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Mendel's Second Law
M.Ed., Stanford University
Winner of multiple teaching awards
Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.
If you’re watching this video, you’re hopefully pretty familiar with Mendel’s basic rules. His First Law of Segregation. If you take a look at my earlobe, you can see I’m detached, which means I’ve got the dominant phenotype. If you knew my mother had attached earlobes like these, you would know therefore that I’m heterozygous for this trait. If these terms heterozygous, dominant, recessive are kind of unfamiliar to you, you should take a look at my first video on Mendel’s first law, and then come back and watch this, and you’ll be good to go.
Now, there’s another trait that I have got that dominant are for, that’s my ability to roll my tongue. If I tell you that I’m heterozygous for this, you could easily use a Punnett Square to figure out what my children might look like, if my wife was also heterozygous for both traits like me. What about if I’m doing both traits at once? How does my ability to roll my tongue, affect my ability to have children with detached or attached earlobes? Get your mind out of the gutter.
What I really mean is that does my dominant tongue rolling ability have to travel along with my dominant detached earlobe ability? Or does it not matter?
The guy who figured this out was Gregor Mendel. He came up with the Second Law of Independent Assortment. Now, when I go through this, I’m going to describe Mendel’s second law. And then I'm going to show you how modern knowledge of how meiosis works, affects Mendel’s First Law and Mendel’s Second Law. So let’s dive right on in with Mendel’s Second Law of Independent Assortment.
Let’s begin with Mendel’s law. Like I said, Mendel’s second law is often called the Law of Independent Assortment. What this means is how one pair of allele separates for a particular gene, is completely independent, not affected at all by how a second pair of alleles for a different gene separates. Now let’s take a look at me with my big R, little r, big E, little e detached earlobes and tongue rolling ability.
So if we look at this, you can see the Rs have to separate and the Es have to separate. Now when we’re looking at just big R, little r, you knew that there was two possible gametes I could form. A sperm that had big R, and a sperm that had little r. If I was just looking at the big E and the little e, then I would have two again; a big E and a little e. Now remember, when I make sperm, I don’t want to just create a sperm that creates a baby with a tongue rolling ability and no legs nothing else. I need to give that baby one copy of each gene.
So what will happen is that, when I do this, my big R could go with the big E making a big RE sperm. I can make that R go with the little e, so I can have a Re sperm. My little r could go with the big E, so I have a rE sperm. Then my last little r with little e, re sperm. So these are the four possible sperm that I could create for this trait.
Now if my wife was also RrEe, then she could make similar gametes on baby eggs, no sperm. So how would I do this Punnett Square? You remember with one trait, I would have all the gametes on one side, all the gametes across the top. And if I was only looking at one trait, it would be 2 by 2. With this kind of one, I have to have 4 by 4. So let’s go ahead and do that Punnett Square.
So I’ll put say my gametes up here, RERe, rEre. Over here down this side, I’ll go ahead and put my wife’s. Since she is identical to me in terms of this combination of traits, we’ll have the same combination of gametes here.
So in here, egg and sperm get together and I do RREE. Then RREe, and then RrEE, I’m bored. Let’s just go ahead and just jump to the end. So here we see the Punnett Square all filled in. Now let’s take a look at some of the possible offspring that I created. This person here what can they do? Well, they’ve got a big R, one or two it doesn’t matter, because it’s a dominant trait and they’ve got big E. So this person is a tongue roller with my detached earlobes. What about this person here? They’ve got the homozygous recessive combination or genotype for the tongue rolling ability. So they’re a non-roller, but they are capable of having the detached earlobes.
What about this person here? Tongue roller, attached earlobes and this little person over here they’re recessive for both traits. rree they are tongue rollers and glued to the jawline attached earlobes. So let’s go ahead and take a look at their phenotypes.
So if we count them up 1, 2, 3, 4, 5, 6, 7...9 of the tongue roller detached earlobes, we’ll see 9 of the offspring out of 16 possible have that phenotype. Let’s count up those who have the tongue rolling ability, but attached earlobes. And when we do that 1, 2, 3 of the orange, we see there’s three of the roller, attached earlobe types.
But second last, let’s take a look at the non-roller, but detached earlobes. We see there’s three of those. Then right here, we see one of the homozygous recessive for both traits; non-roller, attached earlobes. 9:3:3:1 ratio. This is a ratio that you should remember. Memorize it, because it’s the one they’re most likely going to be using on the AP exam. If you see a 9:3:3:1 in the offspring, just go straight to a di-hybrid Cross; RrEe for both parents. So if you get this down, you can handle it. It’s not that hard.
You just put the gametes across the top, put the gametes across the bottom. Don’t forget to put one of each kind of gene in each gamete and then you’ve got it. Now what’s pretty amazing about Mendel is, he figured out all this stuff without knowing a single thing about DNA or even meiosis. It took a long time for people to figure out the connection. But now that we do, let’s take a look at how Mendel’s laws relate to meiosis.
Now when we look at this, let’s begin with Mendel’s first law and meiosis. You recall of course that during meiosis one, the first half of meiosis, the homologous chromosomes pair up during prophase one and exchange parts and then go to the metaphase plate during meiosis one. And then separate to form two haploid cells. If you’re unfamiliar with this stuff, you should go watch my video on meiosis, or just check out that portion of your textbook.
Now this requires that you know what homologous chromosomes are. To represent this I’m going to use shoes. Take a look at this. Is this shoe identical to this shoe? No. Are they very similar? Yeah. One is a left, one is a right, but they pretty damn close, especially if you compare that to these shoes. These two are homologous to each other, but this shoe and this shoe just don’t match.
So I’m going to put the brown ones down for now. I’m going to focus back on these grey ones. When I talk about homologous chromosomes, I’m talking about those similar chromosomes that you got from your mum and your dad. Your mum is human, your dad’s human, I’m hoping. Your mum’s number 8 chromosome is pretty dang similar, not identical to, but similar to your dad’s number 8. You’re a number 9, these are similar your dad’s number 9 and so on and so forth.
What I’m going to do is I’m going to add some alleles to these. Because we know now that the chromosomes are made of segments of DNA with various genes on them. So I can just put a big R onto this and a little R onto that.
So now I have my homologous chromosomes. And you know that during prophase one, they can dance in form and then they pair up, and then move to the middle of the cell, and then separate. That’s Mendel’s First Law. Look, we’ve got 1, 2 pair of shoes, pair of factors. What do they do when we form our gametes? They segregate or separate from each other. That’s Mendel’s First Law, very simple.
What about Mendel’s second Law? Let’s go ahead and take a look at that. Now remember, Mendel’s Second Law is all about how two genes behave. So I’m going to bring in that brown pair of shoes again. This time I’ve got the letter 'e' on them. So I’ve got a 'E' on one, and 'e' on the other. Now you’ll remember during prophase one, the chromosomes will tumble around. Now how this pair of shoes tumbles has no influence on how this pair of shoe tumbles.
So I may eventually wind up during metaphase one, lined up like this. If that’s the case, then I’ll wind up with one gamete that happens to have the big R together with the big E. The other gamete has the little r with the little e. That’s one possible outcome.
Alternately, it could have turned out where I have the big R with the big E, and the little r with the little e. Either outcome is equally possible. So in a guy who’s making tens of millions of sperm a day, you typically find one fourth of his sperm look like this, re. One fourth of his sperm look like this; rE. Another fourth of his sperm will look like RE and then another fourth of it will be Re.
Now some of you who're thinking ahead and may wonder, hey what happens if instead of them being on different chromosomes, what if that’s here together? That does happen and that’s called Blink genes. That’s a major exception to Mendel’s laws. That’s beyond the scope of what I can address today, but if you’re curious about that, read about it in your textbook, or take a look at my video where I go into complications of Mendel’s laws.
So now we know how Mendel’s second law works with meiosis. Now we can figure put that yes, my tongue rolling ability doesn’t affect my ability to have future children with my detached earlobes. No, not according to Mendel’s laws it doesn’t.
So now we know that Mendel’s Second law allows us to use simple probability or those large Punnett Squares to predict the outcome of more than one gene being passed on to the next generation. Because we know that one gene does not affect how another gene is passed along. Decades after Mendel was doing his experiments with pea-plants, scientists figured out that the events during meiosis one explains Mendel's Two laws.
Now using the knowledge that you have about meiosis and Mendel’s laws, you can also now answer those really tricky question in the essay portions, where they’re going to ask you to make those connections. To help cement this knowledge, go ahead and use the bonus materials. There’s a problem set I gave you that addresses both laws, plus you should go online and do the virtual lab for the official AP lab.
Now bear in mind that they’ll have some questions about sex-linked traits which I’ve not covered here today. If you need to go over that, I’ve got another episode where I talk about that. Otherwise, you can just use your textbook and you’ll do fine.
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