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PETER REDDIEN: Starting at the beginning,
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people have been trying to understand
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the principles of genetics for well over a century.
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And I think it's important to start at the beginning
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because there's some ambiguity to how traits are passed on.
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When you just look at the results of crosses,
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that require some sophistication to explain.
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OK, so there's things like Mendel's Peas, where everything
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looks nice and simple, but in real-world crosses,
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things look a lot more complex.
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Including for most of our traits,
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lots of diseases as well.
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All right, so I'm going to go into that,
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then we're going to start simple.
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So here's an example of this, all right,
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so if you cross a white Snapdragon
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with a red Snapdragon.
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The offspring look pink.
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All right, and so there's lots of examples of this.
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Where you take individuals or look at
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crosses that have happened in the wild
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with different species, and where
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there's some variation in some trait within individuals,
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and you look at the offspring, and it
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looks like blending or mixing of some kind.
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So this led to all kinds of blending ideas,
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mixing theories related to how genetics could be explained
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and heredity could be explained.
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Where you could imagine, there's some amount
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of substance, some substance in this white Snapdragon.
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Some other amount in this red substance and they
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inherit some part from both, and you get
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some kind of blended outcome.
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Similarly, if you look at human traits, like height,
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you'll see that it doesn't exist as we all intuitively know
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in discrete states.
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It's not that you're either tall or short.
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There's a continuum of heights if you
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look across a population.
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And if you look at the frequency distribution of people
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in a population and measure their height,
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that distribution will resemble a normal distribution.
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So how do we explain something like height?
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And you can have individuals that
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have offspring where one parent was tall,
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the other parent was short, and the offspring
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are not either tall or short but can
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be some intermediate height.
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So because lots of crosses if you look at it in biology
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and including in human pedigrees and looks
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like this for traits where there's this blending.
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Then it's very hard to try to come up
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with some clean, simple explanation for heredity.
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And we have to deal with this complexity
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when it comes to understanding biology
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and when it comes to understanding
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things like human disease.
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And we will, hopefully, give you a deep understanding
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of this in this class.
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