-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…