Identical Twins and DNA: Are Their Genes 100% the Same?

Do Identical Twins Have the Same DNA?

In the UK, roughly 12,000 sets of twins are born every year, and about one-third are identical. At AssureDNA, we’re often asked whether identical twins share exactly the same DNA — and whether a DNA test could ever tell them apart.

The short answer: not quite.

While identical twins start out with matching genetic blueprints, small differences develop before they’re even born. Understanding these subtle changes helps explain why one twin might develop freckles while the other doesn’t — and why, in DNA testing, identical twins pose a very unique challenge.

What Are Identical Twins?

Identical twins (scientifically called monozygotic twins) form when a single fertilised egg splits into two embryos soon after conception. Because both embryos come from the same egg and sperm, they begin with the same genetic code — the same “instruction manual” for building a person.

That’s why identical twins are almost always the same sex. Rare cases of twins of different sexes arise only through very unusual chromosomal changes during early development.

By contrast, fraternal twins (dizygotic twins) form when two separate eggs are fertilised by two different sperm. They share about 50% of their DNA — just like any other siblings — and can be of different sexes.

How DNA Is Inherited

Every child inherits half their DNA from their mother (via the egg) and half from their father (via the sperm). When the fertilised egg splits to create identical twins, both embryos start with the exact same combination of parental DNA.

From that point on, however, random mutations and epigenetic changes begin to shape each twin’s genome in small but measurable ways.

So although identical twins begin as genetic carbon copies, development ensures they end up with tiny but real differences.

Do Identical Twins Share 100% of Their DNA?

It’s easy to assume so, but even identical twins aren’t completely identical at the genetic level.

Scientists have shown that identical twins differ by dozens to hundreds of single DNA “letters” — tiny variations known as mutations that occur naturally as cells divide in early development. On average, identical twins carry about five unique mutations that the other doesn’t. These are harmless most of the time, but they create a measurable genetic difference.

Even more fascinating: in about 15% of identical twin pairs, one twin carries a noticeably greater number of mutations than the other. That means identical twins don’t just look slightly different over time — their genomes quietly drift apart as they grow.

Epigenetic Differences

Even when the DNA sequence is the same, how that DNA behaves can differ. This is where epigenetics comes in — the system of chemical “switches” that turn genes on or off.

Factors such as stress, nutrition, and environmental exposures (like pollution or smoking) influence these switches. Over years, they help explain why identical twins who share the same DNA might differ in personality, health, or appearance.

Epigenetic differences are why one twin might develop eczema, asthma, or type 1 diabetes while the other doesn’t. The underlying genes are there in both — but they’re expressed differently.

Copy Number Variations and Mosaicism

While point mutations and epigenetic changes are subtle, sometimes larger structural differences occur. Occasionally, whole segments of DNA are duplicated or deleted in one twin but not the other — a phenomenon known as copy number variation (CNV).

In rarer cases, small groups of cells within one twin may carry slightly different DNA from the rest of their body — a process called mosaicism.

Mosaicism can happen during early cell division when a mutation appears in one cell line but not another. From that moment, two genetic “versions” of the same person coexist in different tissues.

These phenomena are uncommon, but they make the genetic landscape of each twin completely unique — even though both began from a single fertilised egg.

Why It Matters for DNA Testing

In everyday life, these differences don’t mean much — but for DNA testing, they matter a great deal.

For example, in paternity testing, if both twins are potential fathers, a standard test will show both as a biological match. That’s because traditional tests examine only a limited number of DNA markers (typically 16–23 STR markers), which are identical between identical twins.

However, at AssureDNA, our advanced SNP-based testing platform analyses hundreds of thousands of markers — a far deeper level of genetic information. While identical twins remain extremely similar, this level of resolution can help identify subtle variations that older testing methods would completely miss.

That’s one of the reasons we upgraded our technology — so customers receive results that are not just accurate, but scientifically future-proof.

Summary: Are Identical Twins Genetically the Same?

Identical twins begin life sharing the same genetic foundation, but nature never makes perfect copies. As their cells divide and grow, small mutations, copy number changes, and epigenetic differences emerge — making each twin a one-of-a-kind version of the same blueprint.

So while identical twins look alike and share nearly all their DNA, no two people — not even identical twins — are ever truly 100% the same.

At AssureDNA, we use the most advanced genetic testing methods available in the UK to analyse even the most complex family relationships. Whether you’re exploring your own DNA out of curiosity or seeking biological confirmation, our accredited team ensures accuracy, discretion, and clarity in every result.

If you’d like to learn more about our at-home sibling or relationship DNA tests, contact us at info@assuredna.co.uk or call 01603 358169 to speak to a member of our team.