is unable to recombine with the X chromosome, almost along its entire length. This non-recombining portion results from a series of inversions that overlap. Decreased recombination rate between sex determining loci and sex-anatagonistic genes is favored by selection. This causes linkage disequilibrium between the male determining locus and an allele at another locus that is beneficial to males. This can happen through inversions resulting in a non-recombining block including both loci, as is the case in the mammalian Y chromosome. Inversions can also be essential in the origination of new sex chromosomes. They can cause linkage disequilibrium between a sex-determining mutation and sex-antagonistic loci and create a new sex chromosome from an autosome. Inversions can be involved in speciation in multiple ways. Since heterozygote inversions can be underdominant, they can cause hybrid fitness loss resulting in post-zygotic isolation. They can also accumulate selected differences between species, causing both pre- and post-zygotic isolation. Inversions often form geographical clines in frequency which can hint to their role in local adaptation. A prominent instance of such a cline is inversion 3RP in Drosophila melanogaster that can be observed in three different continents. When an inversion contains two or more locally adaptive alleles, it can be selected and spread. For example; in the butterfly Heliconius numata, 18 genes controlling colors are linked together by inversions as together they confer higher fitness. Nomenclature 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. 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. References Further reading Chromosomes Chromosomal abnormalities Modification of genetic information