Crossing over or Chromosomal crossover is the process that occurs in cells between the stages of phrophase I and metaphase I in meiosis. It is important because it is responsible in the exchange of genetic material between homologous chromosomes during sexual reproduction. This process ensures the normal segregation of chromosomes during meiosis, and genetic variation. Homologous chromosomes pair up with each other and exchanges segments of their genetic material to form new chromosomes. Thomas Hunt Morgan described crossing over in his theory of genetics.