Short answer? Yes. It happens. It’s rare, but it happens.
Most people think twins are identical copies of each other, but that’s not always the case. When it comes to Down syndrome, one twin can have it while the other doesn’t. It all depends on how the twins formed in the womb.
And this isn’t just some medical anomaly in textbooks. It’s real. It’s been studied. It’s been seen in real families.
And if you’re a parent, a twin, or just someone curious about genetics, buckle up. We’re breaking it all down in plain English—no confusing science talk, no sugarcoating.
How Can One Twin Have Down Syndrome but Not the Other?
Twins come in two main types:
- Identical (monozygotic): One fertilized egg splits into two. They have nearly identical DNA.
- Fraternal (dizygotic): Two separate eggs, two different sets of DNA. Like siblings born at the same time.
Now, logic says that identical twins should both have Down syndrome if one does, right? Same DNA, same everything. But here’s where nature throws a curveball—mosaicism or post-zygotic mutations.
The Science (Simplified)
- Down syndrome happens because of an extra chromosome 21. Normally, people have two copies of chromosome 21, but someone with Down syndrome has three (that’s why it’s called Trisomy 21).
- If this extra chromosome is present from the very beginning—before the twin split—both will have Down syndrome.
- But if the extra chromosome shows up after the split, only one twin gets it. That’s what makes this rare but possible.
For fraternal twins, it’s more straightforward. They’re just like any two siblings. One can have Down syndrome, and the other can be completely typical. Just genetics rolling the dice.
Real-Life Cases of One Twin Having Down Syndrome
This isn’t just some theory. There are actual cases out there.
Case Study: Identical Twins, But Not Identical Outcomes
A study in the National Library of Medicine reported a case of monozygotic twins where only one had Down syndrome.
They ran genetic tests and found that the extra chromosome happened after the embryo split. Their DNA was 99.9% the same, but that tiny difference changed everything.
A Parent’s Story
There are moms who’ve shared their experiences online. Imagine being pregnant with twins, getting the news that one has Down syndrome, but the other doesn’t. It’s confusing, overwhelming, and comes with a million questions.
Some parents say they grieved at first—not for the child, but for the expectations they had. Others describe a deep fear: Will one child resent the other? Will people compare them? But over time, they adjust. They learn. They love.
Raising Twins When One Has Down Syndrome
Alright, let’s get real. Because this isn’t just about science—it’s about real life.
1. Medical Differences Can Be a Big Deal
One twin might need way more doctor visits, therapy, and medical support.
- Many babies with Down syndrome have heart defects. Some need surgery right away.
- Low muscle tone can make things like sitting up, crawling, or walking take longer.
- Speech therapy, physical therapy, occupational therapy—there’s a lot to keep up with.
Meanwhile, the other twin? Hitting milestones, needing fewer doctor visits, maybe even feeling a little “left out” of all the attention. It’s a balancing act.
2. People Say Dumb Stuff
If you’re a parent, you’ll hear it. If you’re a twin, you’ll probably get asked weird questions.
- “They don’t even look like twins!”
- “Are you sure they’re identical?”
- “I thought Down syndrome was genetic?”
People mean well, but they don’t always get it. Sometimes you educate them. Sometimes you ignore them.
3. The “Normal” Twin Takes on a Unique Role
Siblings of kids with disabilities often become protectors, teachers, or even second parents.
- Some kids love that role. They step up, help their twin, and feel a deep sense of connection.
- Others struggle. They might feel jealous of the attention their sibling gets. They might get frustrated if their twin can’t keep up.
One mom shared that her “typical” twin was always defending his brother at school. Any time another kid made a comment, he was ready to fight. Not literally (well, sometimes), but he took on that protector role early.
4. They Have an Unbreakable Bond
At the end of the day, twins are twins. The connection is still there. The laughs, the inside jokes, the sibling fights—it all still happens.
Some parents worry that one twin will feel burdened. Others say their child wouldn’t change a thing. The love is just built different.
Can This Happen With Triplets?
Yep. Same logic applies. If you have three babies in one pregnancy, one could have Down syndrome, and the others might not. The more babies in the mix, the more genetic chances at play.
There are cases of triplets where only one has Down syndrome, just like with twins. Again, it all comes down to when that extra chromosome showed up.
The Emotional Rollercoaster for Parents
Let’s talk about the emotions. Because this isn’t just about genetics. It’s about real people going through this in real time.
1. The Shock Factor
Most parents don’t expect this. It’s rare. And when doctors say, “One twin has Down syndrome, but the other doesn’t,” it’s a shock.
Some feel:
- Fear (What does this mean for their future?)
- Guilt (Did I do something wrong?)
- Confusion (How is this even possible?)
2. The Comparison Trap
Watching one twin reach milestones while the other takes longer can be heartbreaking. Even when you know every child moves at their own pace, the side-by-side development is hard to ignore.
One mom said she cried when one twin started walking and the other was still struggling to stand. Not because she wasn’t proud—just because it reminded her of the differences.
3. Finding Community Helps
Talking to other parents who get it makes a difference. There are Facebook groups, in-person meetups, and Down syndrome advocacy organizations where families connect.
It helps to hear from parents who’ve been through it. Who know the ups and downs. Who can say, “Yeah, it’s tough sometimes, but the love? The joy? Unmatched.”
What Are the Odds?
Alright, so how rare is this, really?
- Fraternal twins: The odds are the same as any pregnancy—about 1 in 700 in the U.S.
- Identical twins: It’s much less common because they start with the same DNA. But if that extra chromosome appears after the split? Then, yeah, one twin can have Down syndrome while the other doesn’t.
There’s no way to predict it. No test can say for sure if this will happen. It’s just biology doing its thing.
Final Thoughts
Can Down syndrome affect only one twin in a pair? Yes. It’s rare, but it happens.
It all comes down to when that extra chromosome appears. If it happens before the egg splits? Both twins will have Down syndrome. If it happens after? Only one twin does.
For families dealing with this, the journey looks different. But the love? That’s the same. Twins are twins—Down syndrome or not.
