 The dihybrid cross is the most complicated cross we're going to have in You know the things that you may be asked to do and there are definitely some shortcuts to doing this kind of thing But we're gonna we're gonna do the long cut today. I Created a whole new scenario for you. We're gonna deal with two heterozygous purple tall Plants and we're gonna go through our little steps First step was what? We're gonna do parent phenotypes and Both parents are the same. They're purple Tall do you agree? That was easy Mostly because I wrote it down for you above there now What are their genotypes? Step two, what are their genotypes? Well, I tell you they're heterozygous So that gives you all the information that you need we will say okay, then their big P little P for the flower color gene and Big T little T for the height gene and Both of them are the same. So we have big P little P big T little T. I Hope you understand me and Now we're done. That's it. That was easy too. Wasn't it our genotypes were easy What about possible gametes? Okay, I Don't know how you want to proceed, but I I just do it my head or whatever with my pen you should push pause and Try it out come up with the possible gametes And Then come back. Okay. Are you back go do it if you didn't do it go really go do it? Okay Gametes are haploid Which means they can only have one copy of each letter of each allele Which means I'm gonna start with big P and big T and Then I'm gonna do big P little T Then I'm gonna do little P Big T and little P little T. Okay Those are my possible gametes for the first one and those are the same possible gametes for the second one I'm gonna do that so that you can see big P's Merniha Merniha Merniha Merniha and Merniha, okay They're the same now Did anybody out there did you write did anybody do this? Here's one possibility and here's one possibility and here's one possibility Did anybody do that? That's probably a it's a really common error But if there are two P's that means that you have two copies of that chromosome and gametes can't have two copies No Gametes only have one copy gametes are haploid. I'm gonna go ahead and erase that for fun Are you ready to make your Punnett square? Reginald Reginald Crundle Reginald Crundle to the rescue Okay Now this is where our Punnett square. This is where the problems become Big because now I have a square that's four by four Four possible gametes on one side four possible gametes on the other. So my little methodical self I'm gonna go. Okay big P big T I'm just gonna put all my gametes here first Okay, and then I'm gonna put all my gametes across the top same set Yes You're cool, huh? This is this is easy Now I'm going to draw in my squares This is just I guess I'm just modeling how I do it But I already see something I don't like Can you guess what it is? My boxes are really small. I'm gonna want bigger boxes. I'm gonna need a bigger box Okay, but I will just write small. Maybe I'll zoom in on this a little bit so you can see it a little bit better Now I'm gonna do all the possibilities if and I know that this is like I'm If I combine gamete one two three and four a B C and D. Do you see how I'm just trying to give you some like Way to track what my crazy brain is doing I'm gonna combine the first two gametes and I'm gonna get two big P's and two big T's You can combine those letters. However you want. I personally need them to be in a methodical order I can't mix them up. I always have to put the big letter first But you don't have to I think it's easier to read if you have a methodical way of doing this I'm just gonna do the hard work of going through and Filling in each one of these boxes You see how like what is it's almost like a times table, right now I'm gonna fill in the second box with gamete number two if gamete number two combines with gamete a we get big P That I can't even I can write them faster than I can say them and Y'all really loud if I make a mistake Because you need to stop me if I make a mistake. Oh I thought that joke was funny. Look I have plenty of room for this Big P little P big T's Are you following this and Look at how I'm putting them in order Capital letter first, but that's only So I can track so it's easier to read Doesn't matter All right This would be a good time to put me on double time, huh? Oh My goodness I'm almost done Are you happy that I'm almost done There I did it Okay, you're done. Just kidding You now know all the possible offspring. This is my possible offspring But does anybody want to report out what that says oh Come on now. I Do not want to report out what that says. That's like horrifying to me Instead I'm gonna tell you we we got to count these guys We got to count our phenotypes. We could count genotypes, but I won't make you do that Because look just for the fun of it and I'm gonna say can you find any two genotypes in there that are the same? I'm gonna guess that the heterozygotes look I'm putting yellow dots in my heterozygotes Boxes It's the same genotype. We could count up all the genotypes that are the same But I only I'm gonna ask you to count up phenotypes. Let's do a phenotypic Ratio And can you tell me what all our possible phenotypes are gonna be? Well, I think I probably would approach this methodically as well. I think I would go okay I'm gonna go into box number one box number one is what's the phenotype of Homozygous dominant big P homozygous dominant little I mean big T Do you follow what I just said? That's why I shouldn't say it. I should just point and then you Know what I'm talking about Both dominant traits are there that guy I'm gonna circle him and I'm gonna say that is purple Tall It's a purple tall plant and I'm gonna say home kids. We have one of those. I'm not gonna count that way I'm gonna count them up afterwards I'm gonna go across the row. So now I have big P big P Big T little T. What is that phenotype same thing purple tall How about if you go through and find all the purple talls in your giddy-up all Of my heterozygotes are purple tall So I'm gonna go through and get all the snack all those guys because those are all purple tall Do you see any other purple tall? Ah, here's a purple tall Here's a purple tall Did I miss any purple talls I Don't think so Shall we count them? How many purple talls do we have one two three four five six seven eight nine? Nine purple tall guys. Okay. Pick a different one. I'm gonna pick a different color. I'm gonna pick pink For our next one and I'm gonna say this guy is the next genotype. We're looking for that one is purple What short how many purple shorts are there? Well, look here's one right here. I Gotta have at least one big P purple short Done how many purple shorts do we have three How about I'm just gonna pick another one now What's that guys? Genotype Little P little P big T big T. That is a white Tall a white tall guy The tall white guy. Here's another tall white guy, and here's another tall white guy. How many total? What do you notice and then? The story is over. We've got one more and who is that? white Short The white shorty is Our last one. I've got good news for you this ratio 9 3 3 1 You can do the entire Punnett square or you can say guess what? I know the phenotypic ratio is 9 to 3 to 3 to 1 9 showing both dominant traits 1 showing Both recessive traits and 3 each showing a dominant and a recessive You just did a dihybrid cross. That's the most complicated cross that we're gonna do I'm gonna do one more thing for you. We're gonna talk about a Type of cross called a test cross and specifically why a test cross can be handy