Mitotic recombination

[1][2] Mitotic homologous recombination occurs mainly between sister chromatids subsequent to replication (but prior to cell division).

Prior to Stern's work, it was hypothesized that twin spotting happened because certain genes had the ability to eliminate the chromosome on which they were located.

If the chromatids containing different alleles line up on the same side of the plate, then the resulting daughter cells will appear heterozygous and be undetectable, despite the crossover event.

If those daughter cells go on to replicate and divide, the twin spots will continue to grow and reflect the differential phenotype.

[6][7] In the budding yeast Saccharomyces cerevisiae, mutations in several genes needed for mitotic (and meiotic) recombination cause increased sensitivity to inactivation by radiation and/or genotoxic chemicals.

These include sister chromatid exchange and mechanisms related to DNA double strand break repair by homologous recombination such as single-strand annealing, synthesis-dependent strand annealing (SDSA), and gene conversion through a double-Holliday Junction intermediate or SDSA.

In another model, two overlapping sister chromatids form a double Holliday junction at a common repeat site and are later sheared in such a way that they switch places.

Alternatively, a crossover can occur during DNA repair[14] if, due to extensive damage, the homologous chromosome is chosen to be the template over the sister chromatid.

[2] For example, Bloom's syndrome is caused by a mutation in RecQ helicase, which plays a role in DNA replication and repair.