 Our genetic system has an interesting way of doing a mix and match to produce new combinations. This is known as chromosomal recombination. Chromosomal recombination refers to the process of recombining genes to produce new gene combinations. Recombination of chromosomes happens during a process within meiosis called crossing over. This is when chromosomes exchange genetic material with one another to produce different combinations. Crossing over increases genetic variability and is an important contributor to the differences in gene expression. As you may recall, meiosis is a type of cell division that occurs to produce gametes or sex cells. Chromosomal recombination occurs in prophase 1 during the first phase of meiosis. In the diagram, you can see a pair of homologous chromosomes in two different colours. Each pair of sister chromatids are identical copies produced after replication and are shown using the same colour. In prophase 1, homologous sister chromatids line up next to each other. Next, the non-sister chromatids cross over or overlap. The genetic material from these sites are cleaved from their original chromosome and inserted into a new location on the homologous chromosome. The sites where the two chromatids cross over are called the chiasma. This produces recombinant chromosomes, where the sister chromatids are no longer identical to each other due to the gene exchange during crossing over. These recombinant homologous chromosomes then line up at the equator or the middle of the cell and segregate during metaphase 1, undergoing the normal processes of meiosis. At the end of meiosis, this results in 4 gametes being produced from the original homologous pairs. Each of these gametes is slightly different to the other due to the process of crossing over. This process is commonly modelled using Drosophila flies, as there are many varieties of these flies, each with different observable characteristics. Crossing over allows new combinations of these characteristics. For instance, the black body and vestigial wing traits are normally inherited together as they are on the same chromosome. However, crossing over allows them to be passed on separately in some cases. So how do we know which genes will cross over? Although crossing over occurs randomly, we can use a process called gene mapping to estimate how likely it is for genes to recombine. Gene mapping is a process of identifying locations of specific genes on chromosomes and their distances. The closer that the genes are to each other, the less likely they are to recombine, as there is a smaller distance between them for recombination to take place. This process of gene mapping can also be used to show changes in DNA due to crossing over. In summary, within this video we covered how chromosomal recombination occurs during meiosis, using a process called crossing over, which exchanges genetic material. This process occurs between non-cystochromatids of homologous chromosomes. It contributes to genetic variability in offspring from their parents. We also looked at a process of gene mapping, which can be used to find locations of different genes on a chromosome.