How does crossing over occur during meiosis?

Genetic variation is important because it allows natural selection to act on the population. Individuals that inherit advantagous alleles are more likely to survive and successfully reproduce. Crossing over events dring meiosis help produce genetic variation. During prophase of meiosis I, the paternal homologous chromosome is paired with the maternal homologous chromosome. This is called a bivalent and is composed of two copies of the paternal chromosome and two copies of the maternal one. Crossing over occurs when a paternal chromatid exchanges part of itself with the maternal chromatid, so some alleles that were on one chromatid are now on the other. This event is random, happens by chance and can occur more than once in a bivalent. Each dipoid germ cell that enters meiosis will undergo different crossing over events, so the chances that, for example, two sperm are identical is very small indeed.