Identical or Fraternal? What a DNA Test Can Settle for Twins

Ask a set of twins whether they're identical and you'll sometimes get a shrug. Maybe a doctor said "fraternal" because there were two placentas. Maybe Grandma insists they're identical because nobody can tell them apart in photos. Maybe they look a bit different and assumed that settled it.

None of those clues is reliable. DNA is. Here's why the usual signs can mislead, and how different kinds of DNA test can settle the question, sometimes cheaply.

Identical vs. fraternal in one minute

Identical (monozygotic) twins come from one fertilized egg that splits into two embryos. They start out with the same genome.

Fraternal (dizygotic) twins come from two separate eggs, each fertilized by a different sperm. Genetically they're ordinary siblings who happened to share a womb, sharing about half their DNA on average.

One shortcut is solid: boy-girl twins are fraternal. The question only really comes up for same-sex pairs.

Why placentas and looks can mislead

The placenta myth

A lot of twins were told at birth that two placentas meant fraternal. That isn't true. Whether identical twins share a placenta depends on when the embryo splits. Twins Research Australia explains that about a third of identical twins split early, before the placenta forms, and so develop with separate placentas, just like fraternal twins. The other two-thirds share one.

It can go the other way too. Two separate placentas can grow close enough to fuse, which can make fraternal twins look identical at delivery.

This mix-up is common enough to drive real demand for testing. A study of zygosity test requests published in Twin Research and Human Genetics found that many came from the mistaken assumption that all twins with two placentas (dichorionic twins) are fraternal. In that group of 405 tested multiples, 93% turned out to be identical.

The "they don't look alike" problem

Identical twins aren't carbon copies. Position in the womb, birth weight, injuries, lifestyle and plain chance all leave marks, and the same study noted that parents sometimes concluded their identical twins were fraternal because of small differences between them. Meanwhile, fraternal siblings can look strikingly similar. Appearance is a hint at best.

How DNA settles it

The logic is simple. Compare the two genomes at enough places where people commonly differ. Fraternal twins, like any siblings, will differ at many of them. Identical twins will match at essentially all of them.

There's no in-between result to puzzle over. Full siblings share about 50% of their DNA on average, and identical twins share close to 100%. That gap is enormous, so any decent test separates them cleanly.

Using a consumer ancestry test

If both twins already test with the same company, or want the ancestry reports anyway, the relative-matching feature will usually answer the question as a bonus. Both twins need to opt in to matching.

• Fraternal twins show up as full siblings, with roughly half their DNA shared, just as any brother or sister would.
• Identical twins show up as a match sharing essentially everything. 23andMe's own help article on relative matching lists "identical twin" as its own row in its table of shared DNA, at 100%.

Not every service has a dedicated "identical twin" label, and wording changes over time, so don't get hung up on what the relationship box says. If the shared DNA is near the maximum the service reports, or the match looks the way the company says your own duplicate test would, that's the tell.

For more on why those estimates move around, see how ancestry percentages actually work and why siblings get different ancestry results.

Dedicated zygosity tests: the simpler option

If all you want is the answer, a twin zygosity test is usually cheaper than two ancestry kits. These are typically cheek-swab tests from DNA-testing labs. Check the current price, since it varies by lab and country.

They work by comparing a selected set of genetic markers. The Washington State Twin Registry explains that markers are chosen because they often differ between fraternal twins. If every marker matches, the twins are very likely identical, with better than a 99% chance. A difference at one or more markers means fraternal. The more markers tested, the more reliable the result. These markers say nothing about health or appearance. They're just there to compare.

The same registry describes DNA testing as the only way to be sure, and keeps a list of labs that offer zygosity testing. A few practical tips:

• Choose a lab that tests both twins in the same run and gives a clear report.
• For newborns, ask the lab about sample timing and collection methods.
• Read the lab's data-retention policy if that matters to you.

Why a whole-genome test can still show tiny differences

Here's the fun twist. "Identical" twins aren't perfectly identical at the DNA level.

After the embryo splits, and sometimes just before, cells keep dividing, and every so often a new mutation creeps in. A 2021 study in Nature Genetics by deCODE genetics sequenced 387 pairs of identical twins along with their parents, spouses and children. On average, the twins differed by about 5.2 early developmental mutations. In roughly 15% of pairs, one twin carried a substantial number of mutations the other didn't.

Put that in perspective: a handful of differences across about three billion letters. It doesn't blur the identical-versus-fraternal line at all. Fraternal twins differ at vastly more positions.

If identical twins each get whole genome sequencing and compare their files, they may see a small number of differences. Some could be real early mutations. Many will be technical: a spot one test read poorly, a call that fell just below a quality threshold, or a difference in how the two samples were processed. Treat any isolated difference with caution, and don't read medical meaning into it without professional help. If you're weighing a full genome against a cheaper chip test, our WGS vs. genotyping comparison explains what each one can see.

Same DNA, different lives: epigenetics

Even with near-identical genomes, identical twins can differ in health, habits and how they age. Part of the reason is epigenetics, the chemical tags that help switch genes on and off without changing the DNA sequence.

A well-known 2005 study in PNAS looked at 80 pairs of identical twins aged 3 to 74. Younger pairs had very similar epigenetic patterns, while older pairs differed more, and twins whose lifestyles had diverged more showed bigger differences. We cover how this works, and what it does and doesn't mean, in epigenetics vs. genetics.

Why knowing can matter

For many twins it's simple curiosity, and that's a perfectly good reason. In the published study of zygosity requests, about half came from curiosity or a wish for certainty. Others were prompted by current or future health questions, or by other twins in the family.

Knowing your zygosity also helps if you join twin research registries, and it can be useful background when a doctor is assessing a condition that runs in families. If a health question is behind your interest, talk it through with a clinician or genetic counselor. They can tell you whether zygosity actually changes anything in your situation.

Common questions

Medical disclaimer. GenomeTesting.org publishes educational content about genetics and consumer genomic testing. Nothing here is medical advice, a diagnosis, or a substitute for care from a qualified clinician or genetic counsellor. Decisions about testing, treatment, or interpreting your own results should be made with a healthcare professional who can see your full history.

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