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 ungergoes 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.
Inversions ususally do not cause any abnormalities in carriers as long as the rearrangment is balanced with no genetic information extra or missing. However, there is a 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 has no deleterious or harmful effects and does not appear to lead to an increased risk for miscarriage or abnormalities in offspring.
Interstitial heterochromatin of chromosome 3 of Ammospermophilus insularis, stained by C-banding.
When the chromosomes later separate, it is important that each chromosome be complete and be free to move away from its partner, and that each chromosome move to the proper location in one of the daughter cells.
The effects of inversions on fertility in heterozygotes depend on the relationship of the positions of the inversion, the centromeres, and the parts of the chromosomes which are exchanged.
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