Structural and Numerical Chromosomal Variation and their implications

Chromosomal Variation

Hereditary material or genes are carried by chromosomes. Any change in the number, location, or sequence of genes in the chromosome is caused by the addition or deletion of chromosomal parts.

Chromosomal aberrations or chromosomal mutations are terms used to describe such structural changes.

There are two types of chromosomal aberrations:

(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 caused by the deletion of a chromosomal segment. Because one or more genes are lost, the chromosome gets shorter (Fig. 5.21).

(ii) Duplication:

Duplication of chromosomes can occur when certain deleted parts of another chromosome are attached to it. This results in the insertion of certain additional genes that do not belong to it.

(b) Changes in the arrangement of genes in a chromosome:

(i) Inversion:

When two breaks in a chromosome occur, the intercalary segment reassembles in reverse order, causing the section to spin by 180 degrees. For example, if the original chromosome’s gene sequence is ABCDEFGH, it may alter to ADCBEFGH (Fig. 5.21). Pericentric inversion occurs when the inverted segment includes the centromere; paracentric inversion occurs when the inverted section does not include the centromere.

Chromosomal Variation

(ii) Translocation:

A chromosomal fragment is translocated when it is moved to a different section of the same chromosome or to a different chromosome. In the latter situation, the transfer may occur across chromosomes that are not identical (Fig. 5.21). The chromosomal aberrations mentioned above are caused by a failure of meiotic division, which results in a change in gene sequence. The phenotypic expression of the genes in a new or modified place may be altered, and the individual may potentially die.

Variations in Chromosome Number (Numerical Changes):

Most organisms are diploid (2n), meaning they have two sets of chromosomes. Ploidy refers to the variation in the typical diploid chromosomal number. Heteroploidy is characterised by numerical changes in chromosomes or variations in chromosomal number. There are two forms of aneuploidy: (i) aneuploidy and (ii) euploidy.

(a) Aneuploidy:

It entails the insertion or deletion of one or more chromosomes from the diploid chromosomal group. During meiosis, the separation of homologous chromosomes of a specific pair fails, resulting in aneuploids. Non-disjunction is the term for it. As a result, two types of gametes are produced: one with more chromosomes than the normal amount, and the other with fewer chromosomes.

Aneuploids are of following types:

(i) Monosomies:

They are caused by the loss of one of the diploid set’s chromosomes, 2n-l. They may produce two kinds of gametes: (n) and (n-1).

(ii) Nullisomics:

These are caused by the loss of a specific pair of chromosomes, in this case 2n-2. They are the result of the fusion of two (n-1) different types of gametes.

(iii) Trisomies:

The genetic formula 2n + 1 results in the addition of one additional chromosome to the standard diploid set, resulting in these. The marriage of a (n + 1) gamete with a regular gamete produces such people (n).

(iv) Tetrasomics:

The addition of an additional pair of chromosomes to the diploid set with the chromosomal formula 2n + 2 results in these. This results in a chromosome being represented in four dosages rather than the usual two.

(b) Euploidy:

Normal organisms have two sets of chromosomes, making them diploid (2n). The addition or removal of a whole set (n) or more than one set of chromosomes has been recorded at times. Euploidy is the scientific term for this.

Euploidy is of following types:

(i) Hapioidy or Monoploidy:

When one of a typical organism’s two sets of chromosomes is gone, the progeny inherit only one set of chromosomes (n).

(ii) Polyploidy:

Polyploids are organisms with more than two typical sets of chromosomes (2n). Iriploids have three sets of chromosomes (2n + n) = 3n, tetraploids have four sets (2n + 2n) = 4n, while pentaploids and hexapioids have five sets (2n + 3n) = 5n and six sets (2n + 4n) = 6n, respectively.

Polyploidy is common among plants, while it is uncommon among mammals. Polyploid grasses account for around a third of all grasses; typical breed wheat is hexaploid (6n), while certain strawberries are octaploid (8n). Polyploids are seen in many commercial crops and decorative plants. Animals’ sex balance, which is more sensitive than that of plants, is thought to be the fundamental reason for their low frequency of polyploids.

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