This article needs additional citations for verification. 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. Genetic mosaicism can result from many different mechanisms including chromosome non-disjunction, anaphase lag and endoreplication. Genetic mosaics may often be confused with chimerism, in which two or more genotypes arise in mosaic 2 oxford pdf individual similarly to mosaicism.
However, 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. 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. 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 occurs in only a selection of the cells. XXY mosaic wherein some of the patient’s cells contain XY chromosomes, and some contain XXY chromosomes. 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.
But mosaicism need not necessarily be deleterious. Revertant somatic mosaicism is a rare recombination event in which there is a spontaneous correction of a mutant, pathogenic allele. Other endogenous factors can also lead to mosaicism including mobile elements, DNA polymerase slippage, and unbalanced chromosomal segregation. X chromosome is expressed in different cells. Somatic mutation leading to mosaicism is prevalent in the beginning and end stages of human life. One basic mechanism which can produce mosaic tissue is mitotic recombination or somatic crossover.
It was first discovered by Curt Stern in Drosophila in 1936. Germline or gonadal mosaicism is a special form of mosaicism, where some gametes—i. 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. 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. 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. Genetic mosaics can also be created through mitotic recombination. More recently the use of a transgene incorporated into the Drosophila genome has made the system far more flexible. It is sometimes inconvenient to use negatively marked clones, especially when generating very small patches of cells, where it is more difficult to see a dark spot on a bright background than a bright spot on a dark background. In 1929, Alfred Sturtevant studied mosaicism in Drosophila.
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