Mosaicism

Overview, Causes, & Risk Factors

There are many causes of mosaicism. For example, an older mother is more likely to have a child with an extra chromosome. If this extra chromosome is only present in a fraction of the cells in the child, that person is mosaic because his or her body has some cells with 46 chromosomes (normal) and some cells with 47 (abnormal).

Another cause of mosaicism is the development of a new mutation during the early embryonic growth of a fetus which affects some, but not all, cells.

Yet another type of mosaicism, called gonadal mosaicism, happens when a mutation occurs in a developing fetus' egg or sperm cells. A male with this condition may not show any signs of a genetic disease himself, but will carry some sperm that have potentially harmful mutations. When he later has his own children, those children carry some risk of inheriting the mutation (depending on whether the pregnancy is the result of a mutant or normal sperm). The same is true for a female with gonadal mosaicism (i.e., some of her eggs develop mutations before she was born).