Science vs. Star Wars: Cousin Kylo and the inbreeding coefficient

Sam A.E.
Sam A.E.
Dec 5, 2019 · 7 min read

If Kylo and Rey are cousins, what does that mean for “Reylo?”

***This story contains Star Wars spoilers***

Hopeless romantics, myself included, wait with bated breath for the final installment of the 9-episode-long saga of the rise and fall (and rise again) of the Skywalker dynasty, but not because we care what happens to the Resistance or to the First Order or even what happens to some of our favorite supporting characters. (But if Chewbacca dies, I walk).

No, we’re really here for “Reylo.”

Actors Daisy Ridley and Adam Driver play Rey and Kylo Ren, respectively, in Star Wars Episodes 7, 8, and 9. Photo credit: Gage Skidmore

A portmanteau for the ages, “Reylo” combines “Rey” — the name of the Resistance’s scrappy heroine — with “Kylo” Ren, aka Ben Solo — the troubled space boi who kills his dad, Han. Star Wars episodes 7 and 8 tease a possible romantic connection between the two characters, through intimate Force-chats and lightsaber-wielding teamwork.

Some fans love to ship “Reylo,” while others love to hate it. With Episode 9 just around the corner, long-dormant “Reylo” reddit threads, fanfic forums, and Twitter memes are waking up, and I’m reminded that many of us are pondering the same question that’s at the heart of Episodes 7–9: are Rey and Kylo related?

Kylo Ren is the son of Leia Organa, Luke Skywalker’s sister, while Rey’s mysterious parentage and her natural aptitude with the Force hint at familial ties to the Skywalker dynasty (what’s her midichlorian count, anyway?).

So, what if — as many hypothesize Rey is actually Luke Skywalker’s daughter, secreted away on a remote desert planet?

This would make Rey and Kylo first cousins and our Reylo dreams would be dashed.

Or… would they?

If Rey and Kylo are cousins, and if they were to have children together, their children would only have 6 great-grandparents, instead of the expected 8. There would be a closed loop in their recent family tree.

Possible closed loop in the Skywalker Family Tree (indicated in red)

So, is that… bad?

Setting aside the “ick factor” (and acknowledging that cousin marriages are common and often encouraged in many parts of the world) we can answer this question with science!

Inbreeding: the background

Inbreeding is when two closely related individuals — who inherited some identical pieces of DNA from the same ancestor(s) — have children.

And, the degree of relationship between the two individuals makes a difference. Children of parents who are brother and sister are more inbred than the children of two people who are first cousins.

For similar reasons, inbreeding within the same family over multiple generations increases the risk for hereditary disorders more than one isolated instance of inbreeding in a family tree.

but they’re usually much bigger loops than those typically associated with inbreeding.

How do we know that? As we look further and further back in our family trees, the number of ancestors in a given generation grows exponentially. We have 4 grandparents, 8 great-grandparents, and over 1,000 10th great-grandparents. Following this logic back, we’d have more than a trillion ancestors just 40 generations ago. But that’s more than 10 times the number of humans who ever lived — i.e. it’s impossible. No matter how you slice or dice it, the numbers just don’t add up the way we’d expect, and it’s because of a phenomenon called “.”

If you go far enough back in time, your family tree is collapsing in on itself over and over again. Many of your ancestors will show up on multiple branches in your family tree, forming “loops.”

What does evidence of recent inbreeding look like in someone’s DNA?

People inherit 23 pairs of chromosomes: one set from mom and one set from dad. Therefore, we generally have two copies of every chromosome…two copies of every gene, and so on, which comes in handy when one of the copies is broken.

It’s also why we have sex. The ability to mix and match slightly different versions of the same genes through genetic recombination helps us adapt to new environments and protects us against disease.

Image explaining how DNA in a chromosome recombines over the generations.
Image explaining how DNA in a chromosome recombines over the generations.
Cartoon depiction of one chromosome pair recombining over the generations (adapted from 23andMe)

For the most part, the DNA in both copies is the same, differing at only a handful of spots. A stretch of completely matching DNA on both copies of a chromosome is called “homozygous,” meaning “same gene,” while a stretch of DNA with a few differences between the copies is called “heterozygous” (meaning “different gene”).

And, if there’s inbreeding in your recent family history, you are more likely to have stretches of homozygous (identical) DNA than someone who is less inbred.

So how can we measure inbreeding?

To quantify inbreeding, scientists look at how much of someone’s DNA is predicted to be completely identical based on their family tree.

It’s called the , and it’s a fairly simple calculation that outputs a score between 0 and 1, where 0 is good and 1 is, well, bad.

In basic terms, someone’s inbreeding coefficient is half of the degree of relationship between their parents, where the parents’ relationship degree is also measured in a range from 0–1, based on how much DNA they share.

If the parents are completely identical clones of each other, then their relationship degree is 1, and the offspring’s inbreeding coefficient would be half that, or 0.5. (This basically means that on average, 50% of the offspring’s DNA would be homozygous).

But, in real life, humans can’t mate with their clones (although, all bets are off in the Star Wars universe).

So, what about Kylo and Rey — possible Skywalker cousins?

First cousins share an average of 12.5% of their DNA, with a relationship degree of 0.125.

If first cousins—Kylo and Rey—had a child, that child would have an inbreeding coefficient of 0.0625 (i.e. 0.125 / 2), meaning roughly 6.25% of his or her DNA would be made up of identical copies from each parent, which they inherited from the same ancestor or ancestors (in this case, Padmé and Anakin).

Turns out, an inbreeding coefficient of 0.0625 is fairly low, at least compared to some individuals in highly inbred families, such as the Spanish Hapsburgs. Because of repeated inbreeding over many generations in these families, the coefficient of inbreeding is more difficult to calculate.

Charles II of Spain. Image in the Public Domain.

The Hapsburg monarch estimated to be the most inbred was the 17th-century Spanish King, Charles II, whose inbreeding coefficient was thought to be more than 0.25.

(His sister’s daughter — who was also his uncle’s daughter — scored higher, at over 0.3, which means she likely had even more homozygous DNA than the child of a brother-sister or parent-child pair would be expected to have).

The silver lining here is that no matter how inbred someone is, it only takes one generation of having children with somebody genetically unrelated — to reset the proverbial inbreeding meter in any children resulting from that union.

So where does that leave us with Reylo?

Assuming Rey is a secret Skywalker (and assuming DNA works the same way in the Star Wars universe), “Reylo” would not be all that unusual, and any children they may have together would probably be just fine. According to a 2002 report published by the National Society of Genetic Counselors (the NSGC):

Romantic relationships between cousins are not infrequent in the United States and Canada, and these unions are preferred marriages in many parts of the world. The offspring of first cousin unions are estimated to have about a 1.7– 2.8% increased risk for congenital defects above the population background risk.

Table excerpt from “Genetic Counseling and Screening of Consanguineous Couples and Their Offspring: Recommendations of the National Society of Genetic Counselors” (2002).

The report’s authors also emphasize:

There is a great deal of stigma associated with cousin unions in the United States and Canada that has little biological basis.

Health providers should provide supportive counseling to these families and respect cultural belief systems. The psychosocial issues for genetic counseling in the case of a cousin union are very different from those for an incestuous union.

We’ll probably learn more about Rey’s ancestry in The Rise of Skywalker (odds are, Kylo and Rey are unrelated), but if Rey and Kylo turn out to be cousins, there’s still hope for Reylo.

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Sam A.E.

Written by

Sam A.E.

Science Writer. PhD Scientist.

The Startup

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