(chromosome inversion) A structural change in a chromosome in which a length of chromosome has detached and then rejoined the opposite way round. If the centromere is included in the inverted segment the inversion is termed pericentric. If the centromere is not in the centre of the inverted segment then the relative lengths of the chromatid arms either side of the centromere will be changed, altering the karyotype. Paracentric inversions, in which the centromere is not included, are the more common. An inversion is homozygous if the same segment is inverted in both homologous chromosomes. Apart from the changed linkage arrangements of the genes and any *position effects
, homozygous inversions behave much as normal homologous chromosomes. Often though the inversion only occurs in one chromosome giving an inversion heterozygote. These can be detected by the characteristic looped appearance during meiosis. Inversion heterozygotes often show reduced fertility because if a crossover occurs within the loop then a dicentric chromosome (a chromosome with two centromeres) and an acentric fragment will result. Such pieces cannot move normally on the spindle and thus only half the gametes (those containing either of the two chromatids not involved in the crossover) will be viable. As the viable gametes will have the same linear sequence of genes as the parent, inversion heterozygotes have been termed crossover suppressors. However they do not suppress crossing over but the resultant recombinants, being inviable, are not apparent. Viable recombinants may be obtained if two crossovers occur in the inversion loop. Inversion heterozygotes may have an adaptive advantage in that any useful combination of alleles in the inverted segment are likely to be held together.