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this videos is on genetic recombination
more specifically linked genes and is section our section 11.5 in your
textbook
so far we've been discussing genes as if they are cards in a deck
where each gene sorts independently. In plants for example, the gene for height
is inherited independently from the gene for pod shape. a plant can be
heterozygous for height
and homozygous for patch that would make it big t
little T are for example but in reality every cromosome actually has more than
one gene on it.
so sometimes shuffling doesn't really work the way that we've been saying that
it does.
genes close to each other on a chromosome
are called linked genes. the reason that we call them linked
is because when gametes are formed during meiosis these linked genes often travel
together
for example if the gene for eye color and the gene for height
are close together on a chromosome they will be put into a gamete
together. it's like when two cards in deck of playing cards
get stuck together and when you are dealing them out,
those two cards go to the same person because they're stuck together. these
linked genes do not
obey the law independent assortment. mendel's law of independent assortment says
that alleles will
divide separately into different gametes during meiosis
some of these alleles are stuck on the same chromosome
are very close together and will not divided into separate gametes.
they will often travel together. so
let's take a look at what this means for meiosis. first we're going to look at
unlinked genes, which is what we've been doing all along when modeling
meiosis. here you can see that gene A
has two alleles right in here.
and gene B also has two alleles.
this cell got it's As and it's Bs from either
it's mom or its dad
and this a and b from either
its mom or its dad. this a could happen mom in this b
been mom in this a good
speaker and or any combination where one of the A's
and one of the b's comes from each parent. That's meiosis.
but since these
chromosomes here have the same
genes, we call them a homologous
pair. You already knew this. One came from mom and one came from dad.
humans have 23 pairs like this
this organism only has two. we're going to briefly review meiosis
with this cell. Sol if you
already understands really well you don't need to write it down, but if you're
still a little bit confused about the process of
meiosis you should be drawing this with me in your notebook.
might help to get some colored pencils and
first I'm going to color each of these
individual sister chromatids
a different color so that we can help keep track each
the sister chromatids is a different color: this one is yellow
this one is pink, green and blue just to help us keep track of the different gametes that we will make.
so the first thing it's going to happen each chromosome is gonna
form sister chromatids and they're going to line up in
meiosis across from their homologous pair. so I'm going to draw
sister chromatids
which is little a chain
make sister chromatid for the yellow, which is lined up
right next to it's homologous pair
and a green on here
for the little b
and a pink one for the little b
Now these homologous pairs have lined up together
big A and the little a, and the big B and the little b and
they are lined up inside this cell
so as you can see we've got
our homologous pairs here lined up and
the first division in meiosis is these
sister chromatids are going to separate
separate down the middle. That is
meiosis one. half are going
here, and half are going to go over here. this was meiosis one.
we separated the homologous pairs, and now
the sister chromatids are going to separate. They're going to do the second division
and that division is gonna happen
right down the middle. this is division 2 of
meiosis. The sister chromatids are going to separate and we're gonna
form four new gametes.
inside those gametes
we're going to have a blue and a green
here
one of these sister chromatids came from this side, one came from this side
then we have the pink ones and yellow ones
on this other set of two gametes.
so this was all just to show that the four possible gamete
combinations shown here are
as follows: big A, big B be which is this
yellow here and here. big A,
little b, which we
do not see right now, we'll talk about why in a second.
Blue and green, which represents little
a and little b
and the ones that we didn't see were big B and little a
and the reason we didn't necessarily see those is that these
can
line up in any direction they want. we could have had this little a
on this side, and then when the little a and the big B
separated from the other side, we would have had a different combination
of gametes. okay someiosis is one way that we can get
variation in offspring. and you can see that
however they line up here will depend, or
will dictate rather, the combinations of alleles that are possible
at the end. now all of that was supposed to be
review of meiosis. and now we're going to talk
about linked genes. And the process of
linked genes. And how we diagram them is a little bit different.
remember that linked genes are found on the same chromosome so here we have gene
a and gene b and they're both found on
chromosome one. So they're both on chromosome 1, and this is still
a homologous pair, but we have a pair of linked genes now
a and b are linked. so let's say hypothetically that gene a
codes for eye color and gene b codes
for height. now we know that since they're linked,
a and b cannot be separated or shuffled. we can't combine the little a
with the big B, or the little b with the big A,
because this little a and little b will always go together
and the big A and the big B will always go together
and that's why the four resulting gametes
that we see are only big A and big B pairing
and little b and little a pairing.
it's because they can't separate from each other and we're gonna go through
that process together
so starting with this original cell let's give them different colors
let's call let's color this one pink and its homologous
pair yellow and let's go ahead
so let's go ahead and draw these
chromosomes we know that our
sister chromatids are going to pair up so we've got our
pink chromosome
and we've got our yellow chromosome, and they are
homologous pairs are paired up because they have the same gene
so we know that they're gonna pair up. the first division is gonna happen
these chromosomes are going to separate. they're going to separate
right down the middle: ones going this way and one's going this way
so let's do that first division
homologous pairs are separated. now
our homologous pairs are gonna separate
into sister chromatids. so
right down the center here our sister chromatids are going to separate
into gametes.
and if we take a look the gametes we have formed, we will see
that we did not get as many combinations as we had expected to
if these genes were not linked. so these yellow ones remember they
represented this chromosome little a little be
and we have two out of four
are little a little b and these pinkones over here represented
the big A big B and you can see in
the combinations that we see in the end
that we have only big A big B and little a
little b, and that again because these genes are linked
they have to assort together
the sort themselves out together: the little a with little b and
the big A with the big B. so we can't get
as much variation when genes are linked this way
because instead of normal assortment, where we would have four possible combinations of
gametes we only have two
and this is where linked genes come in. So
proceed to the next video as soon as you're ready.