crossing over

Imagine two decks of cards, one inherited from your mother and one from your father, each holding the same list of topics but with slightly different cards for each. Before dealing out new hands, you let the two decks physically trade some of their matching cards. The hands that come out are now unique blends of both. Crossing over is precisely this: matching chromosomes physically swap corresponding segments during meiosis, shuffling the parents' genes into new combinations.

Crossing over happens early in meiosis I, when the two members of each homologous pair (the maternal and paternal versions of a chromosome) line up tightly side by side. At points called chiasmata, the chromosomes break at exactly matching positions and rejoin to the other partner, so each chromosome ends up carrying a mosaic of maternal and paternal DNA. Because the breaks happen at corresponding spots, no genes are lost or duplicated — the chromosomes simply exchange equivalent stretches, like splicing equivalent scenes between two copies of the same film.

Crossing over matters because it is a primary source of genetic variety in offspring. Without it, you could only pass on each chromosome intact as you received it; with it, the chromosome you give your child is a fresh mix never seen before. This reshuffling helps populations adapt and is why even full siblings (other than identical twins) are genetically distinct. Geneticists also exploit it: genes that sit close together on a chromosome are rarely separated by crossing over, which lets scientists map where genes lie.