Chromosomal inversion

[3] Inversions can happen either through ectopic recombination between repetitive sequences, or through chromosomal breakage followed by non-homologous end joining.

Pericentric inversions span the centromere, and there is a breakpoint in each arm[5].Inversions usually do not cause any abnormalities in carriers, as long as the rearrangement is balanced, with no extra or missing DNA.

Inversions do not involve either loss or gain of genetic information; they simply rearrange the linear DNA sequence.

Originally, these inversions were noted in polytene chromosomes within the salivary glands of heterozygous Drosophila melanogaster larvae.

[12] The suppressed recombination between inversion heterozygotes provides an opportunity for the independent evolution of the ancestral and inverted arrangements.

[13] Chromosomal inversions have gained a lot of attention in evolutionary research due to their potential role in local adaptation and speciation.

Because non-recombining inversion haplotypes may harbor multiple co-adapted gene variants, inversions are thought to facilitate local adaptation to different environments because natural selection is more efficient in driving such linked adaptive variants to high frequency within a population.

[15] The importance of chromosomal inversions in adaptation to different environments therefore remains an open empirical problem in evolutionary genetics.

Decreased recombination rate between sex determining loci and sex-anatagonistic genes is favored by selection.

This can happen through inversions resulting in a non-recombining block including both loci, as is the case in the mammalian Y chromosome.

They can cause linkage disequilibrium between a sex-determining mutation and sex-antagonistic loci and create a new sex chromosome from an autosome.

Since heterozygote inversions can be underdominant, they can cause hybrid fitness loss, resulting in post-zygotic isolation.

For example; in the butterfly Heliconius numata, 18 genes controlling colors are linked together by inversions as together they confer higher fitness.

A clay model showing why heterozygous inversion loops are visible in polytene chromosome preparations
An inversion loop in the A arm of a chromosome from an Axarus species midge
Three chromosomal abnormalities with ISCN nomenclature, with increasing complexity: (A) A tumour karyotype in a male with loss of the Y chromosome, (B) Prader–Willi Syndrome i.e. deletion in the 15q11-q12 region and (C) an arbitrary karyotype that involves a variety of autosomal and allosomal abnormalities including inversions (abbreviated as inv ). [ 19 ]
Human karyotype with annotated bands and sub-bands as used for the nomenclature of chromosome abnormalities. It shows dark and white regions as seen on G banding . Each row is vertically aligned at centromere level. It shows 22 homologous autosomal chromosome pairs as well as both the female (XX) and male (XY) versions of the two sex chromosomes .
Further information: Karyotype