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Genetic Recombination

Also Known as Cross-over

The animation in the header shows the process of genetic recombination in only two chromosome pairs (tetrads). In reality everything that is happening to these tetrads is happening simultaneously to the other 21 tetrads (or 20 in the case of males). The result is always four cells, each having a single #1 chromosome, and one #2 and one #3 and so on up to one each of 23 chromosomes. Thus each cell has one complete set of chromosomes and is ready to become either a sperm or egg cell.

Each person has 23 pairs of chromosomes in each cell. One member of each pair comes from our mother and one from our father. As part of egg and sperm cell production, the chromosome pairs are split up and distributed independently and randomly in a way that each sperm or egg receives one member of each pair.

When sperm and egg join during fertilization, the resulting gamete receives one member of each pair of chromosomes from its father and one from its mother giving it the 46 chromosomes necessary for human life. As the cells divide to form a new organism, each cell of that organism receives an exact copy of each of the resulting 46 chromosomes.

The sorting process of meiosis insures that the chromosomes received by each sperm and egg cell are distributed independently and randomly.

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. . .

Crossover or Recombination

Between Prophase 1 and Metaphase 1 of meiosis, the members of each chromosome pair (one from the mother and one from the father) team up and entwine (in one last farewell embrace). In the process, they exchange bits and pieces of themselves. "Take this bit to remember me by, my dear."

The result is that each of our chromosomes is a patchwork of DNA received from our father’s parents and our mother’s parents. This 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 between each homologous pair during meiosis.

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 chromosome over which crossover occurs with 1% frequency in a single generation. In humans, 1cM is equivalent, on average, to 1 million base pairs. (

When the chromosomes arrive in the new organism, they are a mosaic of DNA from all four grandparents. 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.

Where Can I Go From Here?

©️2002 - 2017 is a not for profit, educational website.

Where Can I Go From Here?

©️2002 - 2017 is a not for profit, educational website.