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Biology: Crossing Over, Recombination: Map Genes


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About this Lesson

  • Type: Video Tutorial
  • Length: 11:06
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 119 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: Linked Genes and Genetic Mapping (5 lessons, $8.91)

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 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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You've heard me refer a few times if you've been listening in these genetics talks to the fact that we kind of know the locations of many genes and I've mentioned to you that certain chromosomes have thousands of genes. Well, how do we know that? Well, it's a sheer numbers game, number 1, and number 2, we have ways of figuring out where genes are.
Now let me tell you about a little story. A guy, Thomas Hunt Morgan, did a lot of great things back in the early 20th century and he is literally the father of using the fruit fly in genetics. He was the guy who figured out fruit flies were a wonderful way to do genetics experiments, quick life cycles, easy to culture. We now have lots of mutants of these things. And some other things came from Morgan's lab, too.
I want to tell you about a little experiment he did that made him scratch his head a little bit and one of his students, actually, came up with a great answer. Here's what Morgan did. What he did was he took some of these flies and he was doing a dihybrid cross. And so he had a mutation for the body and it was called the "black body fly." And the black body fly was a recessive trait meaning that in the heterozygous condition, it didn't express. So here's what we're going to say. The normal or wild type was kind of grey. So what we're going to do is we're going to say b equals black, and, therefore, B equals grey. So we have a grey-bodied gene and a black-bodied gene. And I want to tell you about a wing mutation. And the wing mutation that they have is vestigial. And, you know, that a vestigial is a structure that is kind of truncated or not functioning and there's a group of mutated flies that have truncated wings. And so we call this the vestigial mutant. And, once again, this is a recessive mutation in the sense that in the heterozygous condition, it has normal wings. So we're going to say that vestigial is equal to v and, sure enough, normal equals V.
Now, here's what Morgan did. He had a lot of stocks, and, you know, I think you guys know enough genetics now that I don't have to tell you where he got each thing and what parents gave rise to what. But he had a fly that he was crossing and this particular fly had the following genotype. It was a heterozygote dihybrid fly. So it was normal. I mean, to all intents and purposes it was normal. And what he did was he was doing something. See if you can tell me what kind of cross this is. And he crossed it with this, the double recessive. What's that called do you remember? It's called a test cross. When you cross something, a heterozygote, known or unknown, with a known homozygote recessive.
Now what would you expect from this? What you would expect is you would only expect 1 gamete from this, bv, and from this you would expect 4 gametes, right? You would expect BVBv. You would expect a bVbv. And therefore, you're going to have 4 possible combinations, right? This with this, you know, we're going to have a combination across here, a combination across there, a combination across there and a combination across there. So let's take a look at their phenotypes. O.K. Let's just look at their phenotypes. BbVv, normal. That's 1. Two, bbVv, black. That's 2. bbvv, that's black and vestigial. These, of course, have this particular one. Let's see. This number 1 is what? Bb, so it's normal color and normal wings. So that's plain old normal. This one is black body, normal wings. So this one is normal, normal. And this one's black normal. And then my 4th one--that's 3. Let's look at 4. Bbvv. So that's normal body, right? Bb, but vestigial wings, vestigial wings. So Morgan expected 4 if it were independent assortment. But that's not what he got. He got a surprise. He got 50/50. Not normal, black normal, black vestigial and normal vestigial. He got 50/50. Let's see why.
Well, what he figured out is that somehow these things must be linked to each other. And therefore, if we take a look at my bbvv parent and we look at its chromosomes, there's only one choice, bv. And therefore, when we look at our heterozygote over here, BbVv, it must be that these genes are linked and that in this case the BV are together and the bv are together because look at what that would give me. In this case, it would only give me a choice of two and that's what he got. He actually got the following from his cross. He got--well, let me tell you what he got. I'll give you some real numbers.
What would you expect from this cross? Let's be tough on you. What would you expect from this cross? Here's what you would expect. You would expect this combination which I'll call 1. You'd expect BbVv and then you would expect a second combination. You would expect this one. You would expect bbvv. So you would expect what? A black body, vestigial wing and a normal and that's all. So the reasoning is I should get 50/50. And he almost got that. Almost.
Here's what he got. So thinking about this, you realize that if it were independent assortment you would have expected , , , , and if it were this linked, you would expect 50/50. Well, here's what he got. He got a total of 2300 flies, give or take one. Here's what he got. He got 975 of these. He got 944 of these. Which is very convincing evidence that it was this, right? How many did I tell you again? 2300? Those numbers don't add up to 2300. Did I make a mistake? I don't make mistakes. If I make mistakes, we cut and redo the shot. No. Here's what he got. He got 206 and 185 of the other 2 genotypes possible if this were independent assortment. All right. Stop and think. Wow, it sure looks good. Look! We had a bunch of these, but we got enough that we can't ignore the fact that we have 206 vestigial normals and a 185--what was it? One vestigial normal and one black normal. This shouldn't be. This is acting like it's independent assortment, but it's not 25/25. What am I going to do? That's why Morgan had students. Because he had a student that said the following. He said, you know, let's assume these genes are linked. Could there be a situation where if they're linked, somehow some of the genes might cross over? I don't think he used those terms. But could there be breakage and reunion and therefore, could some of those genes that were linked become unlinked. Let me just show you here. Let's look at this situation here. Remember the linked situation what he suggested. He suggested this. Well, maybe it was like this. Maybe it was where we have, let's see. What was it? It was--we had bb and a v here. Right? And those were always inherited together. And then we said, uh-huh! Could we have in this case a BV and they would stay together, too. But maybe, just maybe this B might go over here and now look at what you have. It's going to behave in a small amount of cases as if this is a B. Not as if, the B just moved over here and look. Now it's Bv. Not all the time, but occasionally could that happen?
Well, you know? That student went onto fame because we're still talking about him. His name? I can't even spell it. Sturtevant. And we're going to see what his deduction allowed us to do when it comes to gene mapping.
Mendelian Genetics and Mutation
Linked Genes and Genetic Mapping
Crossing Over and Recombination: A Tool for Mapping Genes Page [2 of 2]

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