Mitotic crossover is a rare type of genetic recombination that may occur in some types of somatic cells during mitosis. Mitotic crossover may occur in organisms that do not have a cycle of sexual reproduction where chromosomal crossover would normally occur during meiosis 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 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. For use in experimentation with genomes in model organisms such as Drosophila melanogaster, mitotic recombination can be induced via X-ray and the FLP-FRT recombination system.