Family genetics in Star Wars (Warning: Episode VII spoilers)
So I saw Star Wars VII: “The Force Awakens” the other day. Great movie, which has mostly erased the shame of episodes I-III. Despite even more than the usual suspension of science, it’s a great SF flick.
(Major spoilers below! You have been warned!)
One mystery which will hopefully be resolved in the upcoming episodes is the origin of Rey. She is obviously strong with the force, using a Jedi mind trick on a stormtrooper (played by Daniel Craig!) to escape. Has visions related to the force, fights well with a lightsaber despite no previous training, and is a general badass. However, while it is pretty clear that Rey’s midi-chlorian count is violently high, it is not clear how she relates to the Force-inhabited family (of at all). One idea is that she might be Luke’s daughter, another that she may be a second offspring of Leia, making her a sister or half-sister to Kylo Ren.
Genetics can help us solve this problem. My assumption is that the Sith gene (sitH) is X-linked, recessively inherited. X-linked means that the gene sits on the X chromosome: so for someone to turn to the Dark Side, they need not only a high midi-chlorian count, but also a gene, which happens to sit on the X-chromosome. Recessively inherited means that for the genetic information to be expressed, women need two copies of the gene, but men only need one. Females would need two copies (alleles) of the sitH gene , one on each chromosome (X+/X+ which is why we see few Sith females). But a male inheriting an X chromosome containing a copy of sitH from his mother (X+/Y0) will inevitably turn to the Dark Side, because he does not have a second, x- chromosome to “block” the evil X+. (x- means a copy of X with no sitH gene Y0 means a Y chromosome which cannot carry a copy of the gene sitH).
Under this assumption, let’s examine the genotypes of the Skywalker family:
Anakin Skywalker (Darth Vader): X+/Y0 Anakin has a copy of sitH on his sole X chromosome. He fought for three miserable movies against going to the Dark Side, but failed and gave in to his genotype.
Leia Organa: X+/x- : Leia has to have one copy of sitH, since she has to receive one from Anakin, her father, who is X+. Thankfully, the other copy from her mother Padme Amidala is X-. sitH being recessive, Leia remained firmly in the Light Side.
Luke Skywalker: x-/Y0 Leia’s twin brother. Received the Y chromosome from Vader, and the x- copy from Padme.
Kylo Ren: X+/Y0 the new evil guy. Not quite a Sith Lord, but definitely embedded in the Dark Side. Leia and Han’s son, he received the X+ copy from mom, unfortunately.
Rey: X+/x- or x-/x-. If Rey is indeed Luke’s daughter, she would have received an x- copy from him. Assuming her mother is not from Jedi / Sith descent, that would mean that to be Luke’s daughter she is a x-/x-. Having an X+ copy would mean she got it from a parent who is carrying the X+ allele. Given the age differences, and possible range of parents, that would probably mean that Leia is her mother.
(More on X-linked recessive inheritance).