Mutations that are not inherited are called somatic mutations. They occur in your body’s cells after conception, meaning they weren’t passed down from either parent and can’t be passed to your children. Every person accumulates thousands of these mutations over a lifetime, in cells ranging from skin and blood to brain and colon tissue.
The key distinction is simple: inherited mutations exist in egg or sperm cells and get copied into every cell of a new organism, while somatic mutations arise spontaneously in ordinary body cells and stay there. Understanding the difference matters because somatic mutations drive most cancers and play a central role in aging.
Somatic vs. Germline Mutations
Your DNA can change in two fundamentally different ways. Germline mutations occur in reproductive cells (eggs and sperm) and are present from the moment of conception. Because every cell in the body develops from that initial fertilized egg, a germline mutation appears in every cell and can be passed to the next generation. These are the mutations that run in families.
Somatic mutations happen in any cell that is not an egg or sperm. They occur after conception, at any point during your life, and they only affect the cell where they happened plus any cells that descend from it through division. A somatic mutation in a skin cell, for instance, will never show up in your liver or your children. It stays local.
There is also a middle category worth knowing about: de novo mutations. These are brand-new changes that appear in a child’s DNA but were not present in either parent’s genome. They can arise during the formation of a parent’s egg or sperm, or in the first few cell divisions after fertilization. Each person is born with roughly 100 to 200 of these novel mutations. De novo mutations that occurred in the egg or sperm are technically present in every cell and can be passed on, but they weren’t “inherited” in the traditional sense because neither parent carried them.
What Causes Somatic Mutations
Somatic mutations come from two broad sources: mistakes your cells make on their own, and damage from the outside world.
Every time a cell divides, it copies about 3 billion letters of DNA. The molecular machinery that handles this job is remarkably accurate, but it’s not perfect. Occasionally the wrong letter gets inserted, and while cells have built-in proofreading and repair systems, some errors slip through. These replication errors are the most common internal source of somatic mutations. Cells that divide frequently, like those lining the colon or circulating in the blood, accumulate more of these mistakes simply because they copy their DNA more often.
External factors accelerate the process. Ultraviolet radiation from sunlight is one of the best-studied examples. UV light directly damages DNA in skin cells, creating structural distortions that can lead to permanent letter swaps if not repaired before the next round of cell division. This is why decades of sun exposure raise the risk of skin cancer. Ionizing radiation, tobacco smoke, and other DNA-reactive chemicals work through similar principles: they damage the DNA molecule, and imperfect repair turns that damage into a permanent mutation.
Internally, reactive oxygen species (byproducts of normal metabolism) also attack DNA. Your cells neutralize most of these, but the defense systems become less efficient with age, which partly explains why mutations pile up faster in older tissues.
How Somatic Mutations Accumulate With Age
Somatic mutations are not a one-time event. They build up steadily throughout life, and the rate varies by tissue. In healthy blood cells, researchers have measured roughly 16 to 25 new single-letter DNA changes per cell per year. Skin cells accumulate around 24 per year. Brain neurons pick up about 16 to 21 per year. Colon cells have the highest known rate: approximately 49 to 56 mutations per cell per year.
To put this in real terms, a newborn’s blood cells carry fewer than 500 somatic mutations per cell. By the time a person reaches 100 years old, that number climbs above 3,000. In lung cells from nonsmokers, one study found 464 mutations per cell in an 11-year-old compared to 2,739 in an 86-year-old. This steady accumulation is one of the universal features of aging across all human tissues that have been studied.
Somatic Mutations and Cancer
Cancer is, at its core, a disease of somatic mutations. The transformation of a normal cell into a cancerous one happens through a sequence of acquired genetic changes that give the cell a growth advantage. No tumor can arise without somatic mutations, though the number needed is surprisingly small. What matters is which genes get hit.
The process works like evolution in miniature. A mutation that lets a cell grow slightly faster than its neighbors allows that cell to multiply into a larger group. That larger group then provides a bigger target for the next mutation. Over time, a handful of these “driver” mutations stack up in the same cell lineage, eventually producing a cell that divides uncontrollably. This is why cancer risk rises sharply with age: more years means more accumulated mutations and more chances for the right combination to occur.
Environmental factors contribute by either increasing the mutation rate directly (like UV radiation causing DNA damage in skin cells) or by increasing the rate of cell division (like chronic inflammation from an infection forcing tissue to constantly repair itself). Some factors do both. The randomness inherent in somatic mutation also explains why cancer can strike people with no known risk factors. There is always an element of chance.
Mosaicism: When Somatic Mutations Shape the Body
Because somatic mutations only affect the cell where they occur and its descendants, they can create a patchwork pattern in the body where some cells carry a mutation and others don’t. This is called mosaicism.
Everyone is a mosaic to some degree. Most somatic mutations are harmless and go unnoticed. But when a somatic mutation happens very early in development, during the first few cell divisions after fertilization, it can end up in a large fraction of the body’s tissues. If that mutation affects a gene involved in growth or development, the results can be dramatic.
Proteus syndrome is a well-known example. It’s caused by a somatic mutation in a growth-signaling gene that occurs early in embryonic development, leading to asymmetric overgrowth of bones, skin, and other tissues. The mutation is never inherited because it only exists in a subset of the affected person’s cells, not in their egg or sperm cells. Other mosaic conditions include certain forms of tuberous sclerosis (which causes noncancerous growths in multiple organs) and focal cortical dysplasia (a brain malformation that can cause epilepsy). All of these are caused by mutations that arose after conception and cannot be passed to the next generation.
Somatic Mutations vs. Epigenetic Changes
Not every non-inherited change to your DNA is a mutation. Epigenetic modifications, particularly DNA methylation, alter how genes are read without changing the underlying DNA sequence. These chemical tags accumulate with age in patterns so predictable they’re used as “biological clocks.” While both somatic mutations and epigenetic changes build up over time and are not inherited in the traditional sense, they are fundamentally different: a somatic mutation permanently changes a letter in the DNA code, while an epigenetic change is a reversible chemical modification layered on top of it.
Interestingly, the two processes are biochemically connected. Methylated cytosine (a common epigenetic mark) is chemically unstable and prone to spontaneously converting to thymine over time, creating a permanent somatic mutation. So some somatic mutations are actually a direct consequence of prior epigenetic modifications, linking these two hallmarks of aging at the molecular level.