Somatic mosaicism

In genetics, a mosaic, or mosaicism, involves the presence of two or more populations of cells with different genotypes in one individual who has developed from a single fertilized egg. Mosaicism has been reported to be present in as high as 70% of cleavage-stage embryos and 90% of blastocyst-stage embryos derived from in vitro fertilization. In genetics, a mosaic, or mosaicism, involves the presence of two or more populations of cells with different genotypes in one individual who has developed from a single fertilized egg. Mosaicism has been reported to be present in as high as 70% of cleavage-stage embryos and 90% of blastocyst-stage embryos derived from in vitro fertilization. Genetic mosaicism can result from many different mechanisms including chromosome nondisjunction, anaphase lag, and endoreplication. Anaphase lagging is the most common way by which mosaicism arises in the preimplantation embryo. Mosaicism can also result from a mutation in one cell during development in which the mutation is passed on to only its daughter cells. Therefore, the mutation is only going to be present in a fraction of the adult cells. Genetic mosaics may often be confused with chimerism, in which two or more genotypes arise in one individual similarly to mosaicism. In chimerism, though, the two genotypes arise from the fusion of more than one fertilized zygote in the early stages of embryonic development, rather than from a mutation or chromosome loss. Different types of mosaicism exist, such as gonadal mosaicism (restricted to the gametes) or 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. In rare cases, intersex conditions can be caused by mosaicism where some cells in the body have XX and others XY chromosomes (46, XX/XY). 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 gynandropmorphs. Likewise, a loss of the Y chromosome can result in XY/X mosaic males. The most common form of mosaicism found through prenatal diagnosis involves trisomies. Although most forms of trisomy are due to problems in meiosis and affect all cells of the organism, some cases occur where the trisomy occurs in only a selection of the cells. This may be caused by a nondisjunction event in an early mitosis, resulting in a loss of a chromosome from some trisomic cells. Generally, this leads to a milder phenotype than in nonmosaic patients with the same disorder. An example of this is one of the milder forms of Klinefelter syndrome, called 46,XY/47,XXY mosaic wherein some of the patient's 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. Around 30% of Turner's syndrome cases demonstrate mosaicism, while complete monosomy (45, X) occurs in about 50–60% of cases.