Gonadal mosaicism

Alternative names

Chromosomal mosaicism; Mosaicism

Definition

A person who is a "genetic mosaic" has cells in his or her body with different genetic makeup.

This usually is a variation in the number of chromosomes in the body's cells. Normally, all body cells would have the same number of chromosomes (46). But in mosaicism, some cells may have 47 chromosomes (such as extra chromosome 21 or X chromosome in some, but not all cells).

Causes, incidence, and 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).

Symptoms

The symptoms depend on what genetic change is mosaic.

For example, if a person has two populations of cells - one normal and one missing an X chromosome - the symptoms are usually milder versions of those seen in Turner syndrome. Turner syndrome results when 100% of cells lack an X chromosome. If only a fraction of cells lack this chromosome, the symptoms will vary from milder to none at all. The severity increases as the percentage of mosaicism increases. If 90% of cells are abnormal, symptoms are likely; if only 10% are abnormal, the person may be unaware that anything is wrong.

Gonadal mosaicism is a different situation. In that case, the mosaicism is in the parent's ovaries or testes. Any individul egg or sperm either has the mutation or not. Therefore, if conception involves one of these mutant sex cells, the resultant child will not be mosaic, but will simply have the genetic disease caused by that particular mutation.

Signs and tests

Signs, like symptoms, depend on which genetic change is mosaic. When mosaicism results from mutation during embryonic or fetal growth, one part of the body may have signs or symptoms of a genetic disease, while another part of the body may be normal.