Cheryl takes a chimera test on 'Riverdale.'

Cheryl's "Chimerism Testing" Storyline On 'Riverdale' Is Actually Kinda Legit

by Dylan Kickham
The CW

Cheryl Blossom's story on Riverdale has always been out there, but Season 4 has really gone above and beyond, with corpses, hauntings, and, most recently, a ton of unexpected questions about genetics. Warning: This post contains spoilers for Riverdale Season 4, Episode 8, "In Treatment." Despite her zany storyline, though, her latest subplot on Riverdale, regarding chimerism testing, is actually a real thing.

Let's back up a bit. "In Greek mythology, a chimera was an animal with the body of a goat, the tail of a serpent, and the head of a lion," Lois Regen, a lab manager at Seattle Cancer Care Alliance's Clinical Immunogenetics Lab tells Elite Daily. In real-life, though, a chimera is a person who has two genetically different types of cells in their bodies.

Cheryl's potential chimerism was first mentioned at the beginning of Season 4, when Nana Rose told her granddaughter that she and Jason were supposed to be triplets, and that Cheryl absorbed the third Blossom embryo — who would have been named Julian — in utero. Since then, Cheryl became paranoid that her would-have-been sibling's spirit was haunting her via a doll (named Julian, of course). In the Dec. 4 episode, Cheryl finally explained all this to her school guidance counselor Mrs. Burble, who didn't buy the story at all. At Burble's recommendation, Cheryl took a chimerism test to determine if she actually did carry some of Julian's DNA, or if the story was all a lie to gaslight her.

Much like Betty's serial killer gene debacle, the moment had many fans wondering whether this test is actually based in truth, or if it was just another instance of Riverdale fans needing to suspend their disbelief. It turns out, the process actually is real, though it is pretty rare and not exactly as the show portrayed it.

Mrs. Burble was right to be suspicious of Cheryl's reported condition. A 2007 report from the European Molecular Biology Organization noted that only about 100 cases of chimerism had been reported in medical literature (though the report also surmised that the number may be underestimated since the condition is usually only discovered by chance).

There are three types of chimerism: artificial chimerism, microchimerism, and tetragametic chimerism. Artificial chimerism happens due to medical procedures like transplants, transfusions, and grafts, when the host's body absorbs cells from a donor's DNA. Microchimerism happens when a fetus absorbs cells from the mother, or vice versa. Cheryl's fear that she absorbed her potential brother's embryo in utero is a description of tetragametic chimerism (the rarest form of the condition, according to the National Society of Genetic Counselors), wherein one embryo absorbs another embryo developing in the uterus.

Further, while Burble's statement that the test can be done with just a swab of saliva is true, that's actually not the best way to go about it. The Seattle Cancer Care Alliance points out that DNA evidence is most apparent through blood testing, since a chimera can have two different types of blood.

At the end of the episode, Cheryl's test results confirmed she is not a chimera after all, meaning she only has one type of cells (her own) in her body. But it's not as though she was really at risk of anything grave if she did turn out to be a chimera. The condition rarely shows strong visible signs or adverse effects — and it definitely doesn't result in any creepy doll hauntings.

Now, all Cheryl needs to worry about (aside from getting her cheerleading squad back) is figuring out why Nana Rose lied to her about being a chimera in the first place — and why Julian's supposed spirit is after her.

Riverdale's new season continues to air Wednesdays at 8 p.m. ET on The CW.


Wolinsky, H. (2007) A mythical beast. European Molecular Biology Organization, https://www.embopress.org/doi/10.1038/sj.embor.7400918

Sheets, Kayla. (2019) Chimerism explained: How one person can unknowingly have two sets of DNA. National Society of Genetic Counselor blog, https://www.nsgc.org/p/bl/et/blogaid=1084

Sheets, K., Baird, M., Heinig, J., Davis, D., Sabatini, M., Starr, D. (2018) A case of chimerism-induced paternity confusion: what ART practitioners can do to prevent future calamity for families. Journal of Assisted Reproduction and Genetics, https://link.springer.com/article/10.1007%2Fs10815-017-1064-6

Source interviewed:

Lois Regen, Lab Manager at Seattle Cancer Care Alliance's Clinical Immunogenetics Lab