What happens during crossing over in meiosis?

During crossing over in meiosis, homologous chromosomes exchange segments of genetic material. This process occurs during prophase I of meiosis when homologous chromosomes, each consisting of two sister chromatids, align closely in a formation called a tetrad. The points at which the chromatids cross over each other and exchange portions of their DNA are known as chiasmata.

This exchange increases genetic diversity in the offspring produced through sexual reproduction because it creates new combinations of alleles on the chromosomes. As a result, the gametes (sperm and egg) will have unique genetic configurations compared to their parental chromosomes. This genetic variation is crucial for evolution and adaptation in populations.

The other options do not describe crossing over. Fusion of gametes to form a zygote occurs after meiosis when fertilization takes place. Chromosome duplication happens during the S phase of the cell cycle before meiosis begins, not during the crossing over phase. While crossing over can influence gene expression in a broader context through generating genetic variation, it does not directly refer to the process of genes being expressed differently.