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Hank talks about chimeras, and why Venus the cat probably isn't one - but your mom might be!

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You know what's fascinating, and also, a little bit gross when you think about it? Chimeras. Not the mythological ones, but actual chimeras: organisms that contain the genetic information of more than one individual, and yes this is a real thing!

Chimeras are usually formed in the very early stages of embryonic development when 2 zygotes fuse into one. And the resulting chimera basically ends up being its own fraternal twin.

Which is, yeah, I know, and you end up with all sorts of weird conditions like if you were like a male and female fraternal twin fuses into one, sometimes you get one ovary and one testicle, literally with different genetic information in them.

There’s also this woman who needed a kidney and so they did a blood test on her kids to see if they'd be good donors, and they did it, and not only would they be bad donors, but they aren’t your kids. And she was like "yeah, but, uh they came out of me".

It turned out that her blood and her organ systems were from two different people, they had two different genetic makeups! So she gave birth to a different person than her blood would have given birth to.

And i wager a box of Jujubes bees that you’ve heard of the adorable, two-faced cat with mismatched eyes, Venus. She's often described as a chimera, but it turns out she’s probably not. Instead, she appears to be what most female mammals are: a mosaic.

See, a female has two X chromosomes, one from her father and one from her mother. But as an embryo, each of her cells deactivates one of those two X's at random, so she ends up with two different groups of cells in her body; one with only her dad's X and one with her mom's.

In humans, this differentiation is difficult to detect because not a lot of traits are linked to the X chromosomes. But in cats, that distinctive tortoise shell coloration that Venus has is X linked. So what we’re seeing is probably the arrangement of her different mom X cells and dad X cells that happen to line up down the middle of her very kissable face.

So Venus may not really be a Chimera, but try not to be too heartbroken, because you know who is?! I’m sure you know one, actually, because you came out of one.

Just today, scientists in Seattle said that they found the first evidence of male DNA in the brains of women. After performing autopsies on the brains of 59 women, they found that nearly 2/3 of them contain different DNA, and it was male: XY not female XX. Now the researchers don’t know the history of each woman whose brain was studied, but they suspect that the DNA got into their brains through what’s known as fetal chimerism.

When a pregnant woman receives cells from her embryo which then go on to start lineages in the mother's body that can last for the rest of her life. This phenomenon, also known as microchimerism, was first discovered in the 1990's in women's blood, but this is the first time its been observed in their brains.

This could change current thinking about how widespread chimerism is in women and also what implications it might have for the study of brain disorders, like Alzheimer’s disease.

So, as if those 9 months weren’t enough, mom, you’re still carrying a little bit of me with you wherever you go.

Thanks for watching this episode of SciShow break. if you want to keep up to date on science news you can go to and subscribe. Questions, ideas, and comments are always welcome on Facebook, Twitter, and of course, down in the comments below.

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