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| Ploidy Breeding |
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| An organism, in which number of complete chromosome sets is more than the diploid number is called polyploid. It is usual practice to express the chromosome number of an organism in diploid, symbolically represented by 2n. In all sexually propagated crops, such expression is of specific significance - because pairs of chromosomes indicate the number of chromosomes in each genome. The organisms are able to maintain chromosome numbers over generations due to the fact that gametes have n number of chromosomes, while all other cells of the organisms have 2n number of chromosomes. |
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| However, some organisms have more than 2n chromosomes. They are called polypliods. Polyploids may be triploids (3n), tetraploids (4n), hexaploids (6n). When polyploidy occurs within a species it is called autopolyploidy and when polyploidy occurs due to chromosomes from two species it is called allopolyploidy. Polyploidy may be caused by |
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 fertilization of an egg by more than one sperm |
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 failure of meiosis during gamete formation. |
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| Polyploidy can artificially be introduced by a chemical called colchicine or by crossing diploids with tetraploids. |
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| Although triploids exhibit sterility, polyploidy is useful in some ways due to some advantages. For e.g., |
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 In some plants like apple, pear the fruit size is bigger in triploids. |
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 In sugar beet, high sugar content is associated with triploidy. |
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| Ploidy breeding involves hybridization and selection of plants / varieties /species exhibiting various polyploidy. A great deal of success in wheat breeding was achieved, when origin of hexaploid wheat was discovered. Hexaploid wheat was a result of sequence of natural crossing and chromosome doubling. In the study of crop evolution, many such instances are recorded in crops like potato, paddy and sugarcane. |
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| Wheat is the best example of role of polyploidy in crop improvement. |
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| When number of chromosome is odd (except 1), the condition is called aneuploidy, while the condition with even numbers (2, 4, 6, 8) is termed as euploidy. |
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