Chromosomes are the vehicle of hereditary material or genes. Any alteration, addition or deletion of chromosomal part leads to alteration of number, position or sequence of genes in the chromosome.
Such change of structure is referred to as chromosomal aberrations or chromosomal mutations.
Chromosomal aberrations involve two types of changes:
(i) Changes in number of genes in a chromosome
(ii) Changes involving arrangement of genes.
(a) Changes in the number of genes in a chromosome:
(i) Deletion or Deficiency:
It is due to loss of a part of a chromosome. The chromosome becomes shorter due to loss of one or more genes (Fig. 5.21).
(ii) Duplication:
Duplication of chromosome may take place due to attachment of some deleted part of another chromosome with it. This brings addition of some new genes not belonging to it.
(b) Changes in the arrangement of genes in a chromosome:
(i) Inversion:
An inversion is produced when there are two breaks in a chromosome and the intercalary segment reunites in reverse order i.e., the segment rotate by 180°. For example, if the gene sequence in the original chromosome is ABCDEFGH, it may change to ADCBEFGH (Fig. 5.21). If the inverted segment includes the centromere, the inversion is called pericentric inversion; if it does not include centromere the inversion is called as paracentric inversion.
(ii) Translocation:
Translocation involves transfer of a segment of a chromosome to a different part of the same chromosome or to a different chromosome. In the later case the transfer may take place between non-homologous chromosomes (Fig. 5.21). The chromosomal aberrations described above are the outcome of defective meiotic division and result in changed sequence of genes. The genes in new or changed location may alter the phenotypic expression or may even cause death of the individual.
Variations in Chromosome Number (Numerical Changes):
The organisms are usually diploid (2n), i.e., they possess two sets of chromosomes. Variation in the normal diploid chromosome number is termed ploidy. Numerical change in chromosome or variations in chromosome number (heteroploidy), can be mainly of two types, namely (i) aneuploidy and (ii) euploidy.
(a) Aneuploidy:
It involves addition or deletion of one or few chromosomes to the usual diploid set of chromosomes. The aneuploids arise due to failure of the separation of homologous chromosomes of particular pair during meiosis. It is known as non-disjunction. As a result two types of gametes are produced; one type contain more chromosomes than the normal number and the other type of gamete contain less chromosomes.
Aneuploids are of following types:
(i) Monosomies:
They arise by the loss of one chromosome from the diploid set i.e., 2n-l. They can form two types of gametes, (n) and (n-1).
(ii) Nullisomics:
These arise by the loss of a particular pair of chromosomes i.e., 2n-2. They arise by the fusion of two (n-1) type of gametes.
(iii) Trisomies:
These arise by addition of an extra chromosome to the normal diploid set with the genetic formula, 2n + 1. Such individuals are formed by the union of a (n + 1) gamete with a normal gamete (n).
(iv) Tetrasomics:
These arise by the addition of an extra pair of chromosome to the diploid set with a chromosomal formula 2n + 2. By this a particular chromosome is represented in four doses instead of normal two.
(b) Euploidy:
Normally organism possesses two sets of chromosomes i.e., they are diploid (2n). At times there is addition or loss of complete one set (n) or more than one set of chromosomes is observed. It is called as euploidy.
Euploidy is of following types:
(i) Hapioidy or Monoploidy:
Out of two sets of chromosomes of a normal organism when one set is lost, the resulting offspring’s have just one set of chromosomes (n).
(ii) Polyploidy:
Organisms having more than two normal sets of chromosomes (2n) are called polyploids. Organisms with three sets of chromosomes (2n + n) = 3n are iriploids ; those with four sets of chromosomes (2n + 2n) = 4n are tetraploids and those with five sets (2n + 3n) = 5n and six sets (2n + 4n) = 6n are known as pentaploids and hexapioids respectively.
Polyploidy is generally found among the plants but rarely found among animals. About one third of all the grasses are polyploids, common breed wheat is hexaploid (6n), some strawberries arc octaploid (8n). Vlany commercial fruits and ornamental plants are polyploids. The primary cause lor relatively low frequency of polyploids among animals is attributed to their sex balance, which is more delicate than that in plants.
However, few animals such as the brine shrimp, some annelids, the axolotl larva and the golden hamster show evidence of polyploidy. Some times certain specialized tissues (e.g., liver) within a diploid organism may be polyploid. Polyploidy can be induced by treating the living tissues with a chemical (an alkaloid) called colchicine. Temperature treatment in maize and decapitation in tomato induced polyploidy.
Polyploidy originates in several ways. It may arise due to abnormal mitosis. Sometimes diploid spores and gametes are produced due to defective rneiotic division in which there is no reduction of chromosome number. Such diploid gametes fuse to form tetraploid individuals. Participation of more than two nuclei in fertilization in endosperm tissue of seed plants results in polyploidy.
There are two major kinds of polyploids according to the origin of chromosomes autopolyploid and allopolyploid. Autopolyploids are those polyploids, which have same basic set of chromosomes multiplied. For instance, if a diploid species has two similar sets of chromosomes (AA), an autotriploid will have three similar sets (AAA) and an autotetraploid will have four such sets (AAAA).
Polyploidy may also result from doubling of chromosome number in a F1 hybrid which is derived from two distinctly different species (allopolyploidy). The common examples are autopolyploidy is the common ‘doob’ grass (Cynodon dactylon) and those of allopolyploidy is Raphanobrassica and common wheat.
Significance of polyploidy:
1. Polyploid plants generally have large flowers, seeds and fruits.
2. Polyploidy acts as a conservative process and stabilizes interspecific hybrids.
3. Polyploidy facilitates gene exchange between distantly related species.