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MICHAEL HEMANN: Let's think about another phenotype
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and another cross.
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And this is a cross between flies
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that have different eye colors.
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So in flies, the wild type color is red.
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And this is actually one of the first phenotypes that was
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characterized in fly studies.
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So this was done the turn of the 19th or the 20th century
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by Muller and other geneticists and one
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of the initial genes they characterized
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and the first gene that was really ever mapped,
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that they can actually localize it
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and they could place it on a particular chromosome.
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So flies that are mutant for this gene have white eyes.
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And the gene is actually called the white gene.
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So, typically, we call genes by the names
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of the mutant phenotype.
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And so we're calling this a white gene
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because the mutants are white, not because the wild type
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color is white.
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So let's do a cross here.
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So we have a cross between females that have white eyes,
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and we're going to cross with males that have red eyes.
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And we'll look at the F1 generation.
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Now, of course, these are both true breeding,
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so they're homozygous at all loci,
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and we get an F1 generation.
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And in the F1 generation, we have females that have red eyes
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and we have males that have white eyes.
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Well, this is weird.
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So what's going on here?
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Why do we see this pattern of inheritance?
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We're doing essentially complementation studies
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and we're getting an answer that's totally weird.
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Absolutely.
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So I think some of you have seen this before.
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This is a sex-linked trait.
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It is an X chromosome linked trait.
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And that's essentially how they map the location of this gene.
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They could localize the gene to a sex chromosome
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to the X chromosome.
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So let's draw out how this works.
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So you have a female.
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And a female has two Xs.
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The males have one X or have XY.
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And so we'll indicate this as an X chromosome
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that has a white allele.
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And another X chromosome that has a white allele.
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So true breeding homozygous will cross
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with a male that has the wild type allele and a Y chromosome.
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When we cross, we get resulting females.
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And these females get one X chromosome from the mother.
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So that's a white allele.
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And then have to get the one X chromosome from the father.
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And that's a wild type allele.
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And because red is dominant to white,
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we have they red eye female.
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And the resulting male obtains the one X chromosome
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from the mother.
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So that's a white allele, and then
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the Y chromosome from the father.
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So the males inherit only the mutant copy, only
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the white copy.
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And so therefore, have white eyes.
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So this is characteristic of X-linked genes.
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We call this criss-cross inheritance.
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And it's a characteristic of X-linked genes.
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So it's a kind of complementation study.
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But in essence, we're only seeing this complementation
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with a single X chromosome inherited by the male.
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So, essentially, we're doing a dominant-recessive study
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looking at an X chromosome gene.
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