Chromosomal inversion

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

An inversion is a chromosome rearrangement in which a segment of a chromosome is reversed end-to-end. An inversion occurs when a single chromosome undergoes breakage and rearrangement within itself. Inversions are of two types: paracentric and pericentric.

Paracentric inversions do not include the centromere, and both breaks occur in one arm of the chromosome. Pericentric inversions include the centromere, and there is a break point in each arm.

Cytogenetic techniques may be able to detect inversions, or inversions may be inferred from genetic analysis. Nevertheless, in most species, small inversions go undetected. In insects with polytene chromosomes (for example, Drosophila), preparations of larval salivary gland chromosomes allow inversions to be seen when they are heterozygous. This useful characteristic of polytene chromosomes was first noted by Theophilus Shickel Painter in 1933.

Inversions usually do not cause any abnormalities in carriers, as long as the rearrangement is balanced, with no extra or missing DNA. However, in individuals which are heterozygous for an inversion, there is an increased production of abnormal chromatids (this occurs when crossing-over occurs within the span of the inversion). This leads to lowered fertility, due to production of unbalanced gametes. An inversion does not involve a loss of genetic information, but simply rearranges the linear gene sequence.

Human families that may be carriers of inversions may be offered genetic counseling and genetic testing.

In the species Drosophila subobscura, researchers have been able to track global climate change by measuring the magnitude and directional shift in chromosome inversion frequencies, relative to temperatures at specific global sites.

An example of chromosomal Inversion in organisms is demonstrated in the insect, Coelopa frigida. This particular species of Coelopa have a variation of chromosomal inversions that allow the species to create a series of physical differences. Individual C. frigida that are larger do not undergo a chromosomal inversion, whereas individuals that are smaller do undergo a chromosomal inversion.

Nomenclature

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).

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.

The International System for Human Cytogenomic Nomenclature (ISCN) is an international standard for human chromosome nomenclature, which includes band names, symbols and abbreviated terms used in the description of human chromosome and chromosome abnormalities. Abbreviations include inv for inversions.

Notable cases

• Brenden Adams: former holder of the Guinness World Record for tallest teenager. His height is caused by an inversion of chromosome 12.

Further reading

• Lehtonen S, Myllys L, Huttunen S (2009). "Phylogenetic analysis of non-coding plastid DNAthtjtdjj in the presence of short inversions" (PDF-preview). Phytotaxa. 1: 3–20. doi:10.11646/phytotaxa.1.1.2.