Which of the following best describes what happens during a cross-over event?
a) tetrads change orientation as they line up along the center of the cell.
b) chromatids of homologous chromosomes exchange corresponding segments.
c) x and y sex chromosomes overlap and combine together during metaphase.
d) spindle fibers overlap and attach to the wrong chromosomes during anaphase.

The crossover of chromosomes is the exchange of genetic material between homologous chromosomes. Homologous chromosomes are present in the meiosis I, but not in meiosis the II. After pairing up of in the prophase I, homologous chromosomes are able to exchange some corresponding segments and a cross-over occurs. Thanks to the cross-over event, each chromosome will have a new combination of genes. Without this, parents will produce gametes with only two different gene combinations.