Somatic mosaics have been created in Drosophila using X‑ray treatment and the use of irradiation to induce somatic mutation has been a useful technique in the study of genetics.True mosaicism should not be mistaken for the phenomenon of X-inactivation, where all cells in an organism have the same genotype, but a different copy of the X chromosome is expressed in different cells. The latter is the case in normal (XX) female mammals, although it is not always visible from the phenotype (as it is in calico cats). However, all multicellular organisms are likely to be somatic mosaics to some extent. Gonosomal mosaicism
Gonosomal mosaicism is a type of somatic mosaicism that occurs very early in the organisms development and thus is present within both germline and somatic cells. Somatic mosaicism is not generally inheritable as it does not usually affect germ cells. In the instance of gonosomal mosaicism, organisms have the potential to pass the genetic alteration, including to potential offspring because the altered allele is present in both somatic and germline cells. Brain cell mosaicism
A frequent type of neuronal genomic mosaicism is copy number variation. Possible sources of such variation were suggested to be incorrect repair of DNA damages and somatic recombination. Mitotic recombination
One basic mechanism that can produce mosaic tissue is mitotic recombination or somatic crossover. It was first discovered by Curt Stern in Drosophila in 1936. The amount of tissue that is mosaic depends on where in the tree of cell division the exchange takes place.
In the fruit fly Drosophila melanogaster, where a fly possessing two X chromosomes is a female and a fly possessing a single X chromosome is a sterile male, a loss of an X chromosome early in embryonic development can result in sexual mosaics, or gynandromorphs. Likewise, a loss of the Y chromosome can result in XY/X mosaic males.An example of this is one of the milder forms of Klinefelter syndrome, called 46,XY/47,XXY mosaic wherein some of the patients cells contain XY chromosomes, and some contain XXY chromosomes. The 46/47 annotation indicates that the XY cells have the normal number of 46 total chromosomes, and the XXY cells have a total of 47 chromosomes. Also monosomies can present with some form of mosaicism. The only non-lethal full monosomy occurring in humans is the one causing Turners syndrome. Around 30% of Turners syndrome cases demonstrate mosaicism, while complete monosomy (45, X) occurs in about 50–60% of cases. Mosaicism need not necessarily be deleterious, though. Revertant somatic mosaicism is a rare recombination event with a spontaneous correction of a mutant, pathogenic allele. In revertant mosaicism, the healthy tissue formed by mitotic recombination can outcompete the original, surrounding mutant cells in tissues such as blood and epithelia that regenerate often. In the skin disorder ichthyosis with confetti, normal skin spots appear early in life and increase in number and size over time.Other endogenous factors can also lead to mosaicism, including mobile elements, DNA polymerase slippage, and unbalanced chromosomal segregation. Exogenous factors include nicotine and UV radiation.