An inversion is a chromosome rearrangement in which a segment of a chromosome becomes inverted within its original position. An inversion occurs when a chromosome undergoes a two breaks within the chromosomal arm, and the segment between the two breaks inserts itself in the opposite direction in the same chromosome arm. The breakpoints of inversions often happen in regions of repetitive nucleotides, and the regions may be reused in other inversions. Chromosomal segments in inversions can be as small as 100 kilobases or as large as 100 megabases. The number of genes captured by an inversion can range from a handful of genes to hundreds of genes. Inversions can happen either through ectopic recombination, chromosomal breakage and repair, or non-homologous end joining. Inversions are of two types: paracentric and pericentric. Paracentric inversions do not include the centromere, and both breakpoints occur in one arm of the chromosome. Pericentric inversions span the centromere, and there is a breakpoint in each arm. 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. Inversions do not involve either loss or gain of genetic information; they simply rearrange the linear DNA sequence. Detection Cytogenetic techniques may be able to detect inversions, or inversions may be inferred from genetic analysis. Nevertheless, in most species, small inversions go undetected. More recently, comparative genomics has been used to detect chromosomal inversions, by mapping the genome. Population genomics may also be used to detect inversions, using areas of high linkage disequilibrium as indicators for possible inversion sites. Human families that may be carriers of inversions may be offered genetic counseling and genetic testing. History The first evidence of a chromosomal inversion was found in 1921 by Alfred Sturtevant in Drosophila melanogaster. Since then, inversions have been found in a all eukaryotes. When discovered by Sturtevant, inversions were regarded as areas of recombination suppression. Originally, these inversions were noted in polytene chromosomes within the salivary glands of heterozygous Drosophila melanogaster larvae. In 1970, Theodosius Dobzhansky noted that genes within an inversion had higher fitness than those that are found outside of the inversions, although this is an area