So yesterday in Genetics lecture, we learned about something called “incomplete penetrance”. Basically, it’s when not all individuals with a certain allele display that trait, even if logic says they should. Normally I don’t share my class notes with the Internet because frankly I doubt anyone wants to see them. But the example our professor gave us was polydactyly. Naturally, when he explained this and showed us a picture, my first thought was:

Let me explain why I’m bringing this up.

I just sort of assumed that Ford had six fingers due to an in utero mutation (a spontaneous mutation of an embryo while it’s still developing), because polydactyly, or having additional phalanges, is a dominant trait. If a child has it, one of their parents does too. Like inheriting brown vs blue eyes. But it turns out that polydactyly isn’t that simple.

In order to be a polydactyl, you need at least one copy of the dominant allele (let’s call it “P”; the recessive allele is “p”). If you have extra fingers you either have two copies (PP) or one copy (Pp). If you have no copies (pp), you have five fingers on each hand. However, only about 3/5 of people with either PP or Pp actually have extra phalanges. Roughly 2/5 of people with PP or Pp have the normal number of fingers.

I promise I have a point.

And here it is: Most of us have been assuming that Stan and Ford are fraternal, rather than identical, twins. This is not just because of the whole “polydactyly” thing but also because of this:

Ford has a cleft chin, Stan doesn’t. Now, my first thought was “okay but Stan could have a cleft chin, it could just be understated for the sake of simplicity, or telling them apart”. But after a bit of research, I found something out. A cleft chin is like polydactyly: it’s the dominant trait, but has incomplete penetrance.

The two things that everyone thought meant they weren’t identical have no meaning. They could still be identical genetically, it was just some weird twist of fate that made them appear to be fraternal. And if they are identical after all?

Ford isn’t the only anomaly.

In incomplete penetrance, only a minority of people with a dominant allele do not display the dominant trait. Stan is a part of that minority. Ford is weird as hell (that kind of post-axial polydactyly is rare on one hand, let alone two), and from a scientific standpoint, so is Stan. Not displaying a dominant trait when you have the dominant allele is abnormal.

They’re even more alike than they realize.