Genetics, Mosaicism

Excerpt

Genetic mosaicism is defined as the presence of two or more cell lineages with different genotypes arising from a single zygote in a single individual. In contrast, if distinct cell lines derived from different zygotes, the term is now known as chimerism. Genetic mosaicism is a postzygotic mutation.

Since an adult human being requires countless cell divisions for its development (approximately 10 to the 16th power or, virtually, 10,000,000,000,000,000 mitoses) and every cell division carries a risk for genetic mistakes, each person has at least one genotypically distinct cell in their body; thus every individual is a mosaic. Also, probably all the cells within every single human being encompass numerous mutations that could potentially explain every human genetic disease. Nonetheless, only a minimum of these mutational events affects the individual’s well-being, while most are phenotypically silent (i.e., mutations are not relevant for cell function, mutant cells are eliminated by apoptosis). Mosaicism appears to be responsible for an enormous amount of pathologies, ranging from chromosomal abnormalities, such as Turner syndrome, to a myriad of cancers. Both benign and malignant tumors constitute evidence of somatic mosaicism in the human body.