Aneuploidy means presence of chromosome number which is different than a multiple of basic chromosome number. In other words, aneuploidy is the presence of an abnormal number of chromosomes in a cell . Numerical changes in chromosomes or variations in chromosome number (heteroploidy), can be mainly of two types, namely and aneuploidy and euploidy. Euploidy means that the organism should possess one or more full sets of chromosomes.
Let us imagine that 7 is the basic chromosome number (x) in a particular class of individuals where diploid number (2n) is 14. In this case, chromosome numbers 2n =15 and 2n =13 would be aneuploids, while those having 2n=7, 21, 28, 35 or 42 would be euploids.
Aneuploidy (Numerical changes in chromosomes)
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). Hyperploidy 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 the 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.
Monosomics occur in polyploids and diploids cannot tolerate them. Nevertheless, in tomato (2n=24 ), which is a diploid, rarely monosomics could be produced. Double monosomics or triple monosomics could also be produced in polyploids such as wheat. Double monosomics mean that the chromosome number is 2n-2, like that in a nullisomic, but the missing chromosomes are non-homologous.
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). A 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. The chromosome formula would be 2n-2 and not 2n-1-1, which would mean a double monosomic.
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, 7 trisomics are possible.
Trisomics, where extra chromosome is identical to two homologues, are termed as 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 the Datura stramonium by A.F. Blakeslee and his co-workers. Since haploid chromosome number in this species is n=12, 12 primary trisomics, 24 secondary trisomics and a large number of tertiary trisomics are possible. The size, shape and other morphological features of the fruit were helpful in identifying most of the trisomics.
Trisomics also occur in Homo sapiens . Trisomy for certain chromosomes causes definite morphological abnormalities in human beings. Mongolism (i.e., Down’s syndrome) is one such feature, which is common in children. The characteristic features include short body, swollen tongue and eyelid folds resembling those of Mongolian races.
Production of trisomics
Trisomics may originate spontaneously due to production of n+1 type of gametes due to rare non-disjunction of a bivalent. Selfing triploids (produced by crossing diploids and autotetraploids) or by crossing these triploids as females with diploids as male were helpful in producing trisomics artificially. Both the cases were helpful in producing trisomics in large number. The phenotypic effects of individual chromosomes were helpful in the identification of trisomics.
Cytology of trisomics
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.
Trisomics are also helpful for locating genes on specific chromosomes. If a particular gene is occurs on the chromosome involving 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.
Chromosome segregation will hold good, when the gene is located very close to centromere permitting no crossing over between the gene and the centromere, so that both sister chromatids will be similar. In chromatid segregation gene is located away from centromere permitting crossing over between gene and centromere.
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. Besides these tetrasomics, E.R.Sears was able to synthesise a complete set of compensating nullisomic tetrasomics (2n-2+2). Here addition of a pair of chromosomes compensates for the loss of another pair of homologous chromosomes. Such non-homologous chromosomes which are able to compensate for each other are genetically related and are termed as homoeologous chromosomes.