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Biology: Mendel's Findings: Phenotypic Ratios


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  • Type: Video Tutorial
  • Length: 10:22
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 111 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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To commemorate our discussion of Gregor Mendel I've planted, for your viewing pleasure, some pea plants. That brings us to Mendel's experiment. You know, Gregor Mendel, in many ways besides his scientific knowledge, was a planning genius. Why was Mendel's experiments--why, all these years later, are we talking about this guy like he did some of the most cutting edge science in the history of mankind? Because you know what? In many ways he did. He was one of the first people to use a mathematical analysis in a scientific experiment, which is probably the reason why it took the world 40 years to figure him out.
You see, he was unbelievably detailed. His work, Mendel's work was structured and well planned. That is what led to his success. The fact that everything was well thought out and well planned. Indeed, all he worked with were pea plants. Nothing fancy. Pea plants that even back in those days, you could purchase. See, once again, domestication of plants has literally been around for thousands of years and has always been a business. Back in Mendel's days, you could buy the seeds that you chose. Trading seeds was important and bartering seeds and buying seeds. What Mendel did was he purchased 34 true breeding strains of pea, 34. That number is not as important as the fact that they were what are called true breeding.
Now, what the seed sellers of those days meant by true breeding was this. Guaranteed, if you plant this seed, you will get no other variety. So, if I tell you you're getting a pea that if you want to grow peas in your garden that have yellow pods, this is a true breeding, yellow-podded pea. A true breeding, yellow-podded pea would always give you yellow-podded pea plants. Meanwhile, you could have a green-podded pea. The answer was that he bought a good 34 of these and chose seven of them to go into his experiments. So, he had these true breeding strains.
What was his strategy? What was his experimental plan? His experimental plan was to cross these plants for one trait only, each one in different combinations. So, for example, what he would do is he would take a green-podded plant and he would cross it with a yellow pod. Now, how did he cross it? You remember what he did, right? Quick drawing of a flower. He'd take the flower and in this one, what he would do is he might remove the anther from this one and take the pollen grains. To this one, he would go over to this one and--draw my flower, the flower is a little defective--and he would remove the anthers from this one, so there was no pollen--before they ripened, and he'd put the pollen from here onto this.
But Mendel was no dummy. He knew that if his work did--if, as he tried to explain his work, people would say, "Well, wait a minute, you got those results because you used the pollen from the green pod and put it on the stigma of the yellow pod, but you didn't do it the other way." So, obviously Mendel would set up a second experiment for each of these things. And indeed, he would take from this one, he would take the pollen and he'd put it on the stigma of this one. What's that called? Do you remember what that's called? That's called a reciprocal cross. So that he was showing, like was shown in the 17^th century, that you would get the same results no matter which way you went. Mendel called these crosses a mono-hybrid cross. One hybrid. Hybrid means mixing. So, he would call these a mono-hybrid cross and he would follow them through three generations.
He would call the first generation his parent generation, which was the P generation. He would call the first generation the filial generation 1, or filius Latin for son, filial, which was f[1]. That's the children, if you will. Then he would allow these to self-cross and come up with what he called his filial generation 2 or his f[2]. So, now you know some of the Mendelian vocabulary that was used at the time to do this kind of thing.
Let's talk about the first cross Mendel documented. The first cross that Mendel documented was between plants of a seed type. What Mendel did was he took a plant with wrinkled seed. So, he took pollen from a wrinkled seed plant and he crossed that with the stigma, or he put it on the stigma, of a plant that had smooth, kind of spherical seeds and he did the reciprocal. So, he went pollen to stigma and then the pollen of the smooth to the stigma of the wrinkled. What was his results? Well, this was his parent generation. Then he got his f[1] and he got some very curious results. What were those results? They were all smooth, in other words, round, spherical plants.
Well, wasn't that curious? Wasn't that curious that he took smooth and wrinkled and didn't get some smooth and some wrinkled, and didn't get some that were half smooth and half wrinkled, or didn't get some that were kind of wrinkled? He got all smooth--smooth, spherical. Well, winter set in. There wasn't much he could do, because you can't plant seeds in the winter in a monastery, so he waited until the next year. So, he took these and he planted them and he let them grow. So, there's their flowers and he let them cross with each other. In other words, a self-cross. Now, why wasn't it important that he remove the anther? Well, what were you going to remove anthers from? You literally had these things--you allowed them to mix. So, he did an f[1] f[1] cross. Smooth crossed with a smooth. How bizarre. In his f[2], it turns out, that wrinkled reappeared, wrinkled reappeared. How amazing.
In fact, the next--in his f[2], after--remember what he got. He let this pollinate this and this pollinate this. You got little seeds. You plant the seed. Right? Seeds grow into new pea plants and you look at their seeds. In this second generation he got 7,324 peas. Of those 7,324 seeds, 5,474 were spherical, were smooth, spherical and 1,850 were wrinkled. That's about , . Let's transfer those numbers. That is roughly smooth, spherical and wrinkled. So now we have an interesting question. One thing that became obvious to Mendel was this. In that f[1]generation there must have been some kind of factor that went in there that somehow in that f[1] the wrinkled didn't disappear, it was hidden. The wrinkled was hidden or in other words, was dominated. So, somehow, these guys had the wrinkled factor, he called it, hidden in them, but the smooth was dominating it, was masking it. Later in the next generation this wrinkledness came out somehow.
But what a puzzle. Why was it ? Well, Mendel went on to repeat this experiment with a bunch of different variants. I'll show you some of them, because we're still going to have one more question to answer. The question is, why that ? For example, he would cross green and yellow pods. He would cross-inflated pods, fat ones, or restricted, kind of like sugar snap kinds, the restricted. He would cross what we call axial flowers with terminal flowers. He crossed flower colors, purple and white. He crossed green and yellow and he crossed spherical and wrinkled. In all cases one trait dominated the other, but how do you explain that ? Hint, remember Mendel's math background.
Mendelian Genetics and Mutation
Gregor Mendel
Mendel's Findings: A First Look at Phenotypic Ratios Page [1 of 2]

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