mosaic /mōzā″ik/ [L, Musa, goddess of the arts] , 1. an individual or organism that developed from a single zygote but that has two or more kinds of genetically different cell populations. Mosaicism may result from a mutation, crossing over, or, more commonly in humans, nondisjunction of chromosomes during early embryogenesis, which causes a variation in the number of chromosomes in the cells. The type of chromosomal aberration and the fraction of cells that are affected depend on the cleavage stage at which the causative event occurred. Because monosomic cells are nonviable, except in X monosomic conditions, most mosaic conditions caused by nondisjunction in humans represent a mixture of normal and trisomic cells, regardless of whether an autosome or the sex chromosomes are involved. The degree of clinical involvement depends on the type of tissue containing the abnormality and may vary from near normal to full manifestation of a syndrome, such as Down syndrome or Turner’s syndrome. Compare chimera. See also monosomy, sex chromosome mosaic, trisomy. 2. a fertilized ovum that undergoes determinate cleavage. See also mosaic development. −mosaicism, n.
