The below mentioned article provides a short note on mitotic crossing over.
Crossing over not only occurs during meiosis but also during mitosis. In individuals heterozygous for a gene producing a visible phenotype, somatic crossing over leads to the production of “twin spots” of dominant and recessive phenotypes.
Mitotic crossing over was first discovered in Drosophila by Stern in 1936, between the six-linked genes y (yellow body colour) and sn (singed bristles = bent blunted bristles). The gene sn is located proximal, while y is located distal to the centromere of the X chromosome.
The phenotype of a fly in which both the genes are in heterozygous condition in repulsion phase (trans configuration) (sn+ y/sn y+) is wild type, so long there is no crossing over. There are three possibilities of crossing over.
1. Crossing over between genes sn and y will produce cells with (sn+ y/sn y) genotype and complementary combinations (,sn+y+/sn y+). This would result into the production of a single, yellow spot (actually there is a twin spot of yellow and wild type but the latter is not detectable due to the wild type background).
2. Crossing over between the centromere and gene sn would result in the twin combinations sn+ y/sn+ y and sny+sn these would produce one spot with yellow body colour and the other having singed bristles (Fig. 11.13).
3. Double crossing over may occur between the centromere and gene sn and between the genes sn and y; this will produce the genotype sn y/sny+. and the complementary combinations sn+ y/sn+ y+. These genotypes will produce a single spot having singed bristles.
The size of twin spots varies depending on (he number of subsequent cell divisions in the crossover products.
Mitotic recombination is less frequent than meiotic recombination. However, the relative frequencies for two different chromosomes or two regions of a chromosome may differ. In the fungus Aspergillus, mitotic recombination has been used in linkage studies and a linkage map was prepared for the genes pro (proline requiring mutation), paba (p-amino benzoate requiring mutation) and y (yellow mutation).
It is presumed that mitotic crossing over occurs during mitotic prophase but the exact stage is not known. It is not known whether crossing over occurs during interphase before mitosis or during the transition from interphase to prophase or during the prophase.