A somatic mutation is a genetic mutation which occurs in a somatic cell after conception. These mutations can lead to a variety of medical issues, and lay people commonly associate them specifically with cancers. Somatic mutations can be identified by examining the genetic material in a questionable cell and comparing it to a cell from elsewhere in the body; the DNA in the two cells will be different, despite the fact that it is not supposed to be.
There are two types of cells: somatic cells and germ cells. Germ cells eventually give rise to gametes, while somatic cells give rise to everything else. If a somatic mutation occurs in utero, all of the cells descended from the somatic cell will carry the mutation. This results in a situation called genetic mosaicism, in which some of the cells in someone's body have DNA which is different from other cells.
A somatic mutation in utero can lead to issues such as birth defects, with the impaired cell passing on the damaged DNA to its descendants and causing malformations. In other cases, someone may be left with mosaicism, but no outward problems. After birth, a somatic mutation can lead to the development of a cancer if the growth regulators in the cell are damaged, causing the cell to engage in out of control replication, creating new cells which will do the same.
Somatic mutations cannot be inherited, because they do not involve the germ cells. This type of mutation is sometimes called an “acquired mutation,” referencing the fact that it is not the result of inheriting a mutation from a parent. For example, a woman who develops breast cancer will not pass breast cancer on to her child. However, the risk of a somatic mutation can be increased by the presence or absence of certain inherited genes, which means that a the children of a woman with breast cancer may be at increased risk, and they can be tested to find out whether genes linked with breast cancer are present in their DNA.
Somatic mutations can happen for a variety of reasons. Some appear to be the result of exposure to toxins or radiation which interferes with the cell division process. Others are spontaneous, occurring as the result of a random error in the cell division process. Given the length of the genome, occasional mistakes do happen in individual cells, and in fact the body is coded to destroy somatic cells which have mutated, although it is not always successful.