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Mosaicism refers to the presence of two or more cell populations with different genotypes within a single individual who has descended from a single zygote. This phenomenon can occur in various tissues and at different stages of development. The genetic differences between cell lines can arise from mutations that occur after fertilization. These mutations can be in autosomal chromosomes, sex chromosomes, or mitochondrial DNA.

The clinical significance of mosaicism varies widely. It can range from being completely asymptomatic to causing a range of developmental abnormalities or diseases. The severity of the condition often depends on the specific genetic mutation, the proportion of affected cells, and the tissues in which the mosaicism is present. For example, if a mutation is present in a small percentage of cells in a non-critical tissue, the individual may have no noticeable symptoms. However, if a mutation affects a significant proportion of cells in a vital organ or during early development, it can lead to more pronounced effects.

Mosaicism is often detected through genetic testing, such as karyotyping or DNA sequencing, especially when a genetic disorder is suspected but the typical presentation is not fully observed. The diagnosis of mosaicism can be challenging because the affected cell lines may not be present in all tissues, requiring careful selection of samples for testing. Understanding the extent and location of mosaicism is crucial for genetic counseling and for predicting the potential health outcomes for an individual.