CHROMOSOMAL ABBERATIONS

CHROMOSOMAL ABBERATIONS

19.  CHROMOSOMAL ABBERATIONS

The structural changes or variations in the chromosome is known as chromosomal abberation. Structural changes in chromosome may be loss, gain translocation or inversion. Chromosomal abberations are of the following types.
(i)    Deletion    (ii)    Duplication    (iii)    Inversion    (iv)    Translocation
19.1.    Deletion : Deletion is loss of a part of chromosome. In deletion, there may be loss of some part of chromosome or gene during replication, by base flip page or by template DNA strand flip page and by error in crossing over. Deletion may cause severe genetic diseases. There may be the following type of deletion.


(a)    Terminal deletion: Deletion that occurs at the terminal of the chromosome. 
(b)    Interstitial deletion: Deletion that occurs at the inner side of the chromosome. Deletion may cause severe genetic disease such as Willian's syndrome, Duchenne muscular dystrophy, cri-du-chat etc.
19.2.    Duplication: Duplication is the addition of the same segment of a chromosome. The addition may be in the adjacent region or on the different arm of the same chromosome or on a different chromosome. Duplication creates genetic redundancy. Duplication may be due to unequal cross over, retrotransposition, aneuploidy, polypody and replication flip page. e.g.: Bar eye in Drosophila:- Bar eye is the character, where eyes are narrower as compared to normal eye shape. This phenotypic character is due to duplication in the region of 16A of X-chromosome.
When the duplicated segment is present in the same arm and next to original segments too. Then it is known as tandem duplication. Tandem duplication may be of two types.
(a)    Direct tandem: When the gene order of the duplicated segment is the same as that of the original.
(b)    Reverse tandem: When the gene order of the duplicated segment is inverse to the original segment.

19.3.    Inversion: When a segment of the chromosome is rearranged, rotating 180° or reverse end to end. Inversion is of two types.
    (a)    Paracentric    (b)    Pericentric
(a)    Paracentric Inversion:- These type of inversions do not include the centromere.
(b)    Pericentric Inversion:- These inversions include centromere and have breakage in more than one arm. 
Inversions do not cause any abnormalities if the rearrangement is balanced with no extra or missing DNA.

19.4.    Translocation:- Transfer or rearrangement of chromosomal parts between non-homologous chromosomes. Translocation can be reciprocal or non-reciprocal.
(a)    Reciprocal translocation:- When chromosomal segments are exchanged between two non-homologous chromosomes.

(b)    Non-reciprocal translocation:- When the transfer of chromosomal part occurs between non-homologous chromosomes. Translocation may cause severe membrane disease, like leukaemia, Ewing's sarcoma down syndrome, infertility.

Robertsonian translocation: Is also called whole arm translocations or centric-fusion translocations. It is a type of translocation involving two homologous or non-homologous chromosome. Acrocentric chromosome break at their centromeres and the long arms of both the chromosome fuse to form a single large metacentric chromosome.


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