Video transcript
- [Instructor] In thisvideo, we're going to build on our understanding of Mendelian genetics and Punnett squares by starting to thinkabout two different genes. So we're going back to the pea plant, and we're gonna think aboutthe gene for pea color and the gene for pea shape. So let's say that inthe parental generation, you have one parent thatis hom*ozygous dominant for both of these genes. So their genotype is capital Y, capital Y, and capital R, capital R. So their phenotype for sureis going to be yellow round, and we also see the genotype. We see which alleles it has. Now, let's say that that is crossed with the hom*ozygous recessive parent. So in this case, it's goingto be green-colored peas. It's counterintuitiveto write green with a y, a lowercase y in the color yellow, but the lowercase y represents green. And also these are wrinkled green peas. So I will write that withthe lowercase r here. And so the phenotype here is going to be green and wrinkled for sure. Now, what's going tohappen when they cross? What does the F1 generation look like? Well, we know fromMendel's law of segregation for each of these genes, thatwhen a gamete is created, it randomly gets one copyof each of these genes. So for this first one, it's going to randomly getone of these capital Ys. So it's going to get a capital Y for sure from this first parent. And it's also going to randomlyget one of these capital Rs. So it's going to get a capital R for sure from that first parent. And then by the same logic, it's going to randomly get oneof these two lowercase y's. So it's going to get alowercase y for sure, and it's going to randomly get one of these two lowercase r's, so it's going to get alowercase r for sure. So this is the genotype forall of the F1 generation. This is often known as a dihybrid. It is heterozygous in both genes. Now, what's the phenotype here? Well, we know yellow is dominant, and we know round is dominant. So if we looked at theseplants right over here, their peas would still be yellow round, just like this hom*ozygousparent over here. Now, what's interesting is when you do what's known as a dihybrid cross when you cross one of this F1 generation with itself or with each other. And to do that, I'm gonna create a four byfour Punnett square here. And so one parent hereis going to be hybrid in or heterozygous in the color gene and also heterozygous in the shape gene. And that's going to be trueof the other parent as well. Heterozygous or hybrid in the color gene and also heterozygous in the shape gene. And so that's why this iscalled a dihybrid cross. You're crossing things that are hybrid in two different genes. Now, we've already talkedabout the law of segregation. The gamete is randomly goingto get one copy of each gene. Now, Mendel also has the lawof independent assortment, which tells us the alleles of different genessegregate independently. So for this parent here, whether it contributes acapital Y or a lowercase y is independent of whetherit contribute a capital R or a lowercase r. Now, there is a little bit of a asterisk, a little caveat on there. We now know that genes sit on chromosomes. One chromosome will have many genes on it. And this law of independentassortment only applies to genes that are actuallysitting on different chromosomes. If they sit on the same chromosome, they generally are not goingto assort independently. But let's just assume thelaw of independent assortment 'cause this is true for most genes. So this first parent cancontribute a capital Y out of this first gene and a capital R out of the second gene, or they could contributethe lowercase copy of the first gene and the capital R copy of the second gene, this capital R, the roundallele of the second gene. And we could go throughevery combination here. It could also contribute the yellow allele and the wrinkled allele. Or it could contribute the green allele and the wrinkled allele as well. And the same would be truefor this other dihybrid, this other parent right over here. So let me just write that down. They could contribute capitalY in two of the scenarios. They could contribute a lowercase y or the green allele intwo of the scenarios. And they could contribute a capital R in two of the scenarios, a round allele, or a lowercase r in two of the scenarios, a wrinkled allele. So you have all of thedifferent combinations that each of them can contribute. Once again, whether you get the yellow or the green is independent of whether you get theround or the wrinkled. So these are all equallyprobably right over here. When the two gametes fromthese two parents merge, we can then look at what the genotype of the offspring is going to be, really the genotype of the F2 generation 'cause we're crossing twomembers of the F1 generation. So I encourage you to pause this video and fill in this grid. See if you can figurethe different genotypes that will result. All right, now let's do this together. So this scenario right over here, you're getting a capitalY from both parents, and you're getting acapital R from both parents. This scenario over here, you're getting a capitalY from this parent, lowercase y from that parent, and then you're gettinga capital R from both. This scenario over here,capital Y from both parents. Capital Y, capital Y. And you're getting acapital R from this parent and a lowercase r from that parent. And then this scenario over here, you're getting gonna capitalY allele from this parent and lowercase y from that parent, and you're getting acapital R from this parent and a lowercase r from that parent. And now I'm just goingto speed up the video and just fill in the rest ofthese using the same logic. All right, now that we'vefilled out this Punnett square, let's think about thedifferent phenotypes. How many of these plants are going to produce yellow round peas? Pause the video and think about it. Well, it's yellow and round. It has to have at least onecapital Y and one capital R. So that one's going tobe yellow and round. This is going to be yellow and round. This is going to be yellow and round. That's yellow and round as well. This is yellow and round. That's yellow and round. This is yellow and round. And that one is yellow and round. And then last but not least,I think this is the last one that is both yellow and round. And actually let me makea little color code here. Yellow plus round. And here we're talkingabout the phenotype. You can see we havedifferent genotypes here, but because both yellowand round are dominant, as long as you have atleast one Y and one R, you're going to have a yellow plus round phenotype over here. So you have one, two, three, four, five, six, seven, eight, nine. And I will say there'snine of these over here. Now, how many of these are goingto be yellow plus wrinkled? Pause the video and thinkabout that, that phenotype. So yellow and wrinkled, you're going to have to have a capital Y and two lowercase r'sin order to be wrinkled. So you have at least onecapital Y and two lowercase r's, and least one capital Yand two lowercase r's. Let's see, at least one capitalY and two lowercase r's. It looks like you haveexactly three of them, that phenotype. And then what about the other way around? What if we are looking for,I'll do it in this green color, green plus round? How many of them exhibit that phenotype? Well, to be green and round, you have to have two lowercase y's, and you have to haveat least one capital R. So this would be green and round. This would be green and round. And then this would begreen and round as well. So you have three of those. And then how many of them are going to be both green and wrinkled? Well, I think you see thatone scenario over here that is both green and wrinkled, having that hom*ozygousrecessive phenotype. And so if you were to do this many times, you'd expect the ratios betweenthese various phenotypes to be nine to three to three to one. And when Mendel and many other people since Mendel have donethese types of experiments, they have seen that statistically, this is what you seein that F2 generation. Now, you're unlikely to get exactly a nine to three to three to one ratio. It's all probabilistic. Every one of these 16scenarios are equally likely, so you would expect this nine to three to three to one ratio, but you're not always goingto get that exact ratio. You'll probably get something close to it.
