Chromosomal aberrations

In an organism, any visible abnormality in chromosome number or structure from the diploid set is known as chromosomal aberration. The chromosomal aberrations based on the structure of the chromosome are of four types – deletion, duplication, inversion and transversion.

Structural chromosomal aberrations

Deletion

The loss of a segment of the genetic material in a chromosome is called deletion. It may be terminal or intercalary. When the deletion occurs near the end of the chromosome, then it is called terminal deletion. Eg. Drosophila and Maize. When the deletion occurs in the middle of the chromosome then, it is called intercalary deletion. Most of the deletions lead to death of an organism.

Duplication

When a segment of a chromosome is present more than once in a chromosome then, it is called duplication. For example, the order of genes in a chromosome is a, b, c, d, e, f, g and h. Due to aberration, the genes ‘g’ and ‘h’ are duplicated and the sequence of genes becomes a, b, c, d, e, f, g, h, g and h. In Drosophila, corn and peas a number of duplications are reported. Some duplications are useful in the evolution of the organism.

Inversion

It is another chromosomal abnormality in which, the order of genes in a chromosomal segment is reversed by an angle of 180°. For example, the order of genes in a chromosome is a, b, c, d, e, f, g and h. Due to aberration, the sequence of genes becomes, a, b, c, d, g, f, e and h. There are two types of inversion – pericentric and paracentric inversion.
In pericentric inversion, the inverted segment of the chromosome contains centromere. Sometimes, it is responsible for evolution of the organism. For example the 17t1, human chromosome is acrocentric, while in Chimpanzee the corresponding chromosome is metacentric. In paracentric inversion, the inverted segment of the chromosome has no centromere.

Translocation

It is a kind of a chromosomal abnormality in which the interchange of the chromosomal segments occurs. When translocation occurs between two non-homologous chromosomes, then it is called reciprocal translocation or illegitimate crossingover. It is of two kinds ¬ heterozygous translocation and homozygous translocation.
In heterozygous translocation, one member of each pair of chromosomes is normal and the other member is with interchanged segment. But in homozygous translocation, both the members of paired chromosomes have translocated segments.
They play an important role in species differentiation. Translocations causes hereditary disorders.