Crossing Over

∞ generated and posted on 2016.08.27 ∞

Exchange of genetic material between homologous pairs of chromosomes, particularly as seen during meiosis I.

Crossing over gives rise to a kind of genetic recombination, particularly involved molecular recombination between otherwise separate double helices. Crossing over specifically occurs during prophase I of meiosis and also occurs within the context of the formation of what are known as tetrads. The actual crossing over events, as viewed using a microscope, involve what are known as chiasmata.

Crossing over is only the first of the genetic recombination steps that occur during meiosis, the second being independent assortment. The role that crossing over plays in this genetic recombination is to take chromosomes that are solely derived from each parent, that is, one from your father and one from your mother, and then to molecularly combine those chromosomes into individual, "recombinant" chromosomes. The results are chromosomes that genetically are a combination of the homologues that previously were purely the chromosomes received from one parent but not the other.

We can walk back these ideas by one generation. That is, the homologues found in the cells of your ody are not identical to the homologues found in your parents (half of which are from one parent and half from the other). Instead, these homologues are combinations of homologous pairs of chromosomes that were provided by your grandparents to your parents.

For each homologous pair in your body, one thus is a combination of the equivalent omologues derived from your paternal grandparent's gametes while the other is equivalent to homologues derived from the molecular recombination of chromosomes found in your maternal grandparent's gametes. Our individual chromosomes are combinations of our parent's individual chromosomes, that is, individual homologous pairs, which in turn are combinations of our grandparents' homologous pairs, and so on.