The exchange of genetic material between homologous chromosomes is called crossing over. This process happens during meiosis. Because crossing over occurs in sister chromatids, the process allows for recombination and variety in offspring formed from the resulting daughter cells or gametes. Crossing over is considered a type of genetic recombination and is an important source of genetic diversity in humans.
During meiosis I, the chromosomes are replicated. Homologous pairs of chromosomes match up with one another. During this time, parts of the homologous chromosomes overlap; these parts break off and migrate to the other homologs and reattach. These chromosomes are no longer exact duplicates of the original parent chromosomes. Crossing over occurs with such precision that, most of the time, genes do not get lost or become nonfunctional in any way. At times, this process of crossing over is not precise or becomes aberrant. In these cases, the fetus that results is often spontaneously aborted. In such cases, babies that survive until birth usually do not live beyond their first year.
The X and Y, or sex, chromosomes engage in a special kind of crossing over. A homologous section called the pseudoautosomal region exists between the X and the Y, and these pair up during meiosis. No other part of the X and Y chromosomes experiences any crossing over.