Curt Stern in 1935 assumed that the structural changes in the chromosomes took place as a result of somatic crossing, as a result of which mutations or small chromosomal rearrangements in somatic cells. Thus the inert region causes an increase in mutation frequency or small chromosomal rearrangements in active segments adjacent to inert regions.In the 1930s, Stern demonstrated that genetic recombination, normal in meiosis, can also take place in mitosis. When it does, it results in somatic (body) mosaics. These organisms contain two or more genetically distinct types of tissue. The term somatic mosaicism was used by CW Cotterman in 1956 in his seminal paper on antigenic variation.In 1944, Belgovskii proposed that mosaicism could not account for certain mosaic expressions caused by chromosomal rearrangements involving heterochromatic inert regions. The associated weakening of biochemical activity led to what he called a genetic chimera. Types
Germline mosaicism
Germline or gonadal mosaicism is a particular form of mosaicism wherein some gametes—i.e., sperm or oocytes—carry a mutation, but the rest are normal. The cause is usually a mutation that occurred in an early stem cell that gave rise to all or part of the gametes. Somatic mosaicism
Somatic mosaicism occurs when the somatic cells of the body are of more than one genotype. In the more common mosaics, different genotypes arise from a single fertilized egg cell, due to mitotic errors at first or later cleavages. Somatic mutation leading to mosaicism is prevalent in the beginning and end stages of human life.
A phenotypic character called "twin spot" seen in Drosophila is a result of mitotic recombination. However, it also depends on the allelic status of the genes undergoing recombination. Twin spot occurs only if the heterozygous genes are linked in repulsion, i.e. the trans phase. The recombination needs to occur between the centromeres of the adjacent gene. This gives an appearance of yellow patches on the wild-type background in Drosophila. another example of mitotic recombination is the Blooms syndrome, which happens due to the mutation in the blm gene. The resulting BLM protein is defective. The defect in RecQ, a helicase, facilitates the defective unwinding of DNA during replication, thus is associated with the occurrence of this disease. Use in experimental biology
Genetic mosaics are a particularly powerful tool when used in the commonly studied fruit fly, where specially selected strains frequently lose an X or a Y chromosome in one of the first embryonic cell divisions. These mosaics can then be used to analyze such things as courtship behavior, and female sexual attraction.More recently, the use of a transgene incorporated into the Drosophila genome has made the system far more flexible. The flip recombinase (or FLP) is a gene from the commonly studied yeast Saccharomyces cerevisiae that recognizes "flip recombinase target" (FRT) sites, which are short sequences of DNA, and induces recombination between them. FRT sites have been inserted transgenically near the centromere of each chromosome arm of D. melanogaster.