In what phase of meiosis does crossing over primarily occur?

In meiosis, crossing over primarily occurs during Prophase I. This phase is crucial for genetic diversity because it is when homologous chromosomes pair up closely in a process called synapsis. As the chromosomes align closely, sections of chromatids can exchange genetic material at points known as chiasmata. This exchange creates new combinations of alleles on the chromosomes, which enhances genetic variation in the gametes.

The significance of crossing over cannot be overstated, as it leads to increased genetic diversity, allowing for variation in populations. This process is a key factor in evolution and adaptation, as it results in offspring that may have different traits from their parents.

In other phases of meiosis, such as Metaphase I, Telophase II, and Anaphase II, chromosome behavior involves the movement and separation of chromosomes rather than the exchange of genetic material. Thus, Prophase I is specifically where crossing over is essential and occurs, making it the correct answer.