Crossing over:
Crossing over is a process in which a pair of homologous chromosomes exchange genetic material to allow for variation. It occurs during the first stage of meiosis where a pair of chromosomes (one from each parent) line up next to each other. This process is controlled by a large protein complex known as the recombination nodule. Once the chromosomes are lined up, they will attach together at the chiasma. A chiasma occurs at least once per pair of chromosomes. It is at the chiasma that the corresponding part of each chromosome breaks off and joins the opposite chromosome. After this, the chromosomes separate. This results in a difference in the composition in the chromosomes, and ensures at least two of the four chromatids are unique – that is, differs from the parent chromosome.