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Biology: Mendel: Alternate Alleles, Dominance


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  • Type: Video Tutorial
  • Length: 9:02
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 98 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: Gregor Mendel (4 lessons, $6.93)

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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So what did Mendel have? He grew these pea plants up and he got these very strange results. The P generation, not for pea plants, but for parents he took--what did we have? We have wrinkled crossed with smooth or spherical and he got in his f[1] 100% of them were smooth, spherical seeds. He then let them do what is called self-crossing. So he took the smooth f[1] and crossed it with smooth f[1] and golly gee whiz, so much for blending. Because what he got there was were smooth and were wrinkled and the wrinkled had magically reappeared. Not magic. You know the answer to this I bet you. So, Mendel, with his math background realized this is simply a case of numbers and that some very interesting things were going on here.
So, he came up with his first law. His law was called the law of segregation. I'm going to lay the law out for you in four steps, and then we'll see if we can make it make sense for his data. Okay? Because looking at this, there are some things that may be obvious to you like they were obvious to Mendel. But the bottom line is--well, let's take a look. First of all, what Mendel realized was this. Number one, there are obviously what are called... The Law of Segregation. Here's the thing. There are alternative versions of heritable factors that exist. What I'm going to do here, by the way--this is nice to talk in Mendel talk, but you and I need to talk like 20^th century biologists, so I'm going to tell you some of the things Mendel said. I'm going to put what we call them now in parentheses and then I'm going to refer to them in `90's kind of lingo, instead of 1890's kind of lingo.
Here we go. So what we have is that there are alternative or alternate versions of heritable factors. Heritable, what's that mean? Inheritable factors exist. Now, before I put "exists" down. What do you think this word factors means? Heritable factors--you and I know of as genes. There are alternative versions of heritable factors. Now, there is a word you may not have heard before, the term called alleles. Alleles are heritable factors that control the same trait. So, what Mendel realized was this, that there were two genes, that there were alternative versions of the gene for whether a coat was spherical and smooth or wrinkled.
There was the wrinkled gene and there was the smooth gene. They were alternative versions of the same gene. You see? So, they were alternative versions of the same gene. So, therefore, when we start doing genetics problems, we have to remember that. They are alternative versions of the same gene. So, what's an allele? Two genes are alleles when they control the same trait, but they may not be the same. One might give you a smooth coat; one might give you a wrinkled coat.
Two. Each organism gets one of these from each parent. The brilliance of this--this just gives me shivers when I think about this. Wait until you see how significant this is. Each organism gets one of these from each parent. Now, wait a second, wait a second. If it has two, and it got one from each parent to get two, what's going to happen to him or her or them, since it's a plant, when it becomes a parent? Well the same rule is going to apply. It can only give one of its genes.
What did Mendel discover before we even knew about chromosomes? Before we even knew about chromosomes Mendel described meiosis. The fact that these things are going to come one from each parent, and therefore the parent must somehow segregate them. Whoa, that's amazing. Now that's going to be part of his law that these alleles segregate during gamete formation. These alleles, my word, your word, segregate during gamete formation. How cool is that? Last but not least, four. In combination one allele, our word, heritable factor, his word, one allele may dominate or mask the other.
Well, let's take a look at how this is going to be used to explain our f[1] and I'm going to let you think about the f[2]. F[1] is easy. Now, remember we're going to show two alleles. So, let's say that smooth--okay, once again, quick review. Parents, smooth crossed with wrinkled yielded 100% smooth. All right. Now, we know that each of these have a different variety, they're pure breeding. Remember? That's how we bought them, they're pure breeding. Each of these must have two genes for the trait because they're parents and they're not gametes. So, if we take a look at this parent generation--so here's my parent generation and here's my smooth. Let's see what we have. I'm going to set up a little key over here.
I'm going to say that S equals the factor for smooth and I'm going to make it an upper case S. So, here's the smooth parent and I'm going to give it a S, but wait a minute. How many of these must this parent have? Well, this parent was someone's child and therefore it must have gotten two genes for this trait. So, it's got to have two S's, but is one--are they both the same and what do we know? He bought pure breeding lines, so there's the smooth parent. Now, he realized that smooth must dominate, but this one, the wrinkled, doesn't have any smoothness in it because it's a pure breeding line. To show his experimental results that smooth masks wrinkled, he didn't call the wrinkled W, he used s. We use s for wrinkled. How many? Two.
These are going to cross. Now you know we're going to have sperm and eggs, so let's just draw some sperm and eggs. Let's make this the male. Look at this. Only one possible gene can go into--you can form 12 million of these sperm, each one is going to have S. You can form 12 gazillion eggs, but each one going to have s. Right. Because, remember, one into each gamete. Look at this, you guys, Ss. Smooth--it's explaining how smooth is--how it got it's smoothness and it's explaining the hidden wrinkled. But does it explain the ratio in the f[2]? I'll tell you that one later.
Mendelian Genetics and Mutation
Gregor Mendel
Mendel's Conclusions: Alternate Alleles and Dominance Page [1 of 2]

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