Numerical Changes in Chromosomes
Monoploidy and haploidy
Since monosomics lack one complete chromosome, such aberrations create major imbalance and cannot be tolerated in diploids. These could be easily produced in polyploids. A polyploid has several chromosomes of same type and, therefore, this loss can be easily tolerated. The number of possible monosomics in an organism will be equal to haploid chromosome number. In common wheat, since 21 pairs of chromosomes are present, 21 possible monosomics are known. These 21 monosomics in wheat were produced by E.R. Sears (who died in 1991) in the variety Chinese Spring and are being used for genetic studies all over the world. Monosomics were also isolated in cotton (2n = 52) by J.E. Endrizzi and his co-workers, and in tobacco (2n = 48) by E.R. Clausen and D.R. Cameron.

As indicated above, monosomics are normally found in polyploids and diploids cannot tolerate them. Nevertheless, in tomato (2n = 24), which is a diploid, rarely monosomics could be produced. During the last decade surprisingly a complete set of monosomics has also been produced in maize, which is a diploid crop (See Weber, 1983). Double monosomics (2n-l-l) or triple monosomics (2n-1-1-1) could also be produced in polyploids like wheat. Double monosomics mean that the chromosome number is 2n - 2, like that in a nullisomic, but the missing chromosomes are non-homologous. Same explanation would apply in case of triple monosomics also.

Monosomic condition for a particular chromosome may be associated with a characteristic morphology. Moreover, in progeny of a monosomic we will get a mixture of disomics (2n), monosomics (2n - 1) and nullisomics (2n - 2) and a nullisomic will not possess any of the genes located on this specific chromosome. Therefore, by looking on the morphology of monosamics and that of their progeny, genes can be located on specific chromosomes. For a detailed account of monosomic analysis, which is an important technique for locating genes on chromosomes, please refer advanced book 'Cytogenetics - An Advanced Study by P. K. Gupta'.