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MICHAEL HEMANN: So what happens after a nondisjunction events?
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Well, in general aneuploidy meaning
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having a non-normal chromosome number is not tolerated.
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So one in 800 live births has trisomy 21,
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which is Down syndrome.
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And Down syndrome is really the only aneuploidy,
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is the only abnormal chromosome number
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for an autosome that is reasonably tolerated.
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So people with Down syndrome can grow and have a long life,
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but other trisomies are generally not tolerated.
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So trisomy 13 and 18 are viable, but they don't survive.
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So in general, these kids are not born.
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And if they are born, generally live less than a year.
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Haploidy for autosomes is not tolerated.
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So nobody is born with only a single chromosome,
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a single autosome.
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These are all embryonic lethal.
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We actually in humans, we need two copies of every chromosome
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and rarely, rarely, rarely can tolerate
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three copies of a chromosome.
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There are a lot of interesting explanations
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in terms of dosage, appropriate gene dosage underlying
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the reason why this may occur.
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And there are also explanations why trisomy 21 may be viable.
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I mean, it's the smallest chromosome, so, perhaps--
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or it may have the most minimal effect along
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with chromosome 22, but those are somewhat--
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I think still somewhat speculative.
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So we do tolerate some changes in the numbers
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of X chromosomes.
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So you can survive as an XO, so an X chromosome and no Y
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chromosome.
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This is called Turner syndrome.
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You can be XYY.
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So you can have two Y chromosomes.
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People used to think that this actually
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led to a predisposition for committing crimes.
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I think that's been disproven to having too many Y chromosomes,
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but it was actually argued for a very long, long time
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that it makes people more violent.
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XXY is Klinefelter syndrome.
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You can be XXXY.
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You can be XXXXY.
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So you can tolerate increased numbers of the X chromosome.
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Many sense for why--
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yeah, there's a question what is the difference
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between viability and survival.
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Essentially I think they can be used interchangeable.
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In the case of trisomy 13 and 18, in this case,
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they can be born.
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So they can reach embryonic maturity.
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They can be born, and ostensibly most of the organ systems
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are appropriate, but many of them
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have serious, serious problems.
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And so these kids, again, really never
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survive to one year of age.
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So by survival here, I mean durable survival.
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So it's almost always associated with early lethality, which,
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of course, is a really difficult thing for parents.
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So there's a lot of embryonic testing
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to look for the status of trisomies, particularly
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these trisomy 13 and 18 that can lead
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to really devastating consequences for children.
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So why is it that you think we can tolerate larger
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numbers of X chromosomes?
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Yeah, so it's because of this process of X chromosome--
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well, there's-- yeah, you only express one X in general,
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right.
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So there's this process called Lyonization or X chromosome
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activation where one of the X chromosomes
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is largely transcriptionally silenced.
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And so for people that have two or three or four X chromosomes
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generally have one or two or three
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of these actually silenced so that you only
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have expression off of one, and you can essentially
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regulate dosage.
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And dosage compensation in the sex chromosomes
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is a really interesting thing, sort of,
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how that works overall.
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So lastly, let me just introduce this issue of aneuploidy
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and development, and let's think about having a gamete that
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has two chromosome 15s, right.
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And this would be from a male, and then we
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have a female gamete that has the appropriate just
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one copy of chromosome 15.
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You'd have fertilization here.
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You have a resulting embryo, and this resulting embryo
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has three copies of chromosome 15, right.
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Now one of the ways that this can
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be dealt with during development to actually result
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in a viable offspring-- because trisomy 15 does not
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result in a viable pregnancy-- is that you can actually
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lose one of these chromosomes during development, very early
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in development actually, right.
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So you can get three different possible offspring,
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one with two chromosomes from dad
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and the other with a chromosome from dad and a chromosome
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from mom.
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Do you think it matters which of these outcomes exist?
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Any guesses?
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Yes?
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No?
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Two possibilities?
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It does matter actually so.
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If you have two chromosomes from a parent, a father or a mother,
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you had what's called uniparental disomy, right,
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meaning you just got two chromosomes,
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disomes from one parent, right.
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And this matters in the context of a number of conditions
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called imprinting disorders.
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Now interestingly, you have differential gene expression
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depending upon the alleles that you inherit from your mom
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or your dad for some genes, a subset of genes,
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probably less than 50 genes in people.
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And so sometimes a gene that you inherit from your father
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is turned on, but the maternal allele is turned off, right.
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And so if you have two genes that are turned on
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or two genes that are turned off,
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you have what's called an imprinting disorder, right.
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And in the case of chromosome 15,
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you have Angelman syndrome, which is actually
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a very severe syndrome that has a number
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of different phenotypes.
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So the genes that you inherit from both your parents
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are actually differently regulated.
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You need to actually have a maternal and a paternal allele
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to have appropriate dosing in the embryo.
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And there are lots of really weird explanations,
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anthropological explanations for why males and females have
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different genes off and on.
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There are ideas of sexual antagonism
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that are from fly studies, and I'd
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be happy to tell you about some of these
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if you're interested, but know this does occur.
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So on Wednesday, we're actually going
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to start talking about this issue of recombination
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and how we can link two genes together and think
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about how this is occurring in meiosis
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and how we can use this to actually map
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the fine location of different genes.
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