Crossing Over

A chiasma occurs at least once per chromosome pair. Thus, following crossing over, at least two of the four chromatids become unique, unlike those of the parent. (Crossing over can also occur between sister chromatids; however, such events do not lead to genetic variation because the DNA sequences are identical between the chromatids.) Crossing over helps The exact mechanism for crossing over is unknown. It is believed to involve cutting of one chromatid, removal of nucleotides from both strands, insertion of one strand into the matching region of the nonsister chromatid, and further cutting, swapping of segments, and rejoining of the strands. Adapted from <http://www.ultranet.com/~jkimball/BiologyPages/c/crossingover.html>. to preserve genetic variability within a species by allowing for virtually limitless combinations of genes in the transmission from parent to off-spring.

The frequency of recombination is not uniform throughout the genome. Some areas of some chromosomes have increased rates of recombination (hot spots), while others have reduced rates of recombination (cold spots). The frequency of recombination in humans is generally decreased near the centromeric region of chromosomes, and tends to be greater near the telomeric regions. Recombination frequencies may vary between sexes. Crossing over is estimated to occur approximately fifty-five times in meiosis in males, and about seventy-five times in meiosis in females.