Deletion Chromosomal Aberrations

Deficiency or deletion chromosomal aberrations are chromosomal abnormalities in which either a terminal or interstitial segment of a chromosome is missing. It is the loss of chromosomal material. It was discovered by Bridges (1971) in Drosophila melanogaster on the X-chromosome in the Bar locus.

Types of Deletion Chromosomal Aberrations

The chromosome may break at any two points, releasing intercalary segments, or may break at one point, releasing a terminal segment. This deleted segment does not survive if it lacks a centromere. It may also affect the ends of chromosomes or the bodies of chromosomes.

Deletions are of two types, terminal deletion and interstitial deletion.

Terminal Deletion

Terminal deletion is represented by a single break near the end of the chromosome, giving rise to a small fragment.
This is the most likely type of deletion and the least complicated one as well.
Here, the lost segment includes the telomere.

Interstitial Deletion

Interstitial deletion is produced by two breaks, giving rise to an acentric fragment.
The lost segment here lies between the telomere and centromere.
As the detached segment is released and lost, the remaining ends of the chromosome fuses to form a single genome.
Interstitial deletions can cause birth defects, developmental issues, and intellectual disabilities.

The deletion may be either homozygous or heterozygous. In the former, both chromosomes are deleted type, while in the latter, it is normal type and the other is deleted type. When both chromosomes are deleted, it is known as a homozygous deletion. If only one is deleted and the other remains normal, it is a heterozygous deletion.

Identification of Deletion Chromosomal Aberrations

A homozygous deletion (if the individual survives) is difficult to identify morphologically at the chromosomal level. However, if the normal chromosome has been studied previously, the deleted chromosome can be pointed out because of its reduced length and a change in shape. Since a terminal deletion in a metacentric chromosome would reduce the length of the arm and produce a submetacentric chromosome.

Heterozygous deletion, on the other hand, is easy to identify cytologically during meiosis. When they pair, they produce a specific configuration. In a deletion heterozygous chromosome pairing is normal, but at the region of deletion, the normal chromosome is pushed out in the form of a loop, where the genes are lost in the other homologous chromosome (in interstitial deficiency). In the case of terminal deficiency, the deleted chromosome will be shorter in length, hence, there is no pairing in that portion.

Pic Credit: Dr. Arindam Mandal

Impact of Deletion on Individuals

Deletions have been reported in a wide variety of organisms, both in plants and animals—maize, Drosophila, grasshopper, man, etc., where several phenotypic traits have been associated with deletions.

Terminal deletions have been reported in maize but are rare in Drosophila. A deficiency can vary in size from a minute segment to a major portion of a chromosome. The larger the loss or deficiency, the greater the genetic effect. If a very large segment is lost, the effect can be lethal to the cell, if not to the organism.

If the deficiency or deletion is sub-lethal, it may cause some functional or morphological change in the organism. Organisms with homozygous deficiency usually do not survive to adulthood because a complete set of genes will be lacking. This suggests that most of the genes are indispensable for the development of viable organisms.

Deletion in Drosophila

A variety of Drosophila called notch produces a notched margin of wings. It is inherited as a sex-linked dominant character in females. But it is lethal in males because the male carrying the notched wings dies.

Mutants are perpetuated by breeding females with notched wings to normal males. When normal red-eyed females with notches were crossed with white-eyed males with normal wings, the F1 female with notched wings had all white eyes as though the genes for white eyes were dominant in the presence of a notch.

Thus, the notch white-eyed progeny inherited the deficient chromosome. Here, the recessive allele ‘w’ is expressed because its dominant allele ‘W’ is lost along with the deleted segment. Progeny with deleted chromosomes die in the case of males.

Deletion in Humans

In humans, several chromosome deletions have been identified and associated with many congenital abnormalities in heterozygous conditions.

Eleugine et al discovered a chromosomal deletion in man, associated with Cri-du-chat syndrome or cat’s cry syndrome in babies. The affected babies cry like cats.
Other characteristics are microcephaly (small, broad head), broad face, saddle nose (concave), etc.
There is general mental retardation as well.
Generally, the affected children die early and have short arms due to a deletion in chromosome 5.
Individuals with heterozygous deficiency fail to survive because a complete set of genes is absent or a segment of the chromosome is lost.
Even if the individual survives, they may show unusual phenotypic effects due to the genes that are present on the non-deleted chromosomes and not due to the deleted chromosomes.
Thus, if a dominant gene is lost by deletion, the recessive one, which remains in the deficiency heterozygote, becomes expressed.
Deletion is significant if it removes some genes that suppress some other genes, which can then express themselves.
It may lead to new and desirable phenotypic variations being preferred by natural selection.

References

Agarwal, P. V. |. V. (2004). Cell biology, Genetics, Molecular Biology, Evolution, and Ecology: Evolution and Ecology. S. Chand Publishing.
https://ramsadaycollege.com/upload/eclassroom/Botany/GDM_Botany_Chromosomal%20Aberration_sem4_02.pdf
https://kvmwai.edu.in/upload/StudyMaterial/Chrmosomal_Abere.pdf
https://bnmv.ac.in/images/uploads/Unit-4%20Chromosomal%20Aberations.pdf