DNA Test Differences Surprise Identical Triplets, Uncover Misconceptions

Identical twins have identical DNA, right? That's what makes them identical. So why did these triplets get different results on their DNA test?

Meet the Dahm triplets.

Nicole, Erica, and Jaclyn Dahm are indistinguishable from one another.

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The identical triplets are so similar that Erica has no trouble getting into Nicole's fingerprint-lock safe. Even the whorls of their fingertips are the same.

That shouldn't be surprising. Look at the way identical twins (and by extension, triplets, quadruplets, and so on) are formed in the first place. A single fertilized egg spontaneously splits in two—or, in this case, three. Each of these embryos then develops into a full fetus.

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Because triplets like the Dahms share identical genetic information, common knowledge dictates that their DNA should match. Although environmental factors can switch certain genes on and off throughout a twin's lifetime, the DNA itself shouldn't differ among the sisters at all.

At least that's what the science textbooks said about monozygotic twinning when we were in high school.

So what was the deal with the at-home DNA kit that the Dahm triplets sat discussing on 2017 episode of The Doctors?

"How would you feel if your ancestry was different?" Dr. Travis Stork, host of The Doctors, asked the Dahm triplets.

"I don't know how that could happen," said Nicole. "We're one egg that split, and we all came out of our mother, so maybe a little different DNA, but we still have the same ancestry, right?"

YouTube/The Doctors

Note that Nicole expects to find slight differences in DNA among her and her sisters. She could be referring to the environmental effect on genetic expression—what doctors call "epigenetic factors"—or she might mean something deeper. Either way, the sisters seem willing to bet anything that they'll share the same ancestral background.

The Results of the DNA Test

Investigative reporter Lisa Guerrero of Inside Edition arranged a little experiment with the Dahm triplets. She gave each of them two home DNA tests and then analyzed the results. She shared the news with the triplets on the set of The Doctors, and what she discovered left the triplets confused and even shocked.

The first test was no surprise. It verified that the Dahms are, in fact, identical triplets. But as Guerrero delved deeper into the second test, things started to get surprising.

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"Nicole, you're 18 percent British and Irish," Guerrero told the stunned sisters. "Erica, you're 15 percent British and Irish. But Jaclyn, you're almost 19 percent British and Irish."

The differences kept on coming. The test found that Nicole was 11 percent French and German, whereas her sister Erica had more than twice the Germano–Gallic blood that Nicole has. The test said Erica was 23 percent French and German.

"I am surprised, though, because we came from the same egg," said Nicole. "And we have the same DNA."

Do they, though?

The Truth About Identical Twins and DNA

High school biology classes were very clear on this fact in the 1990s: Identical twins have identical DNA. It was the accepted wisdom.

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In 2008, though, researchers from the University of Alabama at Birmingham, along with universities in the Netherlands and Sweden, conducted a new round of studies on monozygotic twins. The researchers expected to find epigenetic differences. What they actually found went much deeper.

"What we found are changes on the genetic level, the DNA sequence itself," Jan Dumanksi, a genetics professor at the University of Alabama at Birmingham and an author of the paper, told the New York Times shortly after the research was published in The American Journal of Human Genetics.

The research team found several copy number variations—in which genes occur multiple times within one individual, or in which certain DNA coding letters are missing—between pairs of twins they expected to have identical genetic profiles.

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Scientists don't know whether these slight variations were present in the womb or if they develop as the twins age. All they know is that the textbooks were wrong. Identical twins don't necessarily have the same DNA, and that also holds for triplets.

Explaining the Differences Between the Dahm Triplets

After breaking the odd news to the Dahm triplets, Guerrero turned to Stork for answers.

"So, we found a few discrepancies," Guerrero said that day on the set of The Doctors. "Dr. Travis, what does that say to you? When you look at the fact that they're identical triplets, what is your takeaway from these tests?"

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Ever the diplomat, Stork equivocated for a moment.

"I'm not a geneticist," he said. "But I love the idea of these at-home tests for fun."

Later, Stork told his audience that "we're not to a place yet where you can just spit in a cup and have every single answer that you're looking for."

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The implication seems to be that the test was flawed. Stork never said this outright; his is not the show to cast doubt on another company's product. But through his carefully worded assessment, it seems pretty clear he was warning viewers not to take self-administered DNA tests too seriously—yet.

On the other hand, maybe copy number variations in the sisters' genes could account for the minor differences from one sister to the identical next. Genetics is a complicated business. Just ask the Dahm triplets.

Would you trust a home test kit to tell you about your family history?

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