Trisomy

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 twohomologues, 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 (Fig. 20.2). 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 is 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 (Fig. 20.3).

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 causes 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 (for details, see Human Genetics). 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 (Fig. 20.4) 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 Iriploids as females with diploids as male (3x x 2x). In either case trisomics are obtained in large number and can be identified through phenotypic effects of individual chromosomes.

Cytology of 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 20.5. In a tertiary trisomic a characteristic pentavalent is observed.


Trisomic analysis.
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. The expected ratios in trisomics can be worked out, and are given in Table 20.1. In Table 20.1, ratios based on chromosome segregation as well as those based on chromatid segregation are given. 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.