Before making an egg or sperm, your chromosomes trade pieces
Meiosis is the cell division behind sex. It halves the chromosome count so egg and sperm can combine, but first each maternal and paternal chromosome pair swaps segments, shuffling genes into combinations never seen before. That reshuffle is why siblings differ, and mistakes in the process are the leading known cause of miscarriage.
Meiosis happens only in germ cells of sexually reproducing organisms, from single-celled eukaryotes to animals, plants and fungi. It starts like ordinary division: DNA is copied, so each chromosome becomes two identical sister chromatids held together. Then comes a long prophase in which matching chromosomes from mother and father pair up and recombine, cutting and repairing their DNA so that genetic material crosses between them. Some of these exchanges leave physical links called chiasmata, named after the Greek letter chi because of their X shape.
Two divisions follow with no further copying in between. In the first, the spindle apparatus pulls the paired homologous chromosomes into separate cells. In the second, the sister chromatids are released and parted, much as in mitosis. The result is four haploid cells, each carrying a single set of chromosomes. In humans, whose body cells have 23 pairs for a total of 46, each egg or sperm carries 23; fertilisation restores the full count in the zygote, half from each parent. In mammals, meiosis actually finishes only after fertilisation.
Sexes can differ in the outcome. Males, and many plants and fungi, may turn all four products into sperm, pollen or spores. Female animals usually keep just one as the egg and discard the other three as small polar bodies.
The payoff is variety. Thanks to recombination, a single chromatid can be a patchwork of maternal and paternal DNA, and each gamete carries its own mix of maternal, paternal and recombined chromosomes. Offspring therefore differ genetically from both parents, supplying the variation natural selection acts on. The cost of so much choreography is risk: when chromosomes are distributed wrongly, the resulting abnormal count, called aneuploidy, is also the most frequent genetic cause of developmental disabilities.
Source: Meiosis
