Polyploidy can be induced artificially by application of colchicines and granosan. Polyploidy is the three types – autopolyploidy, allopolyploidy and autoallopolyploidy.
Autopolyploidy occurs within a species and improves the numerical increase in the same chromosome set e.g. autotriploid (AAA) autotetraploid (AAAA). Allopolyploidy develops due to hybridization between two species followed by doubling of chromosomes. Allotetraploid (AABB) is the common type. Autoalloploidy develops both by numerical increase of chromosome etc. within a species as well as hybridization between two species followed by chromosome dounling (AAAABB) is common type of autoallopoloidy.
Polyplioids are also called euploids because they possess exact multiple of haploid chromosome set. The plants having few or extra chromosome than the multiple of haploid set are called aneuploids. The condition of having few or extra chromosomes than the multiple of haploid genome is termed as aneuploidy or homologons pairs so that one type of gametes come to have extra chromosome (N + 1) while the others become deficient in one chromosome (N – 1 ). Fusion with similar or normal gametes give rise to aneuploids.