Final answer:
Crossing over involves the exchange of genetic material between non-sister chromatids of homologous chromosomes during meiosis and is not a feature of mitosis or in haploid organisms with a transient diploid stage.
Step-by-step explanation:
Crossing over does not occur between sister chromatids. This process specifically involves the exchange of genetic material between non-sister chromatids of homologous chromosomes during the first prophase of meiosis. Crossing over is a crucial source of genetic variation and does not happen in haploid organisms during their transient diploid stages, nor does it occur in cells undergoing mitosis, as mitosis is involved with the replication and division of cells without genetic recombination.