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Biology: Dominant Gene? Intermediate Inheritance


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About this Lesson

  • Type: Video Tutorial
  • Length: 10:08
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 109 MB
  • Posted: 07/01/2009

This lesson is part of the following series:

Biology Course (390 lessons, $198.00)
Biology: Mendelian Genetics and Mutation (36 lessons, $54.45)
Biology: Genetic Dominance (3 lessons, $4.95)

Taught by Professor George Wolfe, this lesson was selected from a broader, comprehensive course, Biology. This course and others are available from Thinkwell, Inc. The full course can be found at http://www.thinkwell.com/student/product/biology. The full course covers evolution, ecology, inorganic and organic chemistry, cell biology, respiration, molecular genetics, photosynthesis, biotechnology, cell reproduction, Mendelian genetics and mutation, population genetics and mutation, animal systems and homeostasis, evolution of life on earth, and plant systems and homeostasis.

George Wolfe brings 30+ years of teaching and curriculum writing experience to Thinkwell Biology. His teaching career started in Zaire, Africa where he taught Biology, Chemistry, Political Economics, and Physical Education in the Peace Corps. Since then, he's taught in the Western NY region, spending the last 20 years in the Rochester City School District where he is the Director of the Loudoun Academy of Science. Besides his teaching career, Mr. Wolfe has also been an Emmy-winning television host, fielding live questions for the PBS/WXXI production of Homework Hotline as well as writing and performing in "Football Physics" segments for the Buffalo Bills and the Discover Channel. His contributions to education have been extensive, serving on multiple advisory boards including the Cornell Institute of Physics Teachers, the Cornell Institute of Biology Teachers and the Harvard-Smithsonian Center for Astrophysics SportSmarts curriculum project. He has authored several publications including "The Nasonia Project", a lab series built around the genetics and behaviors of a parasitic wasp. He has received numerous awards throughout his teaching career including the NSTA Presidential Excellence Award, The National Association of Biology Teachers Outstanding Biology Teacher Award for New York State, The Shell Award for Outstanding Science Educator, and was recently inducted in the National Teaching Hall of Fame.

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Well, you know, if you guys are good at this whole idea of Mendelian inheritance and some genes masking others and this concept of dominant genes and recessive genes and you're feeling really confident about that, that's too bad. Because I got to tell you, we have to start thinking in a different light. Now let's go back to Mendel's time. You know, it always surprises me and perplexes me and I certainly don't want to cast dispersions on a dead guy, but it always amazing me that Mendel got so lucky. He was very lucky. Think about it. All of his genes, clearly one dominated the other. None of his genes, they all sorted independently. And when you look back in the history books, it is said that he ordered 34 breeds or strains of these things. Chances are that he got a lot of data that didn't make sense. And that doesn't mean he cheated because I'm not even suggesting that. But the bottom line was Mendel gave data that worked perfect and as we've learned more and more about genetics, we found out that it's not always a perfect world, or it certainly is not always a black and white world or it's not always a dominant and recessive world.
You see, here's the thing. There's no such thing as a dominant gene. Sorry, I have to tell you that. Now we have to fix that a little bit, but you know what? Let me say that another way. Let me ask you a question. What makes a gene dominant? Why would yellow hide green? Why would smooth hide wrinkled? Why can I do things and here's one for you. This one I don't get. You know, in genetics classes all the time we use this example of like this. If you can roll your tongue, that's supposed to be a dominant gene, the ability to roll your tongue. I don't get that one because how can one gene control all these muscles. But it's really inherited like a dominant gene. Two people will have children and if neither of them can roll their tongue, their kids can't roll their tongue. I mean, it's really inherited like a dominant recessive gene. But if you know molecular genetics, which I think you probably know, you know something called the central dogma. And in the central dogma, you know that DNA makes RNA and RNA makes proteins. So when we're talking about a gene, at least a coating gene and you know also that there's a lot of different genes. There's regulatory genes which you passed on. There's a lot of different genes. But if you talk about a coating gene and therefore a protein, now I want you to start thinking about what is a dominant gene? How can a protein mask another protein?
Well, that is something that now I want to start talking a lot about, because genes don't mask each other. Genes make proteins. At least expressing genes do. How could one protein mask another and the answer is that really from what you know about biochemistry, it can happen. So now we've got to start looking at some of the other aspects of genetics and the pieces will start to fall into place. And I want to start out with a thing we call intermediate inheritance. In other words, inheritance that's kind of like doesn't seem to be dominant or recessive. And I want to give you an example of something called blending and another one called co-dominance and they're both called intermediate inheritance. Different books refer to them in different ways, but here's the point I want to make. Generically speaking, this is a case where neither gene dominates, or maybe both genes dominate. Let's see what I mean.
We'll start out with one kind of intermediate inheritance that occurs quite commonly in snapdragons, in flowers, okay? And this is often referred to as blending. Blended inheritance. And here's the thing. If I take a red snapdragon and I cross it with a white snapdragon, I get--now you're thinking, ah-ah! One's going to dominate the other. I get 100% pink. Blending, kind of like they believed before the days of Mendel. Are we going in this ugly circle? Well, no. Genes make proteins. Wow! If you think of this in terms of proteins, well then wait a second here. Then there's a red protein, a red pigment, and a white pigment. If I mix red and white, I get pink. This works. Well, that must mean there's a gene to make red and a gene to make white. But how are we going to make these genes? We can't call them Rr because one's not dominant over the other. We can't call them Ww so we've come up with a way to show intermediate inheritance types of genes. And here's the way.
We're going to say that C^R equals red and C^W equals white. C for color. All right. So let's do this. Let's look at the red. If we took that red and crossed it with the white, what's going to happen? Well, let's look at our pinks first. Maybe we should look at our pinks first. There was a cross between the red and the white so it must have gotten one red gene and one white gene because Mendel's laws are not going away just because we're going to call them something else. So we're going to say, Uh-uh. Pink must have C^R, C^W. That means that the red must have at least C^R and certainly the white must have a C^W, but if the red was a heterozygote, would it be red? No, it would be pink. And so we come to this situation right here. A case where neither red nor white dominates, but their protein products blend.
Let's take a look at that a little bit prettier. All right. So here's my red, C^RC^R. Here's my white, C^WC^W. What's going to happen is the gametes we are going to be only C^R from the red, only C^W from the white and therefore, we're going to get an F1 pink. Okay? Let's take a look at how that works in a Punnett square. So what we have is--oh, I'm going to draw the Punnett square for you. I almost gave away what I was going to do next. C^R, C^R C^W, C^W. Well, you can see we're going to get all--if I can take a short cut and you guys won't yell at me, R^WR^W. But now I have a question for you. Suppose we cross a C^RC^W with a C^RC^W. I want you to take a minute and do that right now.
So you think you've got an answer, huh? Let's take a look. Well, here's the thing. If we cross a C^RC^W, let's think of the gametes we're going to have. This is kind of cool. We can have two different gametes from Dad. We can have a C^RC^W, and for Mom, we can have a C^RC^W. Wow! How cool is this? Because now this is going to show us something that this is still Mendel's laws. Look, you guys. This is showing a way--remember back in the days when no one understood why certain genes reappeared? Well, look at what's happening here. Red and white reappear. From the C^RC^W, there's one parent; C^RC^W, there's the other parent. Well, look at what we can get. We're going to bring the Punnett square down and there's a C^RC^R. So from two pinks, we're getting a red. And we're getting a white because the C^WC^W, and we're getting 50% of our offspring a pink, C^RC^W. So we have C^RC^W right here and C^RC^W right here. Two pinks. So Mendel's laws of segregation and of alternate alleles work. It's just that we have to understand that one gene is not dominating the other.
Let me give you one more example of this. I'll show it here in cattle. There's a kind of a cool situation there where there's this kind of roan color. And it turns out that when you take a reddish--some of my students ask me, "I've never seen a red cow. What are they like? Fire engine red?" No, it's kind of a reddish maroon. Okay? And you cross it with a white, you get this very interesting combination called roan which is an intermediate color. But now, here's what we find. This doesn't blend. If you go up to that cow and look very carefully while it's asleep because you don't want to go up to a cow or bull and look at them while they're awake and look at their hairs, you find out that their hairs, some of their hairs are red and some of their hairs are not red. Sorry I don't have white. And so what happens is you have an expression of both genes. So the genes are co-dominating, they're both coming out.
So now this is just the first step. This is the baby step we're going to take as we start to look at modern genetics and the genetics beyond Mendel. But don't forget, his laws still hold.
Mendelian Genetics and Mutation
Genetic Dominance
What Is a Dominant Gene? Intermediate Inheritance Page [2 of 2]

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