Recombination or cross-overs is what occurs during meiosis when creating the reproductive cells. Instead of creating identical copies of each chromosome strand (or non-sister chromatid), a mixed or merged copy is created.

We each have a homologous pair of autosomechromosomes. That is, one copy or strand from our father and one from our mother. If recombining did not occur, that copy would be identical to the one they got from one of their parents. But this would lead to less genetic diversity. So during meiosis, when creating the copy of an autosome, a cut occurs where all of a sudden, instead of copying the paternal chromosone it starts copying the maternal one.

This is a key feature that greatly impacts the properties studied in genetic genealogy and is only more recently taught in schools. (That is, older adults may have missed this learning from their high school biology class as it was not generally known yet.)

Recombining causes a few key features:
  1. Chromosomes are broken up when the reproductive cells are created by each parent.

Studies have shown that only 30 to 50 cross-overs occur across all the autosomes in each reproductive cell. Therefore maybe 2 to 3 cuts in chromosome 1 and maybe none in chromosome 22, on average. Or a cross-over every 100cM.

Due to the statistical variance of these cuts, segments of DNA shorter than about 20cM have a small chance of being broken up by recombination. Hence, once shared segment lengths reach this size, they tend to stick around intact for a few generations and then simply disappear completely due to the chance of simply not being in the portion of DNA passed down. This it is not uncommon to see 3 tested generations to have a single, same length segment in common with a distant cousin. Realize it is more common to not share any segments at all with the more distant cousin.

As an example, consider your mother’s Chromosome 1. If she only passed one or the other copy she had, in its entirety, then the grandchild would only have all of the maternal grandmothers chromosome 1 or all of the maternal grandfathers chromosome 1. So if recombining did not occur, as the above implies, the “discreteness” in passing down the full chromosome or not would lead to a greater variation in DNA sharing between siblings and subsequent generations; thus a greater variance in features and such between them as well. But instead, because of recombination the mother creates a unique chromosome 1 mixture of her two strands of chromosome 1 in each reproductive cell (i.e. egg). This leads to a finer grain. more equitable mixing of the DNA passed down to a child from the grandparents.

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