How Identical Are Identical Twins?

It’s something we all learn about in science class—the way a fertilized egg becomes an embryo, which then forms fraternal or identical twins. But these educational experiences just scratch the surface when it comes to the fascinating phenomena of twins. Once identical twins start growing up, it's natural to wonder about the question“nature vs. nurture." After all, no two people are truly identical in every way, right? So, how identical are identical twins?

Disappointed in their children facts Shutterstock

Double Trouble

As identical twins mature, they start to express differences in both their personality and appearance. Even so, one thing never changes: their genetic makeup. In the case of fraternal twins, two eggs are fertilized at the same time by two different sperm cells. But in the case of identical twins, there’s just one sperm cell that fertilizes one single egg.

When that egg splits, the two parts form identical twins. Each twin will have different fingerprints, while other facts remain identical. Identical twins are always the same gender and they always share the same blood type. But is their genetic makeup the same? For a long time, scientists thought yes—but it may be more complicated than that.

Female Scientists facts Pixabay

Sign up to our newsletter.

History’s most fascinating stories and darkest secrets, delivered to your inbox daily. Making distraction rewarding since 2017.

The Question of Genetic Makeup

When a fertilized egg splits, each part contains the same genome, or set of genetic instructions. Twins keep those genomes, which many doctors believed were completely identical. However, a recent study contradicts this assumption. In the study, scientists compared the genomes of adult identical twins very, very closely. Lo and behold, they found a very specific difference.

Each twin may carry a different number of a specific type of gene. So, for example, one twin may carry six copies of the gene for brown hair, while his or her twin could carry eight (copies of genes can appear between zero and 14 times). Identifying this difference could give geneticists the ability to more closely identify particular regions of a gene that coincide with particular diseases.

Strangest Coincidences Facts Shutterstock

The Limits of Identical Thinking

Another point of similarity between identical twins, historically, was gender, but there have also been reported cases of one of a pair of identical twins identifying as transgender. So, between that and the recent discoveries about genetics, that leaves just one way in which every pair of identical twins are 100% identical: blood type.

Sources: 1, 2, 3, 4, 5

More from Factinate

Want to tell us to write facts on a topic? We’re always looking for your input! Please reach out to us to let us know what you’re interested in reading. Your suggestions can be as general or specific as you like, from “Life” to “Compact Cars and Trucks” to “A Subspecies of Capybara Called Hydrochoerus Isthmius.” We’ll get our writers on it because we want to create articles on the topics you’re interested in. Please submit feedback to contribute@factinate.com. Thanks for your time!

Do you question the accuracy of a fact you just read? At Factinate, we’re dedicated to getting things right. Our credibility is the turbo-charged engine of our success. We want our readers to trust us. Our editors are instructed to fact check thoroughly, including finding at least three references for each fact. However, despite our best efforts, we sometimes miss the mark. When we do, we depend on our loyal, helpful readers to point out how we can do better. Please let us know if a fact we’ve published is inaccurate (or even if you just suspect it’s inaccurate) by reaching out to us at contribute@factinate.com. Thanks for your help!