What is Crossing over?
[[Image:Crossover 2.jpg|thumb|right|300px|Crossing over takes place between the [[chromatid]]s of two [[homologous chromosomes]]. Maternal chromatids are red; paternal chromatids are blue. Lines point to the chiasmata (cross overs)]] '''Crossing over''' is a basic concept of [[genetics]] and [[cell biology]], often called '''recombination'''. It occurs during [[meiosis]]. Crossing over is the exchange of [[chromosome]] segments between non-sister [[chromatid]]s during the production of [[gamete]]s. The effect is to assort (shuffle) the [[allele]]s on parental chromosomes, so that the gametes carry ''combinations'' of [[gene]]s different from either parent. This has the overall effect of increasing the variety of [[phenotype]]s present in a population. The process can be seen directly in stained [[cell]]s, and indirectly if there are genetic markers or not on the chromosomes. The visible '''crossovers''' are called '''chiasma''' (plural: '''chiasmata'''), which is [[Ancient Greek|Greek]] for a cross. F. A. Janssens was the first to suggest what chiasmata meant.<ref>Janssens F. A., 1909, La théorie de la chiasmatypie. ''La Cellule'' '''25''', 389–411.</ref><ref>Sturtevant A. H., ''A history of genetics''. Harper & Row, New York, pps. 77–79</ref> The large-scale effect of crossing over is to spread variation through a population. This is the main result of sexual reproduction compared to non-sexual modes of reproduction. The main advantage to the parents is the greater variety in their offspring. This, it is thought, increases the chance that some of the offspring will survive and reproduce. This has been the subject of much investigation and discussion.<ref>Thompdon J. N., 2013. Recombination and reproduction in ''Relentless evolution'', pgs. 144{{ndash}}164. University of Chicago Press. {{ISBN|978-0-226-01875-1}}</ref> == References == {{Reflist}} [[Category:Chromosomes]]