BOTANY Higher Secondary Second Year - Textbooks Online

3.6. 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 17
126
th human chromosome is acrocentric, while
in Chimpanzee the corresponding chromosome is metacentric. In
paracentric inversion, the inverted segment of the chromosome has no
centromere.