During prophase I, crossing-over of parts of chromosomes can occur; this increases genetic variability.
Prophase is the first stage of cell division. During meiosis, the type of division that results in haploid (half the usual number of chromosomes) cells for sexual reproduction, the cell undergoes the entire process, with variations, two times. There are differences in how the chromosomes are arranged between prophase in mitosis, and prophase I in meiosis. In prophase I, the chromosomes line up with their homologous chromosomes. Because of the fact that they are very close to each other, crossing-over can occur. This is transfer or switching of part of one chromosome of the pair, with the same section of its partner. This increases genetic variability, as the genes that were previously on one of the pair are now on the other. When the egg or sperm cell is produced, the genes that were linked due to being on the same chromosome as each other no longer go to the same cell.