Origin of many of our most important cultivated plants was through polyploidy. A polyploid plant has more than two sets of chromosomes (genomes) in somatic cells. There are two kinds of polyploids, viz., autopolyploid and allopolyploid. In the autopolyploid each set of chromosomes, is identical or at least very closely similar to each other. On the other hand, in allopolyploid the basic sets of chromosomes, making up the multiple sets of chromosomes are differentiated from each other. Allopolyploid is the plant derived from hybridization between two differentiated species followed by doubling of the genomes. In sonic cases cultivated varieties have arisen by autopolyploidy. This is true of timothy, Phleum pratense. The 42 chromosomes in this species are all derived from the 14 chromosomes occurring in the small semi-wild pasture timothy, Phleum nodosum (Nordenskiold, 1949). Similar spontaneous autopolyploid origin is known for a large number of ornamental plants, potato, banana, apples, etc. A large proportion of cultivated plants are allopolyploids. Prominent among these are wheat, cotton, tobacco, and many forage grasses, Evolution of cultivated varieties of wheat is via allopolyploidy. In wheat and Aegilops only allopolyploid species have been found in nature, while no autopolyploids have been ever known (Kihara, 1966). According to Kihara (1966) the common wheat, Triticum aestivum, which is hexaploid (i.e. possesses six sets of chromosomes per somatic cell) has arisen from hybrids between the cultivated emmer i.e.( a tetraploid wheat, such as Tritiriun dicoccum) and Aegilops squarrosa, The evidence suggests that amphidiploid between the einkorn, Triticurn urariu (possessing 14 chromosomes or two sets of chromosomes, AA ) and a species presumably of sitopsis section of Aegilops, gave rise to wild emmer wheat, T. dicorcoids with genomic constitution AABB, which in turn gave rise to most cultivated hexaploid wheats (Jain, 1989).
Haploids are with a single set of chromosomes. The haploids are gametic cells formed after meiotic division. Diploids are with double set of chromosomes. They are formed after the fusion of male and female gametes. The resulted cell is called as zygote and it further divides mitotically.
Some cells have an abnormal number of chromosomes that is not a whole multiple of the haploid number. This condition is called aneuploidy. Most aneuploids arise by nondisjunction, a failure of homologous chromosomes to separate at meiosis. When a gamete of this type is fertilized by a normal gamete, the zygotes formed will have an unequal distribution of chromosomes.