Biology: Segregation and Independent Assortment

dihybrid crosses

10/28/2010

~ just_choy

i wish he's my teacher on biology class

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Once Mendel had solved the problem of monohybrid crosses, crosses that hybridized or mixed one trait, he was on to bigger and better things. When you read Mendel's paper you'll see he did di-hybrid and indeed tri-hybrid crosses. We're going to stick with di-hybrids. A di-hybrid cross is when you cross something for two different traits. Now let's think about the implications of that. Let's take one of the experiments Mendel did. Say, for example, Mendel took a--here's the two tricks he used. Code color or pod color, yellow versus green. Actually, this was seed color. He also--and by the way yellow was dominant, so we'll call that Y and green was y. Obviously, they're not express one, the green code color. The idea of a round seed versus, say, a wrinkled seed. We're going to say round, R, and wrinkled or not round, r. Round being the dominant one.
So, Mendel said, "Okay, using this whole concept of dominant genes, or genes that will mask the other, let's take a look at the genotype." Remember genotype, the genes that are present in this two-hybrid situation. Well, let's take for example a plant that has yellow and round seeds. Well, let's say it's true breeding. So, we haven't crisscrossed it with anything. Well, since it's true breeding we know that its yellow genotype is going to be YY and its round genotype is going to be RR. Therefore, if we take something that is, let's say something that is green and wrinkled. What's that genotype going to be? That one's genotype is going to be yy, rr.
Now, I'm setting something up here. Obviously I wrote it all the way over here. Let's cross them. YY, RR. In other words, let's take this yellow round and this green wrinkled and see what we get. But, this is science so I want to predict what I think I might get in any number of situations. Now, let's do the whole gamete thing. Let's not worry. Let's not do pun and squares yet. Let's do the gamete thing.
Well, again, if we look at these, we're going to say, "Ah ha, that's pretty obvious that this one, if it's the male, it has to pass on two of it's traits." Right? I mean, we can't just say one trait, it's going to pass on all it's traits. If we had a thousand of these, we'd have to pass on a thousand of the alleles. So, we're going to pass on a y and an r. That's the male. This particular female would have to pass on a yr. Well, it seems very obvious that's what we would expect. What we would expect would be Yy, Rr, piece of cake. Yy, that's going to give me yellow and Rr, that's going to give me round. Make sense? Bet you're asking the same question Mendel did. I hear you. You're saying, "What happens when you go to an f[2]?"
Now we have an interesting situation where we can predict an answer. Whether it's right or wrong, we'll get there. You're going to take a Yy, Rr, and cross it with a Yy Rr. Now, what would you predict? Well, it kind of depends. Let's go to scenario one. In scenario one let's just say that because this came from a parent--think back to the parents. The parents had associated together the Y and the R. Right? Remember, the parents looked something like this. We'll put the parents over here. The parents of this cross were YY, RR crossed with yy, rr. So the parents had always the Y and the R together. So, if the Y and the R came together we can assume that, maybe in this hypothesis, that the Y and tri-hybrid R are going to stay together. So, this particular one will produce two different forms of gametes. A YR gamete and a yr gamete. Why? Because this parent gave the YR, and this parent gave the yr and why would it be any different for the offspring, in this hypothesis? Remember hypothesis. No answers yet.
Well, same thing here, you guys. Right? YR, yr, won't they still stay together. So, this is looking like it's going to be a pretty simple case. Let's do a Punnett square. So, now, if this is a gamete and this is a gamete and this is a gamete and this is a gamete, just like we did before, we can make a Punnett square. With that Punnett square--okay, this gamete goes here, because remember that's what you show on the Punnett square. Punnett squares can make life easy. I can even make it easier still, but that will come later. yr, right? YR, yr and golly gee whiz, let's see what we'd expect.
It's real easy when you have someone print up the Punnett square, but that's not what I was talking about. So, here we go. There was our parents--in this hypothesis, what the parents gave. In this hypothesis that's what the parents gave and there was no question about that. We knew that was going to happen. But now, in this hypothesis, what we're expecting it looks like--here's what we're expecting. Let's see what this YR and this YR is going to give. Well, that's the genotype. What's the phenotype? Well, this particular phenotype, right here, is going to be--remember what phenotype means, it means physical. This particular phenotype in this situation is going to be what? Yellow, round. This is expected in hypothesis one. Expected. So, we've done this one.
Let's take a look at this one Yy, Rr. Yellow, round. Let's take a look at this one. What do we have here? Let's see, we have Yy, Rr--these should be the other way around. There is going to be a yellow, round. This one is the interesting one, yy, rr, yy, rr, yy, what is that? That's green. What's rr? Wrinkled. So, guess what, you guy, we would expect yellow, round and green, wrinkled. This is what we expect. Let me give you Mendel's number, let's see if he's right, if this hypothesis is right.
Here's what he got. He got 315, 315 yellow, round. He got 108 yellow, wrinkled. Uh-oh. He got 101--uh-oh, this isn't looking like , to me. He got 101 green, round and he got 32 green, wrinkled. Wow. Does that look like , to you? What does this tell you about this hypothesis? History. They were not inherited together. So, Mendel realized that genes, alleles, factors must assort independently and he came up with his second law. What is that second law? The law of independent assortment. Genes sort themselves independently of each other.
Now, there's a lot more to it than meets the eye. What we'll do, soon, is we'll take a look at how we can explain this and go into 90's Biology and look at meiosis. Cool.
Mendelian Genetics and Mutation
Segregation and Independent Assortment
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