Aneuploidy can be either due to loss of one or more chromosomes (hypoploidy) or due to addition of one or more chromosomes to complete chromosome complement (hyperploidy). Hypoploidy is mainly due to loss of a single chromosome, monosomy (2n-1), or due to loss of one pair of chromosomes, nullisomy (2n -2). Similarly hyperploidy may involve addition of either a single chromosome, trisomy (2n +1) or a pair of chromosomes, tetrasomy (2n+2). In representing chromosome number of aneuploids, here we are using 2n as the euploid chromosome number, even though 2n actually represents the somatic chromosome number of any organism, whether euploid or aneuploid. It is for this reason that in the preceding paragraph aneuploids are shown as 2n= 15 or 2n – 13 and not as 2n + 1 = 15 or 2n-1 = 13.
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-1-1) 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 two 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 the progeny of a monosomic we will get a mixture of disomics (2n), monosomics (2n-1) and nullisomics (2n-2) and the nullisomic will not possess any of the genes located on this specific chromosome. Therefore; by looking on the morphology of monosomics and that of their progeny, genes can be located on specific chromosomes.
Nullisomics are those individuals, which lack a single pair of homologous chromosomes, so that the chromosome formula would be 2n – 2 and not 2n -1 -1; the latter is used for a double monosomic. Sears had isolated all the 21 nullisomics in wheat.
Trisomics are those organisms, which have an extra chromosome (2n + 1). Since the extra chromosome may belong to any one of the different chromosomes of a haploid complement, the number of possible trisomics in an organism will be equal to its haploid chromosome number. For instance, we know that haploid chromosome number in barley is n = 7, consequently, seven trisomics are possible. Trisomics, where extra chromosome is identical to its homologues, are called primary trisomics. Besides these, there are secondary and tertiary trisomics. While a secondary trisomic means that extra chromosome should be an isochromosome (both arms genetically similar), a tertiary trisomic would mean that extra chromosome should be the product of a translocation. Trisomics were obtained for the first time in Datura stramonium (jimson weed) by A.F. Blakeslee and his co-workers. Since haploid chromosome number in this species in n = 12, 12 primary trisomics, 24 secondary trisomics and a large number of tertiary trisomics are possible. Most of the trisomics were identified by size, shape and other morphological features of the fruit.
One of the most extensively studied trisomic series is that produced and studied by T. Tsuchiya (who died in May, 1992) in barley. Trisomics are also known in Homo sapiens (human beings). Trisomy for certain chromosomes cause definite morphological abnormalities in human beings. Mongolism (Down’s syndrome) is one such feature, which is common in children and is characterized by mental retardation, a short body, swollen tongue and eyelid folds resembling those of Mongolian races. Other cases of trisomy are also known in a number of different plant and animal species.
Production of trisomics-
Trisomics may originate spontaneously due to production of n+1 type of gametes due to rare non- disjunction of a bivalent. However more often trisomics are produced artificially either by selfing triploids (produced by crossing diploids and autotetraploids) or by crossing these triploids as females with diploids as male (3xx 2x). In either case trisomics are obtained in large number and can b identified through phenotypic effects of individual chromosomes.
Cytology or trisomics-
A trisomic has an extra chromosome which is homologous to one of the chromosomes of the complement. Therefore, it forms a trivalent. This trivalent may take a variety of shapes in primary and secondary trisomics as shown in Figure. In a tertiary trisomic a characteristic pentavalent is observed.
Trisomics are also used for locating genes on specific chromosomes. If a particular gene is located on the chromosome involved in trisomy, segregation in the progeny of this trisomic will not follow a mendelian pattern, but the ratio will deviate from normal 3:1 F2 and 1:1 test cross ratios.
Tetrasomics have a particular chromosome represented in four doses. Therefore, general chromosome formula for tetrasomics is 2n + 2 rather than 2n+1+1, the latter being a double trisomic. All 21 possible tetrasomics are available in wheat.
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