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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. Special cytogenetics techniques are often needed to detect inversions. In the "giant" polytene chromosomes found in some organisms, inversions can be easily visualized when they are heterozygous. This is a very useful characteristic of polytene chromosomes that 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 genetic information. However, there is an increased chance for the production of unbalanced chromosome rearrangements in the offspring of carriers. Families that may be carriers of inversions may be offered genetic counseling and genetic testing.

The most common inversion seen in humans is on chromosome 9, at inv(9)(p11q12). This inversion is generally considered to have no deleterious or harmful effects, but there is some evidence it leads to an increased risk for miscarriage for about 30% of affected couples.


  • Painter, TS (1933). A new method for the study of chromosome rearrangements and the plotting of chromosome maps.. Science 78: 585-586.

See also

External links

de:Inversion (Genetik)
it:Inversione (cromosoma)
nl:Inversie (chromosomen)
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