Inversion Chromosomal Aberrations

Inversion chromosomal aberrations refer to the rearrangement of the chromosome fragment due to two break points on the same chromosome, followed by inversion and reinsertion of the fragment. Inversion is a kind of intra-chromosomal aberration where there will be no change in the quality or quantity of the genes, but there will be an alteration in their sequence.

To bring about an inversion, there will be two breaks with a segment getting separated from the chromosome. The broken segment, after rotating an angle of 180 degrees, joins back to the chromosome in reverse sequence. For example, in a chromosome with a gene sequence of ABCDEFG, the segment BCDE is inverted and rejoins the chromosome to form the altered chromosome with a sequence of AEDCBFG.

Just like deletions and duplications, inversions have also been reported in a wide variety of plants and animals. Inversions have a wider distribution than the other types of aberrations in the natural population. Among plants, inversions have been reported in Tradescantia maize, etc, and in animals, it is seen in Drosophila, which exhibits quite many types of inversions. Anopheles mosquitoes, amphibians are many species of grasshoppers are where inversions have been reported.

Types of Inversion Chromosomal Aberrations

Inversion chromosomal aberrations are classified into two basic categories, ie, paracentric and pericentric. When the centromere is not involved in the inversion, it is called a paracentric inversion, and when the centromere is involved in the inversion, it is called a pericentric inversion.

Paracentric Inversions

Here, the homologous sequences pair up during prophase I.
The crossover at the inverted region causes the formation of an unusual structure.
The outer chromatids carry the original non-recombinant genes as they did not take part in the crossing over.
The inner chromatids that underwent crossing over show an abnormality.
Each chromatid has copies of some genes, while other genes do not have copies.
Moreover, one of the four chromatids has two centromeres, forming a dicentric chromatid, and the other one lacks a centromere, forming an acentric chromatid.
The pulling towards the poles during anaphase stretches the dicentric chromatid across the centre to form a dicentric bridge, which will eventually break when the centromeres are pulled apart.
The acentric fragment does not get segregated to one nucleus during meiosis and is lost.
In meiosis II, when the sister chromatids separate and gametes are formed, two of them carry original non-recombinant chromosomes, and the other two show abnormalities.
One of them will have missing genes and become non-viable.

Such inversion heterozygotes were studied in detail in many plants by Stebbins and Darlington.

Pic Credit: Pla-Victori, J.

Pericentric Inversions

Pericentric inversions may change the morphology of inversions. Sometimes, a second change in the chromosome that already has one gives a compound inversion. Inversions may be of different types.

Adjacent Inversions: Here, the first inverted segment is separated from the second by a non-inverted region.
Overlapping Inversions: One breakage point of the second inversion is within the already inverted segment, and the other one is outside of it.
Included Inversions: Both the breakage points of the second inversion are within the already inverted segment.

Chromosomal inversions always involve two breaks, and it is always the internal segment that becomes inverted. Only the broken end will be sticky and can be attached together after breakage. Inversions may vary greatly in size. Some inversions involve just a small segment, while others have a larger segment.

There are homozygous and heterozygous inversions. In the homologous inversion, both chromosomes are of inverted type, so there will not be any difficulty while painting. In the heterozygous inversion, one chromosome will be inverted, so it is difficult to have a point-to-point pairing. The pairing of homozygous pair will be in a peculiar configuration.

This is referred to as an inversion heterozygote. During pairing, the normal chromosome bulges out into an open groove or an open loop to enable point-by-point pairing. Sometimes, the compound inversions may have compound loops at the time of pairing. The subsequent fate of crossing over chromatids will be different according to the position of the centromere (whether it is inside or outside the segment).

A double crossover within the limit of paracentric inversion does not form chromosome bridges as an inversion heterozygote having a single crossover. Here, one of the double chromatids is ABEDCF, and the other is ABCDEF. Both are monocentric in this case. All will be viable, whereas in the previous case of the four gametes, two will be non-viable. There is partial sterility in inversion heterozygotes with a single crossover.

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://bnmv.ac.in/images/uploads/Unit-4%20Chromosomal%20Aberations.pdf
Pla-Victori, J. (2019). The role of genetic counseling in the infertile patient. Human Reproductive Genetics, 295-316. https://doi.org/10.1016/B978-0-12-816561-4.00019-3