Answer: When two homologous chromosomes transfer genetic information in prophase 1.
Step-by-step explanation:
Meiosis is a process in which the a single parent (diploid) cell divides twice to produce four haploid daughter cells which have half the number of chromosomes as that of their parents.
Crossing over can be defined as exchange of genetic material that occurs between the chromosomes of opposite germ line. This process helps in transfer of mixing of traits to the offspring from both parents. This occurs in prophase 1 stage of meiosis. This process is the basis of formation of gametes or sex cells that are eggs in female and sperm cells in the male. The paired homologous chromosomes from single parent align to form a bivalent so that cross over each other and a unique combination of trait develop after this process.