Inversion

The first evidence of chromosomal inversion was provided by Alfred Sturtevant in Drosophila melanogaster in 1921. Since then, inversions have been found in all eukaryotes.

Inversion is a genetic term used to describe a chromosome rearrangement. If two breaks occur in a chromosome, a segment of the chromosome can be inverted in its original position and reinserted at this position. The break sites of inversions are often located in regions with repeating nucleotides, and these regions can be reused in other inversions. Chromosome segments in inversions can be very small, e.g. one kilobase, or very large, e.g. 100 megabases. The number of genes affected by an inversion can range from a few to hundreds of genes.

Basically, two types of inversions can be distinguished:

• paracentric inversions
• and
• pericentric inversions.

Paracentric inv ersions do not include the centromere, and both breakpoints occur in one arm of the chromosome.

Pericentric inv ersions include the centromere; there is a breakpoint in each arm of the chromosome.

Inversions do not usually cause abnormalities in carriers as long as the rearrangement is balanced. However, in individuals who are heterozygous for an inversion, there is an increased production of abnormal chromatids (this occurs through crossing-over within the inversion span). This leads to reduced fertility, as unbalanced germ cells are produced.

In inversions, there is neither a loss nor a gain of genetic information; only the linear DNA sequence is rearranged.

Inversions were of crucial importance for the evolution of sex chromosomes. In mammals, the Y chromosome is unable to recombine with the X chromosome along almost its entire length. This non-recombining part results from a series of inversions that overlap.

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4. Villoutreix R et al. (2021) Inversion breakpoints and the evolution of supergenes. Mol Ecol 30:2738-2755.