Crossing over involves the exchange of chromosome segments between non-sister chromatids during the production of gametes. The effect is to assort (shuffle) the alleles on parental chromosomes, so that the gametes carry combinations of genes different from either parent. This has the overall effect of increasing the variety of phenotypes present in a population.
By suitable means, the process can be observed directly in stained cells, and indirectly by the presence or absence of genetic markers on the chromosomes. The visible crossovers are called chiasma (plural: chiasmata), which is Greek for a cross. F.A. Janssens was the first to suggest what chiasmata meant.
The large-scale effect of crossing over is to spread variation through a population. This is thought to be the main advantage of sexual reproduction over non-sexual modes of reproduction.
- Janssens F.A. 1909 La théorie de la chiasmatypie. La Cellule 25, 389–411.
- Sturtevant A.H. A history of genetics. Harper & Row N.Y. p77–79