 The last section, I think the last one is the hardest because it's sort of mathematical. There are problems involved. You have to apply your understanding of Punnett squares and two traits like dihybrid crosses, super complicated math, but mathematical nonetheless. So if you can get through all the bookkeeping and all the pieces to keep track of, for the most part, you can figure it out. This section, this section deals with two concepts that muddy up the phenotypes we see. So I don't even know how to say this. They both relate to muddy phenotype, genotype relationships. That says relationships. Oh, we can do this. All right. I probably, oh man, let's see how we're gonna describe it. Let's do definitions first. Penetrance is a measure of expression of a trait. So it's the measure of here's the genotype and this is the phenotype that is expressed as a result of this genotype. Whereas expressivity is a measure of variation what in trait expression. Honestly, we are gonna have to do some heavy duty, brainstorming, sharing. How are you gonna remember the difference between these two? Penetrance can actually be a number because the range of penetrance is if you have this genotype and there is 50% penetrance, then you have a 50% chance of expressing the expected phenotype. Okay, if that doesn't blow up the whole system for you, this is the thing that where all of our planning and all of our what are the expected gametes that you would produce and what are the phenotypes and what's the genotype related to this and how many babies are we gonna get? This is the piece that says, well, you can have that genotype and not express the expected phenotype. So 50, oh geez, 50% penetrance means that only 50% show the expected phenotype and some of them just randomly don't show it. Expressivity, you have a genotype that causes a phenotype, but sometimes the phenotype is like really big and sometimes the phenotype is really small. It's still the same phenotype, but there is great variation in expression. And then, I mean, is it a new trait? How do you know if this is a whole new trait? I'm gonna show you a picture. This came from a video and I'll try to find the link to the video. I know I have the link somewhere, but I really, really like this visual because this human, this person who made this video to compare penetrance and expressivity says, okay, look, let's pretend like the trait is mouse ears on a human and you can go through and you can see like, oh, look, all the yellow circled images are mouse ears, like you are expressing mouse ears. Not everybody expresses mouse ears, do you see that? But they are, that's the trait that we're looking at. Now, let's look down here. You can have narrow expressivity, which means that there's no variation. There's very little variation in expression. And look at the mouse ears in all of these situations. There's no variation. If you have mouse ears, you have big old mouse ears. Awesome, it's narrow expressivity for that trait. But check out broad expressivity. We have big mouse ears, little mouse ears, medium mouse ears, bigger mouse ears. Everybody's expressing, everybody who is expressing mouse ears is expressing them, but there's a lot of variation. That's expressivity. Penetrance is the top line. Look, we have either incomplete or complete penetrance. And you can imagine we have a big old range with complete penetrance. Every single person in this situation has mouse ears. Complete penetrance. If you have the mouse ear genotype, you're gonna express it. You could have broad expressivity and lots of variation or narrow expressivity, not very much variation, but everybody has it because you have complete penetrance. However, you could have incomplete penetrance, in which case you have these home kids. How'd they make it out without the mouse ears? They have the genotype, but they don't have the phenotype. Why? Because the trait is incompletely penetrable. The trait has incomplete penetrance. Incomplete penetrance, if you get the mouse ears, you're all gonna look the same because it has narrow expressivity. I really like this chart. We should make our own charts like this to show, to illustrate this concept because once you have your chart, you can go like, okay, okay, I can solve problems with this. I can answer questions about this. I understand the concept, but pulling it out of your head, I think it's super complicated. Okay, we can do it.