Every mutation is a change in DNA, and the difference between the two kinds comes down to which cells carry it. A germline mutation sits in an egg or sperm cell, so it can be passed to children and appears in every cell of their body.1MedlinePlus. How are gene variants involved in evolution? A somatic mutation happens after conception in an ordinary body cell, affects only that person, and dies with them.2Genome.gov. What is genome editing? Everyone builds up somatic mutations through life, so for most people the question is not whether they have them but which kind a test found.
Which Cells Carry Each Kind
In animals, only a select few cells, called germ cells, pass DNA to the next generation.3PubMed Central. Somatic cell conversion to a germ cell lineage – a violation or a revelation? These cells are set aside early in development, later move into the ovaries or testes, and eventually make eggs or sperm.4PubMed Central. Somatic cell conversion to a germ cell lineage – a violation or a revelation? The rest of the body, such as nerves, skin and heart, dies with the individual.5PubMed Central. Somatic cell conversion to a germ cell lineage – a violation or a revelation? In most animals, body cells cannot turn back into reproductive cells.6Current Opinion in Systems Biology. A pan-metazoan view of germline-soma distinction challenges our understanding of how the metazoan germline evolves7PubMed Central. Somatic cell conversion to a germ cell lineage – a violation or a revelation?
That is why only changes in egg or sperm cells can be passed on and contribute to evolution.8MedlinePlus. How are gene variants involved in evolution? A mutation found in a tumor sample alone usually tells you about the tumor, not about your children.9MedlinePlus. How are gene variants involved in evolution?
A mutation in a father’s sperm is carried in every cell of the child, including the child’s own eggs or sperm, so it can keep traveling down the family tree. A sun-caused mutation in a mother’s skin cell stays in that patch of skin, and her child is unaffected. This is why doctors ask whether a change is inherited before they talk about relatives.
It also helps to know that “genetic” does not mean “inherited.” All mutations are genetic changes, but only germline ones run in families. Cancer is genetic in the sense that DNA changes drive it, yet most cancer is not inherited. On a report, “hereditary” and “germline” usually mean the same thing for practical purposes, while “acquired” or “somatic” means the change happened during your lifetime.
How Often Each Kind Happens
In both humans and mice, body cells mutate almost 100 times faster than reproductive cells.10PubMed Central. Differences between germline and somatic mutation rates in humans and mice Body cells only need to last one lifetime, while the reproductive line must keep its DNA intact across generations.11PubMed. Genome Instability and DNA Repair in Somatic and Reproductive Aging The stem cells that make sperm had the lowest mutation rate of any human cell type examined, partly because they divide slowly.12PubMed. The mutational landscape of human somatic and germline cells
Different cells also face different damage. Each cell type collects its own pattern of mutations, reflecting different kinds of DNA damage and repair.13PubMed Central. The Evolutionary Interplay of Somatic and Germline Mutation Rates Skin cells depend heavily on repair systems for sunlight damage, which reproductive cells, hidden inside the body, rarely need.14PubMed Central. The Evolutionary Interplay of Somatic and Germline Mutation Rates
By middle age every person’s tissues hold many cells with small somatic changes. Most do nothing noticeable, because they hit unimportant stretches of DNA or the cell dies. This background of changes is normal, and a routine test does not look for it.
Even with a low germline rate, each child carries some new changes not found in either parent. Most are harmless, but rarely one lands in an important gene and causes a genetic condition with no family history.
Why a Father’s Age Matters Most
A de novo mutation is a germline change that appears for the first time in a child, not inherited from either parent’s body. A child gains about 2 more new mutations for each extra year of the father’s age at conception.15PubMed Central. Rate of de novo mutations, father’s age, and disease risk The father’s share of new mutations roughly doubles every 16.5 years.16PubMed Central. Rate of de novo mutations, father’s age, and disease risk Once the father’s age is accounted for, the mother’s age adds little to the count.17PubMed Central. Rate of de novo mutations, father’s age, and disease risk The reason is that sperm-making cells keep dividing throughout a man’s adult life, adding a few mutations each year.18PubMed Central. Timing, rates and spectra of human germline mutation
In real families, children with a genetic disease caused by a new mutation had fathers about 1.3 years older on average than children whose disease was inherited.19PubMed Central. Paternal age effect in autosomal dominant or X-linked de novo variants identified by genome-wide sequencing So the effect is real but small for any single family.20PubMed Central. Timing, rates and spectra of human germline mutation21PubMed Central. Evolution of the germline mutation rate across vertebrates
For prospective parents, most children of older fathers are born without any genetic condition. The added new mutations mostly land in DNA that does nothing harmful. A genetic counselor can explain screening options if age is a concern.
To tell a new mutation from an inherited one, doctors test both parents’ blood to see whether either carries the change. If neither does, it is called a new mutation in the child. That usually means siblings have low risk, though not zero, because a parent can carry the change in some eggs or sperm only.
Mutations That Arise in the Early Embryo
Mosaicism means a person’s body contains a mix of cells, some with a mutation and some without. A change in the fertilized egg’s very first divisions can end up in all of the child’s tissues even though neither parent has it.22PubMed Central. De novo mutations, genetic mosaicism and human disease A change later in development may be in many cells of several tissues, or only part of one tissue like the brain.23PubMed Central. De novo mutations, genetic mosaicism and human disease
This makes mosaic changes easy to miss. A blood test may overlook a change present mainly in skin or brain. A parent can carry the change in some sperm or eggs without having it in blood, which is one reason a second child can occasionally inherit a “new” condition. Doctors sometimes test a second tissue such as skin or saliva.
If the mixed cells include eggs or sperm, the change can be passed on. If not, it behaves like an acquired (somatic) change affecting only that person.
How Each Kind Leads to Cancer
Cells pick up mutations as they divide during growth and normal tissue repair throughout life.24PubMed Central. Aging and the rise of somatic cancer-associated mutations in normal tissues A change in one of roughly 140 known cancer genes can give a cell a growth edge, letting it build a cluster of copies.25PubMed Central. Aging and the rise of somatic cancer-associated mutations in normal tissues These clusters are a normal part of aging and only sometimes gather enough further changes to become cancer.26PubMed Central. Aging and the rise of somatic cancer-associated mutations in normal tissues27PubMed Central. Aging and the rise of somatic cancer-associated mutations in normal tissues
Inherited changes give a head start. In hereditary cancer, a person is born with one damaged copy of a protective gene, and cancer needs a second somatic hit to the other copy.28PubMed Central. Hereditary cancer: two hits revisited In non-inherited cancer of the same type, both hits happen in body cells.29PubMed Central. Hereditary cancer: two hits revisited In children with cancer who carried an inherited mutation in a cancer-protective gene, about two-thirds of tumors showed the second hit.30PubMed Central. Germline Mutations in Predisposition Genes in Pediatric Cancer An inherited mutation therefore raises risk without guaranteeing cancer. Certain patterns suggest a cancer may be hereditary: cancer at an unusually young age, several relatives with the same or related cancers, or more than one separate cancer in one person.
A germline cancer result matters to relatives too. With dominant syndromes like BRCA, each child of a carrier has about a one-in-two chance of inheriting it. Relatives can test for the specific change, which is simpler than a first test, and carriers may get earlier or more frequent screening.
Reading Germline and Somatic Results on a Test Report
Doctors tell the two apart by comparing tumor DNA with normal DNA from blood or saliva. A change in both is usually inherited, and a change only in the tumor is usually acquired.
A liquid biopsy is a blood test that reads small pieces of tumor DNA floating in the blood. In patients with advanced tumors who had such a test, about 1.4% showed a suspected inherited cancer mutation.31PubMed Central. Identification of Incidental Germline Mutations in Patients With Advanced Solid Tumors Who Underwent Cell-Free Circulating Tumor DNA Sequencing So a test done to choose treatment may unexpectedly raise a question for your family. Combining tumor, blood and inherited testing found useful changes missed by routine testing in about a third of patients in one group.32PubMed Central. Combining germline, tissue and liquid biopsy analysis by comprehensive genomic profiling to improve the yield of actionable variants in a real-world cancer cohort
A change in blood is not always from the tumor. With age, blood stem cells can gain cancer-linked mutations and form clusters without any cancer, called clonal hematopoiesis (CHIP) on some reports.33PubMed Central. Clonal hematopoiesis in human aging and disease The same blood sample can hold tumor DNA, inherited DNA and these blood-cell changes at similar levels, so the amount alone cannot tell them apart.34PubMed Central. Cellular origins and evolutionary dynamics of plasma cfDNA in solid tumors: integrating germline, tumor-derived, and hematopoietic variation for MRD and treatment guidance Labs compare the blood DNA with white blood cells or use extra analysis to sort changes by origin.35PubMed Central. Fragmentomic-based algorithm to computationally predict tumor-somatic, germline, and clonal hematopoiesis variant origin in liquid biopsy
After a tumor result, it is reasonable to ask whether the change was checked against normal DNA from blood or saliva, and whether it is labelled somatic, germline, or suspected germline. If germline is suspected, ask for confirmation with a dedicated inherited test. Ask also whether to see a genetic counselor before telling relatives.
If CHIP appears on your report, it is not cancer and is common with age. It can make a blood test for tumor DNA look confusing, and doctors may pay closer attention to heart health. No treatment is usually needed for the finding alone.
Somatic Mutations That Help You
Not every somatic mutation is damage.36PubMed. Somatic hypermutation of immunoglobulin genes: merging mechanisms for genetic diversity
When a germ enters the body, immune cells switch on a mutation process in their antibody genes.37PubMed. The role of somatic hypermutation in the generation of antibody diversity The cells making the best-fitting antibodies are selected and multiply, giving a stronger defense.38PubMed. The role of somatic hypermutation in the generation of antibody diversity
This process explains why antibodies improve over weeks after an infection or vaccination, and why booster doses give the system another round to refine them. These improvements stay in that person and are never passed to children.
Somatic Versus Germline Gene Editing
Somatic gene therapy changes non-reproductive cells and affects only the person treated.39Genome.gov. What is genome editing? Germline editing changes eggs, sperm or embryos, so every cell and future generations carry the change.40Harvard Gazette. Harvard researchers share views on future, ethics of gene editing Most scientists agree germline editing should not be done now, and many countries regulate against it.41Genome.gov. What is genome editing? Germline editing raises particular concerns: mistakes could not be undone in later generations, future people cannot consent to the change, and safety cannot be fully tested within one lifetime.
For patients, somatic therapy targets cells such as blood or eye cells in one person. A treated parent can still pass the original disease gene to children, so families should still consider genetic counseling after therapy.
Exceptions in Mitochondria and Plants
Mitochondria, the cell’s energy units, have their own DNA inherited from the mother. The share of mitochondria carrying a variant can shift much more between mother and child than within one body.42PubMed Central. Bottleneck and selection in the germline and maternal age influence transmission of mitochondrial DNA in human pedigrees Whether somatic mitochondrial changes cause disease or aging remains uncertain.43PubMed Central. Human mitochondrial DNA: roles of inherited and somatic mutations Plants blur the line differently: they lack a separate reproductive line, so mutations in a growing tip can reach seeds.44Nature Research Intelligence. Somatic Mutation Dynamics in Plant Systems Larger, longer-lived plants have more chances to pass such changes on.45PubMed Central. Population-level consequences of inheritable somatic mutations and the evolution of mutation rates in plants
For families, a mother with a low level of a mitochondrial variant may have children with higher or lower levels. Fathers generally do not pass on mitochondrial DNA.