Chimeras, Mosaics, and Other Fun Stuff

I heard it is possible for two twins to have different fathers. If these two twins, each with a different father, conjoin together, would the conjoined baby have two fathers? Is this even possible? If so, how would the genetics work for the conjoined baby?

-A high school student from China

September 23, 2015

First off, yes, it is possible for twins to have different fathers. This goes by the exciting name heteropaternal superfecundation. It is much more common in other animals but it can and does happen in people too.

And of course twins can be conjoined. But these twins are identical which means by definition they have the same mom and dad. In fact, they have the same DNA!

So no, conjoined twins with different fathers is not possible. But something close to this just might be.

While twins of the nonidentical kind can’t become conjoined, they can become something called chimeras. This is where fraternal twins fuse together very early on in development.

The end result is not two separate, connected people but one person with a mix of both twins. Some of the chimera’s cells have the DNA of one twin and the rest have the DNA of the other.

Theoretically then we could have someone with chimerism who has two different dads. Basically the first step would be that two eggs get fertilized by different men’s sperm. Then these twins would fuse together to create a chimera with two different dads.

Since both steps are rare, it will be really rare for both to happen at the same time. But it is definitely possible.

Very Close Brothers or Sisters

Like I said, chimeras happen when fraternal twins fuse together to make one person. This usually isn’t a big deal except sometimes when it happens with a brother and a sister. Then there can be issues like having one testicle and one ovary for example.

Most people with chimerism have much more subtle signs like patches of different colored hair or skin. Each twin’s cells are expressing their own genetics and so the chimera has each twin’s skin tone, hair and eye color in the different patches where they are.

And sometimes the differences are so subtle that there are essentially no outward signs of being a chimera. They don’t know they are a chimera until they get a weird genetic test or blood typing. Then all sorts of bad things can happen.

This is because when someone with chimerism passes down his or her DNA, it is from one twin or the other. And if that twin’s DNA doesn’t happen to match the DNA of the cells getting tested which happen to have the other twin’s DNA…

Duncle or Mont

Imagine you go to a fertility clinic and your sperm and your wife’s eggs are combined and then the fertilized eggs are implanted into your wife. Nine months later she gives birth to a beautiful baby boy.

A routine blood test on the baby shows he is AB and a red flag goes up. There isn’t any easy way to explain how you and your wife produced the baby’s blood type since you are both A. The clinic must have implanted the wrong embryos!

You do DNA testing and find that you aren’t his father. The DNA is not a good enough match.

But then you do one more test, an ancestry test that can see more distant relationships, and it shows that the two of you are definitely related. Instead of father/son, though, it shows uncle/nephew or grandfather/grandson. 

If you think about it, this is the result you might expect if dad was a chimera. Fraternal twins are as related as any brother and your brother’s son is your nephew.

So imagine you are a chimera. Some of your cells have one DNA and the rest have a second set of DNA. The DNAs are related to each other as brothers.

Now the DNA that fertilized the egg is from one brother and the DNA from the tested cells (usually cheek cells) is from the other. When the DNA from the cheek cells is compared to the baby’s DNA, it will look like a brother is the dad.

In other words, it looks like the baby is the man’s nephew when he is really his son!  He is the baby’s duncle (dad + uncle). (A mom would be a mont, mother + aunt.)

This isn’t all just made up either. It is very similar to what happened recently to a family.

When they took a standard DNA test, they could not tell the baby and the dad were related. It really looked like the clinic had made a really big mistake.

But an ancestry DNA test was able to show their true relationship. In this case, the baby’s DNA was about 25% related to the dad’s cheek cells. 

Who’s Your Daddy?

As you tell, things can get pretty confusing pretty quickly. While there may be a natural tendency to identify one set of DNA as mine and the other as my twin, this isn’t really how it works.

A chimera is a wonderful mix of both twins. Neither is the true self even if the cells that first got tested happened to have a certain DNA. A chimera is really a fusion of two sets of DNA that make one unique person.

This also means that each child is equally related to the chimera. One child is not more or less related.

One child might be 50% related to one of the twins and 25% related to the other and the situation might be reversed for the second child. But this doesn’t mean either is more related to dad genetically.

Maybe we need to think about them both as 37.5% related to dad. They are 50% related to half of him and 25% related to the other half.

Well, that may be too simple. There is no reason to think that it is an even 50-50 split with each twin. Depending how many cells there were at the start and which ones happened to become which parts, one set of DNA could be more common than the other. Of course, that is a tough one to figure out!

Still, the kids are the chimera's. Is genetics cool or what?

By Dr. Barry Starr, Stanford University

Conjoined twins cannot have different fathers but chimeras could. (Wikimedia Commons)

If these boys had fused together very early in development, they'd be one person. They'd be a chimera. (Pixabay)

Whichever part of a chimera a child is related most to, he or she is still a chimera's son or daughter. (Wikimedia Commons)

Chimeras, Mosaics, and Other Fun Stuff