Hey y'all, I just wanted to talk to you guys a little bit today about Mendelian genetics The main tool in Mendelian genetics is a Punnett square and this is a simple tool to help us visualize simple genetic crosses Today's geneticists have amazing and advanced tools that can tell us all kinds of stuff about ourselves We'll talk about that more later but for now Let's just play with this basic tool in order to help us understand how Mendelian autosomal traits are passed from one generation to the next Okay So let's use a hairline For example, this is an autosomal trait and each of us has a gene for hairline that has two alleles. So two traits widow's peak hairline or straight hairline. A widow's peak as seen here on the left is a v-shape point in the hairline in the center of the forehead. The picture on the right shows a straight hairline. For this particular trait the widow's peak hairline allele is dominant and the straight hairline is recessive. So now let's take all of this information and put it into a key to help us keep things organized. Okay, so we're doing hairline, right? And we have a genotype and a phenotype So we've got we're gonna use big T big T which is homozygous dominant Which means they appear with widow's peak. All right, then we're going to do heterozygous And they also appear with widow's peak and then homozygous recessive Which is straight hairline. We're organized now, let's draw a Punnett Square And since this is an autosomal trait There will be only three genetic or genotypic options for each parent. The father could be big T, big T; or big T, little t; or little t, little t. And the same is true for the mother But what's weird is there are only two phenotypic options for each parent They either show widow's peak, or they show a straight hairline. So let's say we see a man who has a straight hairline. He's with a woman with widow's peak. And we want to try to guess or predict what types of hair lines their kids might have. We would take what we can see the phenotype or the appearance of the hairline and plug that into the punnett square. Now according to our key the straight hair line is homozygous recessive and we will use little t, little t to show that, so we would put a little t here and a little t here. Now the woman has widow's peak, but there's no way for us to look at her appearance and tell whether she is homozygous dominant. Where her second allele is also dominant or a big T or heterozygous, Where her second allele is recessive or a little t, so what we do is plug in what we do know and that is she has to have at least one big T in order to show widow's peak. And we can add those to the Punnett square. You see, even without knowing the second allele we would be able to predict that if this couple were to have kids, at least 50% of them will have the heterozygous genotype, and at least 50% will have the phenotype of the widow's peak hairline. You with me here? So for me, you see that I have a straight hairline, but my father had widow's peak. So where did I get my straight hairline? My mother, right? And it means that I show the little t or straight hairline allele, but I carry the big T or widow's peak hairline -- and as such I could pass it on to my kids. Are you with me? Well, just in case let's try one more. This time we'll use hitchhiker's thumb. Each of us has a gene for whether or not our thumb bends backward at the tip that has two alleles -- either hitchhiker's thumb or a street thumb. Now hitchhiker's thumb as seen here on the left allows the distal joint to hyper-extend or bend backwards. The picture on the right shows a person who has a straight thumb. For this particular trait, the Hitchhiker's thumb allele is recessive, And the street thumb is dominant. So let's take this information and put it into the key and help us keep things organized again. So we're looking at thumbs this time -- We're gonna use H's just for fun and the genotype of homozygous dominant would be a person who appears with a straight thumb and we'll use big H big H -- a person who also appears or has a phenotype of a straight thumb but could be heterozygous genotype Which we're going to use big H, little h. And then of course little h little h will represent our homozygous recessive, which appears with the Hitchhiker's thumb. Again this is an autosomal trait. So there are only three genotypic options here for each parent Just like before the father could be big H, big H or big H, little h or little h, little h, and the mother could be the same. And again, there are only two phenotypic options. They either have a straight thumb or they have a Hitchhiker's thumb So let's say we see a man who has a hitchhiker's thumb and he's with a woman who also has a hitchhiker's thumb. And we want to try to guess or predict what types of thumbs their kids might have. Again, we would take what we can see, the phenotype or the appearance of the thumbs, and plug that into the Punnett square. Again, according to our key, hitchhiker's thumb is homozygous recessive and so we use the little h, little h to show that, so we would Put a little h here and a little h here. The woman also has hitchhiker's thumb so we know she too is homozygous recessive for the allele and we again use little h, little h to show that, and we plug this into the Punnett square and what do we predict? Well, that all of their kids 100% will have hitchhiker's thumbs. So, which do you have? Can you run a Punnett square or two and try and predict what your kids might receive from you and a potential mate? Just for fun? Anyway, I hope this has helped to explain a little bit about how to use the Punnett square to predict some simple human traits, and I hope also hope you will check out another of my videos where we look at how sex-linked traits are inherited I'll see you next time
