Each sperm or egg is produced independently of the others. Each sperm or egg cell produced by an individual will share a significant amount of DNA with all the others produced by that person but none will be exactly the same.

It's all very simple. . . Except there is a one little complication. . .

Between Prophase 1 and Metaphase 1 of meiosis, two copies (chromatids) of the maternal chromosome and two copies of the paternal chromosome team up and entwine to form a tetrad (a farewell embrace). In the process, they exchange bits and pieces of themselves. "Take this bit to remember me by, my dear." Cross-over can occur between all four members of the tetrad.

This results in chromosomes that are a patchwork of DNA received from our father’s parents and our mother’s parents. It is called genetic recombination or more commonly crossover. Crossover can occur at any location on a chromosome. In fact, it can occur at several locations at the same time. It is estimated that during each meiosis in humans, there is an average of two to three crossovers for each pair of homologous chromosomes.

Crossover is mandatory! In order for sperm or egg formation to occur, there must be at least one cross over event for each homologous chromosome pair during meiosis.

There are two possible outcomes of crossover (recombination).

The crossover may occur between the identical maternal chromatids or the two identical paternal chromatids. These crossover events are invisible to sequencing because the reconfigured chromosomes will be still be complete copies of the maternal or paternal chromosomes. Their occurrence, however, does satisfy the requirement for at least one crossover event for each homologous chromosome pair. This type of crossover results in a grandchild receiving one fully identical chromosome from a one grandparent.

For example, click on the above tab for the Mapping Grandchildren’s Chromosome Project an find Cathryn’s chromosomes #6 (all red so completely from her maternal grandmother). In contrast, Cathryn’s paternal chromosome #6 is a blend of three segments where two crossover events made that chromosome mostly from her paternal grandfather (light blue) with two smaller segments from her paternal grandmother (dark blue). Similarly, Cathryn’s sister Isabelle has received an intact chromosome #1 from her paternal grandmother (dark blue).

The second outcome, of crossover between a maternal and a paternal chromatid, is of more interest to the genetic genealogist. This is the process that results in the patchwork of segments from both the maternal and paternal lines. As time goes on, these exchanges at each generation result in smaller and smaller segments from more distant ancestors. Because the same events are occurring in other lines from common ancestors, this frequently lead to shared chromosome segments between distantly related cousins.

Curiously, crossover rates are 1.7 times higher in female meiosis than in male meiosis. Crossover rates in males are 5 times lower near the centromeres but 10 times higher near the telomeres compared with those in females. (Buard J, de Massy B: Playing hide and seek with mammalian meiotic crossover hotspots. Trends Genet. 2007, 23: 301-309. 10.1016/j.tig.2007.03.014.)

The centiMorgan (cM) is a unit of chromosome "length" used in genetic genealogy to describe the amount of DNA shared between two individuals. It is a measure of genetic recombination (crossover) frequency. One cM is equal to the length of a chromosome over which crossover occurs with 1% frequency in a single generation. In humans, 1cM is equivalent, on average, to 1 million base pairs. (http://www.medicinenet.com/script/main/art.asp?articlekey=2665)

When the chromosomes arrive in the new organism, they are a mosaic of DNA from all four grandparents (click the Grandchild Chromosome Mapping Project tab above). And since the grandparents received their DNA in the same way, the chromosomes are actually a mix of DNA from many ancestors who lived far back in time. Of course, as the DNA is passed through the generations, the contribution from more distant ancestors becomes increasingly fragmented and smaller. In many instances, we share no DNA from some of our more distant ancestors. On average, studies are showing that as many as 10% of third cousins and 50% or fourth cousins share now identical DNA segments.