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Recombination

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    There are over 20,000 genes in the human genome, and only 23
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    pairs of chromosomes for them to fit. This means that there're going
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    to be a lot of genes that end up on the same
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    chromosome. Now, if we only focus on, let's say, two genes, they
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    each need two alleles apiece, and they're on the same chromosome. And
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    we look at the allelic combinations we could get after myosis in
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    our gametes, we'd find that instead of getting four, we would only
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    end up with two. And this would appear to break the rule
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    of independent assortment that says all these alleles can segregate
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    independently and we get all these neat combinations. Well, if
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    the point of sex is to increase this genetic variability,
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    then we need a way around this problem, and our germ
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    cells have a really cool trick to do this and
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    maker sure that we can mix and match all of
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    our alleles. It's time we move back to visual representations
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    of our chromosomes because this
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    really cool trick called chromosome recombination
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    is a bit more complicated than what we
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    can do with our deck of playing cards. Again
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    we're not going to show all 46 chromosomes or all
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    23 pairs, I'm just going to show a representative set
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    of chromosomes here. Here we have 1,2,3,4,5,6,7,8 pairs of
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    chromosomes. Of course each pair has one chromosome from
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    mom in pink here and one chromosome from dad
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    in blue. Now Chromosome Recombination is a very special
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    kind of event. Now in the exercise we
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    just looked at we had an individual Heterozygous
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    for lactase persistence in dwarfism. But we hypothetically
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    imagine that these alleles were on the same chromosome,
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    chromosome two and because of this after meiosis
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    occurred we only got two possible allelic combinations because
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    these alleles are on the same chromosomes. So
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    the law of segregation still applies but independent assortment
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    where alleles from one gene are so independently from alleles of another gene
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    doesn't seem to hold true. Instead, these
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    alleles appear linked. What a special trick
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    does, called chromosome recombination, is it allows
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    regions of a chromosome to actually swap
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    information. What this means is that physically,
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    literally these chromosomes are touching each other,
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    right, one of these chromosomes came from mom, one
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    of these chromosomes came from dad. They're going to get
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    so close that they're going to connect at regions
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    along the chromosomes and where they connect they're going to
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    exchange pieces of information. So that what happens is
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    a bit of mom and dad's chromosomes get mixed
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    up they get swapped. So now this one linear
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    chromosome that used to be the whole chromosome that you
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    got from mom is now a chromosome that starts as
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    a piece you got from mom, then includes a bunch
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    of sequence you got from dad, and more sequence you
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    got from mom. So when you pass these chromosomes on to
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    your children, you're not just passing down mom's chromosome that
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    she gave you, or dad's chromosome that he gave you,
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    you're passing down a mix of your mom and dad's
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    chromosomes. Now, think about what this means for the allelic compositions.
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    Let's just imagine for a second that the
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    placement of our lactase and dwarfism alleles are
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    here on the chromosomes. And we know already
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    that if recombination doesn't occur we only get two
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    possible allelic combinations here but if recombination does
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    occur look what happens. Now where's the dominant
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    and recessive alleles? We're on the same chromosome
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    here. We have the two dominant alleles for each
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    trait on the same chromosome and the two
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    recessive alleles together. So that when segregation occurs, we
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    get the two missing combinations that we didn't
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    have before. So just to make sure you're with
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    me again, I want you to tell me
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    how many gamete genotypes are possible in this case,
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    even when genes are on the same chromosome,
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    but far enough apart for chromosome recombination to occur.
タイトル:
Recombination
Video Language:
English
Team:
Udacity
プロジェクト:
BIO110 - Tales from the Genome
Duration:
03:49

English subtitles

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