http://ghr.nlm.nih.gov/glossary=somaticmutation
Your use of "somatic" in this context seems to be different from the standard definition (e.g. see this definition .
"Germline DNA" is typically read to mean "the DNA sequence you were born with". The term "somatic mutation" is usually read to imply that the germline DNA sequence was altered somehow after you were born; that's where the "somatic" part comes in. Part of the definition, in my mind, is that this mutation is not present in every cell of the body. Somatic mutations are typically identified by comparing the sequence of a particular pool of DNA-- let's say DNA extracted from a lung tumor-- to DNA thought to be representative of the germline DNA. For people with epithelial tumors, DNA extracted from blood is usually used to get the germline status. You can identify novel somatic mutations by comparing the target sequence to germline sequence. Somatic mutations are catalogued in databases like COSMIC.
This definition is distinct from the question of "what's the difference between a common polymorphism and a rare polymorphism?", since the context for that question is the DNA the person was born with. That question is usually a matter of commonly accepted definitions for "rare" vs. "common" and the population being used for comparisons, rather that being a difference in kind. Whatever the definition, I think these unusual variations are usually termed "rare variants" rather than "somatic mutations".
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.
Germinal and Somatic Mutations
Eukaryotic organisms have two primary cell types --- germ and somatic. Mutations can occur in either cell type. If a gene is altered in a germ cell, the mutation is termed a germinal mutation . Because germ cells give rise to gametes, some gamete s will carry the mutation and it will be passed on to the next generation when the individual successfully mates. Typically germinal mutations are not expressed in the individual containing the mutation. The only instance in which it would be expressed is if it negatively (or positively) affected gamete production.
Somatic cells give rise to all non-germline tissues. Mutations in somatic cells are called somatic mutations . Because they do not occur in cells that give rise to gametes, the mutation is not passed along to the next generation by sexual means. To maintain this mutation, the individual containing the mutation must be cloned. Two example of somatic clones are navel oranges and red delicious apples. Horticulturists first observed the mutants. They then grafted mutant branches onto the stocks of "normal" trees. After the graft was established, cuttings from that original graft were grafted onto tree stocks. In this way the mutation was maintained and proliferated.
Most tissues are derived from a cell or a few progenitor cells. If a mutation occurs in one of the progenitor cells, all of its daughter cells will also express the mutation. For this reason, somatic mutations generally appear as a sector on the mutated individual.
Cancer tumors are a unique class of somatic mutations. The tumor arises when a gene involved in cell division, a protooncogene, is mutated. All of the daughter cells contain this mutation. The phenotype of all cells containing the mutation is un controlled cell division. This results in a tumor that is a collection of undifferentiated cells called tumor cells.
http://www.ornl.gov/sci/techresources/Human_Genome/faq/snps.shtml
