Crossing Over
This animation shows crossing over, which is the process by which homologous chromosomes exchange portions of their chromatids during meiosis. As you can see, crossing over results in new combinations of genetic information, thus affecting inheritance and increasing genetic diversity. Genes that are present together on the same chromosome tend to be inherited together, and are referred to as linked. Crossing over can cause genes that are normally linked to be inherited independently.
Crossing over occurs in prophase I, the stage where the chromosomes first pair off as tetrads.
Although DNA sequencing is a more precise technique, biologists also use crossing over to map genes...the closer two genes are to one another - the less likely it is that these genes will be separated. By observing how frequently crossing over separates two genes, researchers can determine these genes' relative positions on their chromosome.