Mutations

A mutation is a random change in a living thing's DNA. Mutations happen all the time. Every time a cell copies its DNA before dividing, there is a tiny chance of a copying mistake. Mutations can also be caused by radiation, chemicals or just bad luck. Most mutations are harmless, some cause illness, and a very few are surprisingly helpful: these rare beneficial mutations are the raw material that natural selection uses to drive evolution. Without mutations, no new species could ever exist.

• Cell copy accuracyapprox. 1 mistake per billion basesVery accurate, but not perfect
• Mutations per person per generationapprox. 70 to 100New DNA changes inherited from parents
• Most areNeutralHave no effect at all
• Some areHarmfulCan cause genetic diseases
• A few areHelpfulThe raw material of evolution
• Main causesRandom + radiation + chemicalsPlus viruses and copying errors

What is a mutation?

The DNA in your cells is essentially a long sequence of letters (A, T, G, C). A mutation is any change to that sequence:

• Point mutation: one letter is swapped for another. The smallest possible kind of mutation.
• Insertion: extra letters are added to the sequence.
• Deletion: letters are removed.
• Duplication: a section of DNA is accidentally copied twice in a row.
• Chromosome change: whole sections of chromosomes get swapped, flipped or fused.

The smallest mutation might change just one letter out of three billion. The biggest can change whole chromosomes at once.

What causes mutations?

Mutations come from a few main sources:

• Copying mistakes: every time a cell divides it has to copy all 3 billion letters of its DNA. The cell's copying machinery is incredibly accurate but not perfect. One mistake per billion bases means roughly 3 new mutations per cell division.
• Ultraviolet light: UV rays from the Sun can break DNA bonds in skin cells, which is why too much sun exposure causes mutations that can lead to skin cancer.
• Other radiation: X-rays, gamma rays and other high-energy radiation can also damage DNA.
• Chemicals: certain chemicals (especially the tar in tobacco smoke) react with DNA and cause mutations. This is one of the main reasons smoking causes cancer.
• Viruses: some viruses (especially retroviruses) insert their own DNA into the host's genome, which is a mutation.

Most mutations do nothing

The vast majority of mutations have no effect at all. There are several reasons:

• About 98% of human DNA does not code for any protein, so a mutation there usually changes nothing.
• The genetic code has built-in redundancy: many different DNA letter combinations produce the same amino acid, so a mutation in the third letter of a codon often makes no difference.
• Cells have repair systems that fix many mutations before they cause problems.
• Most mutations happen in cells that will not be passed on to children (skin, muscle, gut cells). Only mutations in egg or sperm cells get inherited.

When mutations cause problems

A small number of mutations land in important genes and have noticeable effects. Most of those effects are harmful.

• Genetic diseases: cystic fibrosis, sickle cell disease, haemophilia and many others are caused by specific harmful mutations in single genes.
• Cancer: mutations that break the genes that control cell division can lead to cancer.
• Birth conditions: some mutations cause changes in how an embryo develops, like Down syndrome (caused by an extra copy of chromosome 21).

It is important to remember that almost every person carries some harmful mutations without ever knowing it, because they only have one bad copy of a recessive gene and never meet the second.

When mutations are helpful

Occasionally, a mutation turns out to be beneficial. Some famous examples in humans:

• Lactose tolerance: most adult mammals cannot digest the milk sugar lactose. About 10,000 years ago a mutation appeared in some northern Europeans and East Africans that kept the lactose-digesting gene switched on into adulthood. People with this mutation can drink milk all their lives without problems.
• High-altitude survival: Tibetans carry a mutation in a gene called EPAS1 that helps them get enough oxygen in the thin air of the Himalayas. The mutation appears to come from an ancient extinct human cousin called the Denisovans.
• Resistance to HIV: a small percentage of Europeans carry a mutation in a gene called CCR5 that makes them resistant to HIV infection.

Fact You were born with about 70 to 100 new mutations that neither of your parents had. Most of them are completely harmless. They are simply the random mistakes that happened while your parents' cells were copying their DNA in the egg and sperm that became you. The same will happen to your children, and theirs, generation after generation.

Mutations and evolution

Mutations are the only way for living things to get genuinely new features. Sexual reproduction just shuffles the existing genes around; only mutations can create something genuinely different. That is why mutations are the raw material of evolution: natural selection then picks out the helpful ones and gradually filters out the harmful ones. Over millions of years, this process builds the astonishing variety of life on Earth.

Did you know? Modern medicine uses mutations on purpose to make sure vaccines work. Many vaccines (like the flu vaccine) contain weakened versions of the virus that have been mutated to be unable to cause disease but still able to train your immune system. The mRNA Covid-19 vaccines work in a different but related way: they give your cells a short piece of synthetic mRNA that tells them to make a harmless copy of the virus's spike protein, training the immune system before any real virus arrives.

Deeper dive: superpowers and the myth of "good" mutations

Films and comics often portray mutations as the source of dramatic new abilities: X-ray vision, super-strength, telepathy. Real biology is much more modest. Most beneficial mutations in humans give tiny, subtle improvements: slightly better digestion of one kind of food, a small increase in resistance to one specific disease, a modest change in eye colour or hair colour. There is no known mutation that gives a person any kind of superpower.

The reason is that the human body is already extraordinarily well-tuned by 4 billion years of evolution. Almost every possible "obvious" improvement has already been tried. Most random mutations that change the body in any noticeable way break something that was working, rather than improve it. The few mutations that ARE beneficial are usually tiny tweaks to existing systems.

Even the most famous "beneficial mutation" stories are subtle. Lactose tolerance is just one switched-on gene that produces an enzyme already present in all babies. The Tibetan high-altitude adaptation is a single base change that adjusts how a particular gene responds to oxygen levels. These are not superpowers; they are tiny edits to existing machinery.

What evolution can do given enough time is much more impressive than any single mutation: through billions of years and trillions of small changes, it has shaped bacteria into trees, fish into birds, and shrew-like mammals into elephants, whales and humans. But each individual step along the way was just a tiny random tweak that happened to work slightly better than what was there before.

For how mutations drive evolution, see natural selection. For the rules of how genes get passed on, see inheritance.