 We're back at blood typing and this time we're going to talk about a concept called epistasis. I'm going to write epistasis down here, epistasis, because here's the thing, here's the thing I didn't tell you, the secret that I did not tell you about blood typing. Blood typing is so crazy cool and it's even cooler than you thought it was. There's another gene, it's called the H gene. And the H gene codes for a protein that holds the antigens into the red blood cell. What, how cool is that? Okay, now H is epistatic to the I gene, which means that if H is Wackadoosy, let's pretend H is dead, long live H, H is dead. If it's a non-functional H, it doesn't matter what the I gene says. The I gene can say, dude, you got to rock the A antigens and B antigens in this red blood cell. Who cares? You're going to show an O phenotype that those antigens can't stick in the red blood cell wall. Can you imagine that? So basically, I'm going to show you my IA, IB genotype, but wonky H genotype. And now you'd look at that and you'd be like, oh yeah, you're going to have antigens embedded in the red blood cell and you're going to have type AB blood. My wonky H can't grab ahold of the antigens. My body's going to make the antigens. It just can't embed them in the red blood cell. So now the genotype, the blood typing genotype is irrelevant. The H gene trumped it. H was epistatic to I. Your phenotype in this case, your genotype says you should have type AB blood. Your phenotype is type O blood. This is why you never say my mom isn't my mom because she has type whatever blood my blood can't be possible because maybe you have a wacky, wonky, wookie H gene, which is epistatic to the I gene. Did you follow that? Honestly, I mean, I could talk for 50 years. You probably have figured that out by now about all of these, but really you should settle in and like draw some pictures and make yourself a really nice little cheat sheet that helps you visualize each of these specific situations. The next situation, this one is just straight up stone cold, weird, and makes everything like, oh, okay, really? We can't draw any conclusions anymore after we talk about the next one. So let's talk about penetrance next.