In human genetics, the exchange of DNA between paired homologous chromosomes during meiosis is called crossing over.
Crossing over occurs during prophase I of meiosis, when homologous chromosomes pair along their lengths. Matching DNA segments can be exchanged at visible contact points called chiasmata. The resulting chromosomes contain combinations of genetic material from both parental chromosomes.
This process increases genetic variation in offspring. It works alongside the independent assortment of chromosome pairs, another feature of meiosis that produces many possible combinations of parental chromosomes in reproductive cells.
Crossing over is not the same as ordinary DNA replication. Replication copies DNA before cell division, while crossing over rearranges corresponding segments between homologous chromosomes. It can also occasionally produce unequal exchanges, which may cause duplications or deletions when the chromosomes do not align correctly.