genetic mosaicism

In genetic medicine, a mosaic or mosaicism denotes the presence of two populations of cells with different genotypes in one individual, who has developed from a single fertilized egg. [1] Mosaicism may result from a mutation during development which is propagated to only a subset of the adult cells.

Different types of mosaicism exist, such as gonadal mosaicism (restricted to the gametes) or tissue mosaicism; one of them is chimerism, where two or more genotypes arise from the fusion of more than one fertilized zygote in the early stages of embryonal development. In the more common mosaics, different genotypes arise from only a single fertilized egg cell, due to meiosis errors.

In rare cases, intersex conditions can be caused by mosaicism where some cells in the body have XX and others XY chromosomes.[2]

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, there are cases where the trisomy only occurs in 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.[3] Generally this leads to a milder phenotype than in non-mosaic patients with the same disorder.

An example of this is one of the milder forms of Klinefelter's syndrome, called 46/47 XY/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 47 total chromosomes.

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, such as in calico cats. Contents [hide] 1 Gonadal mosaicism 2 Use in experimental biology 3 See also 4 References 5 Further reading //

Gonadal mosaicism

Gonadal mosaicism is a special form of mosaicism, where some gametes, i.e. either sperm or oocytes, carry a mutation, but the rest are normal.[4]

The cause is usually a mutation that occurred in an early stem cell that gave rise to all or part of the gonadal tissue.

This can cause only some children to be affected, even for a dominant disease.

Use in experimental biology

Genetic mosaics can be extraordinarily useful in the study of biological systems, and can be created intentionally in many model organisms in a variety of ways. They often allow for the study of genes that are important for very early events in development, making it otherwise difficult to obtain adult organisms in which later effects would be apparent. Furthermore they can be used to determine the tissue