Mitotic crossover is a rare type of genetic recombination

Genetic recombination

Genetic recombination is a process by which a molecule of nucleic acid is broken and then joined to a different one. Recombination can occur between similar molecules of DNA, as in homologous recombination, or dissimilar molecules, as in non-homologous end joining. Recombination is a common method...

 that may occur in some types of somatic cells during mitosis

Mitosis

Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...

. Mitotic crossover may occur in organisms that do not have a cycle of sexual reproduction where chromosomal crossover

Chromosomal crossover

Chromosomal crossover is an exchange of genetic material between homologous chromosomes. It is one of the final phases of genetic recombination, which occurs during prophase I of meiosis in a process called synapsis. Synapsis begins before the synaptonemal complex develops, and is not completed...

 would normally occur during meiosis

Meiosis

Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....

 to generate genetic variation. It can only occur in diploid cells and a pair of chromosomes is required for the crossover to occur.

The mitotic crossover results in the production of homozygous allele combinations in all heterozygous genes that are located on that chromosome arm distal to the crossover. Thus when a mitotic crossover occurs, genes that were previously recessive

Recessive

In genetics, the term "recessive gene" refers to an allele that causes a phenotype that is only seen in a homozygous genotype and never in a heterozygous genotype. Every person has two copies of every gene on autosomal chromosomes, one from mother and one from father...

 are expressed creating a new phenotype.

Mitotic crossover is known to occur in some asexually reproducing fungi and in normal human cells, where the event may allow normally recessive cancer-causing genes to be expressed and thus predispose the cell in which it occurs to the development of cancer

Cancer

Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...

. For use in experimentation with genomes in model organisms such as Drosophila melanogaster

Drosophila melanogaster

Drosophila melanogaster is a species of Diptera, or the order of flies, in the family Drosophilidae. The species is known generally as the common fruit fly or vinegar fly. Starting from Charles W...

, mitotic recombination can be induced via X-ray and the FLP-FRT recombination

FLP-FRT Recombination

In genetics, FLP-FRT recombination is a site-directed recombination technology used to manipulate an organism's DNA under controlled conditions in vivo. It is analogous to Cre-Lox recombination...

system.