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  • Hi. It's Mr. Andersen and in this video I'm going to go through chromosomal inheritance.

  • Most students understand Mendelian genetics and could do a simple Punnett square like

  • this, but in this video we're going to dig more deeply and see that those genes are actually

  • carried on chromosomes and packaged into gametes. And so it's built on the work of Thomas Hunt

  • Morgan. And so we're going to go through a monohybrid and dihybrid cross. Not only the

  • Punnett square but let's dig into how the chromosomes are actually delivering those

  • genes. And so in this monohybrid cross, this first Mendelian cross, he crossed remember

  • yellow with green and got just yellow seeds. And then he selfed yellow with themselves

  • and found that 3 to 1 ratio. So if we were to add letters to that that represent the

  • genes, the parents are going to be homozygous dominant and recessive. And the F1 generation,

  • they are going to be heterozygous. And then we produce a Punnett square that shows us

  • all the alternatives that we could get from that F1 cross. And so what do these two sides

  • really represent? This big Y represents the odds of delivering that big Y allele or that

  • dominant allele. And this is the odds of representing that little allele or that recessive allele.

  • And so there's a 50 percent probability. And so these are really the odds of producing

  • gametes that have the dominant gene or the recessive gene. But remember you don't just

  • produce two gametes, you produce four. And so we're going to dig into that in just a

  • little bit. These would be the gametes from the other offspring. Or the other parent.

  • And then these are going to be the four possible offspring that we could get. Now remember

  • those genes are actually delivered on chromosomes. And so if we look at it at the level of Thomas

  • Hunt Morgan, and in this model I just have 2 chromosomes here. We find that that dominant

  • gene is actually found on a chromosome. And the recessive gene is found on a homologous

  • chromosome. In other words it's the same length. Now where did these chromosomes come from?

  • They came from the P generation. And so in the P generation you could imagine that parent

  • would have 4 chromosomes. They would all be red. And each of their chromosomes too are

  • going to have those dominant Y genes. And likewise for the other parent. And so now

  • we have to figure out what possible gametes could come from this. And so we have to dig

  • into meiosis. Remember meiosis starts with duplication of the DNA and then two divisions.

  • And so let's duplicate the DNA and see what happens. We've got two that have the big Y

  • and then two that have the little y. And so you could imagine we're going to make 4 gametes.

  • And you can almost visualize those. So each of these vertically is going to make a gamete.

  • And so how many possibilities do you see so far? I just see the big Y, here's another

  • big Y. Or the little y. Now remember during meiosis they're going to independently orient

  • themselves at the metaphase plate. So they could line up like that. Or they could line

  • up like that. Or they could line up like that. Or they could line up like that. There are

  • four arrangements. But it doesn't really matter how they are orienting themselves. We're still

  • seeing those 4 possible gametes. But two of them are going to be exactly the same. Each

  • sex cell is either going to get a big Y or a it's going to get a little y. And so let's

  • see those two divisions in meiosis. So there's one division. And there's another division.

  • So remember you produce four cells in meiosis. But you can see that two of those are going

  • to be the exact same genes, the exact same chromosomes. And so we really only have two

  • alternatives. Big Y or little y. Now how does fertilization work? Remember we're going to

  • have those gametes of the other parent. And then they're going to combine. And since there

  • are two possibilities on each of the parent we could only have four possibilities of the

  • offspring. Let's kind of visualize those. We could get a big Y from each of the parent.

  • So that would be one possible zygote. And that's going to be a yellow seed. We could

  • arrange it like this. Get a dominant from one and recessive from the other. Or a dominant

  • from this one and a recessive from the other. Those are each going to be yellow as well.

  • Or we could get two recessive genes or recessive chromosomes. And so how many possibilities

  • do we get? We get a 3 to 1. Now the Punnett square worked great. We didn't learn anything

  • new really by looking at the level of the chromosome. But that's about to change. Now

  • let's look at a dihybrid cross. And so in this cross Mendel crossed yellow round with

  • green wrinkled and he found in the F1 generation that they were all yellow round. So yellow

  • and round seemed to be dominant. And so if we were to add letters to that these would

  • be homozygous yellow yellow round round versus green green wrinkled wrinkled. And so what's

  • going to be our F1 generation? They're each giving one of each of those letters so there

  • going to be hybrid for both. But now Mendel is presented with this problem. And so do

  • those genes travel together? Are they dependent on one another? Or are they independent of

  • one another? Because those two alternatives could produce different possibilities. You

  • could get a Punnett square that looked like this where we only have two possible phenotypes.

  • Or we could have 4 possible phenotypes. And tons of different ratios. And so you probably

  • know what's the right answer. But let's go through the Morgan way. Look at the chromosomes

  • and figure out what's going on with those genes. And so if we go back to the F1 generation,

  • and let's just put the yellow and the green on our respective chromosomes, now the question

  • is begging, is the R gene, is that round gene going to be found on the same chromosome as

  • the yellow or is it going to be found on a different chromosome? And so let's just play

  • out those two possibilities. Let's just say that the round is found on the same chromosome.

  • So let's put it right here. Again, you're getting the round and the wrinkled gene from

  • that one parent, the dominant parent we could say. And then the two recessives from the

  • recessive parent. And so let's go through all the possibilities that we could get. And

  • so now we've got interphase where we copy the chromosomes. And now we have to figure

  • out all of those different orientations. It could like up like this. This would be a gamete

  • that's going to be round and yellow. And this would be a gamete that's going to be green

  • and wrinkled. And so let's figure out if they orient themselves in a different way. Does

  • that change the different gametes that we could get? So they could line up like that.

  • That's not really changing the possibilities. They could like up like that. It's not really

  • changing anything. You can see that the round is going with the yellow. And the wrinkled

  • is going with the green. They could orient themselves like that as well. And so really

  • we only get two possible gametes in this case. And so if we were to go through meiosis, let's

  • grab one like that. This would be one parent. This would be the other parent. So how many

  • possibilities could we get? Well since this parent only has two gametes and this does

  • as well, there's really only four boxes in our Punnett square. So you could have an individual

  • made from round and yellow. So dominant for both traits. You could get a dominant from

  • this side and a recessive from the other. Or vice versa. Or we could get both of those

  • recessive genes and it would look like that. So if we were to do this one alternate reality,

  • how many possible phenotypes would we get? These are all going to be round yellow except

  • one-fourth of them are going to be wrinkled and green. And when Mendel did that cross,

  • that didn't happen to be. And so we now know that those are going to be found on different

  • chromosomes. And so the round and the yellow are found on different chromosomes. So let's

  • go through meiosis and figure out how we're getting all these different possibilities.

  • And so again during interphase they're going to copy all of their DNA. So we've duplicated

  • the chromosomes. And so they could line up like this. And so what kind of a gamete would

  • this be? It's going to be a round with a yellow. And a wrinkled with a green. But if it lines

  • up like that, nothing changes. We've still got round with yellow and wrinkled with green.

  • But let's say it lines up like that. Now what do we have? You can visualize now we've got

  • a wrinkled with a yellow. And we're also getting a round with a green. And so how many gamete

  • possibilities do we have here? Four possibilities. This again, three and four, are going to be

  • the exact same thing. And so now there are four possible gametes. And so if we were to

  • play that out, using just a Punnett square we'd have to put all those possibilities on

  • one side. All those possibilities on another. And we'd find that all these individuals are

  • going to be round and yellow. These three individuals are going to be round and green.

  • These three are going to be wrinkled and yellow. And only one of those 16 possibilities are

  • going to be wrinkled and green. Now Mendel was able to figure this out not understanding

  • what's going on with chromosomes. And so it took decades before scientists looked at this

  • and found a flaw in his model. And let me show you that. So in this Thomas Hunt Morgan

  • cross we're dealing now with fruit flies rather than peas. But in this cross it's a simple

  • test cross. So what he has is a wild fruit fly. But it's hybrid. So what does that mean?

  • It has one of the wild genes, and this is for normal wings with a vestigial gene, which

  • would normally create these small little wings. And then it's also hybrid for coloration.

  • So it's got the normal coloration. And then it has one of the genes for this black coloration.

  • Likewise this other parent over here is going to be recessive for both. And so when Thomas

  • Hunt Morgan, because he understood Mendelian genetics, set up his Punnett square he set

  • it up like this. And so what are going to be the possible gametes that this fruit fly

  • could give? Well it could give VG+ and it also could give B+. It could give that. It

  • could also give each of the recessive genes. So it could give that recessive vestigial

  • wing gene and the black gene. It could also then give a combination of those two. So there

  • are four possibilities that we could get from that one parent. Since this one is recessive

  • for both, it really can only give one gamete. And so Morgan knew that there would only be

  • four possibilities and we should get 25% of each of these. And so if you get these gametes

  • you should get a hybrid for each and so you're going to look exactly like this parent. You're

  • going to be wild type for both. If you're this, you're going to get all recessive genes

  • and so you're going to look like this parent. And so we call those parental phenotypes,

  • because they look just like the parent. This one here looks like that parent. This one

  • looks like that parent right here. You could also get what are called recombinants. And

  • so this one right one right here is going to have the normal wings but it's going to

  • have that black coloration. And this one over here is going to have normal coloration but

  • it's going to have vestigial wings. And so Morgan knew that it would be 50% parental

  • types, 50% recombinants. But when he actually did the cross that's not the number he got.

  • What he got was 17% recombinant, 83%. So Medel was wrong or his model didn't go deeply enough.

  • And so in the next video on genetic recombination and gene mapping I'm going to answer that

  • and show you not only did Morgan figure out now the genes are carried on chromosomes.

  • But the importance of crossing over producing those recombinants. And so that's chromosomal

  • inheritance. It's understanding genetics not at the level of Mendel, but also at the level

  • of Morgan. And I hope that was helpful.

Hi. It's Mr. Andersen and in this video I'm going to go through chromosomal inheritance.

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B1 中級

染色体の遺伝 (Chromosomal Inheritance)

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    李掌櫃 に公開 2021 年 01 月 14 日
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