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A chromosomal disorder in which some cells are normal and some cells contain the chromosomal abnormalities of the disorder, in contrast to a complete distribution of the abnormal chromosomes throughout all cells. The distribution of abnormal cells in mosaicism is usually random and unpredictable. Mosaicism most commonly occurs in autosomal trisomy, in which there is an additional copy of one chromosome that appears in some cells and not in others.
The result generally is a milder presentation of symptoms when only some cells express the abnormality (mosaic disorder) than occurs when all cells express the abnormality (complete disorder). People who have a mosaic expression of the autosomal trisomy disorder Down syndrome, for example, typically have milder symptoms than people who have a complete expression. Mosaicism may also affect genetic expressions other than health disorders.