Importance of Crossing Over

Crossing over is not a minor genetic curiosity; it has real practical and evolutionary weight precisely because it occurs, in some form, in essentially every group of organisms studied, from bacteria and yeast through fungi to higher plants and animals. It generates entirely new combinations of alleles that neither parent possessed, which is an important raw material that natural selection can act on during evolution. The fact that recombination frequency between two genes scales predictably with the number of crossovers occurring between them was also the key clue that let geneticists infer that genes are arranged in a strict linear order along a chromosome, rather than in some more complicated three-dimensional cluster. That same relationship is what makes it possible to build genetic maps in the first place, since the observed frequency of crossing over between two loci is used directly to estimate the physical distance separating them. Understanding crossing over also sheds light on the deeper molecular mechanism of how genes are expressed and regulated, because the DNA-breakage-and-rejoining machinery it depends on is shared with other cellular processes. Finally, whenever crossing over happens to gen …