- [Voiceover] One of the
very interesting things about genetic recombination is that you can actually use genetic recombination to figure out the distance between genes on a chromosome. And if you were to do this to
all the genes on a chromosome, you could actually map out the chromosome, figure out exactly where the genes are. And we're going to explore that concept. So we're looking at a pair
of homologous chromosomes, and let's just say that
the orange chromosome is the paternal chromosome,
so it comes from the father, and let's say that the yellow one is the maternal chromosome. And, just to remind ourselves,
these are sister chromatids. That means that they are
identical chromatids, they have identical genes on them. So those colored circles that I drew represent just some genes
that I randomly picked. And you can see that on
the two sister chromatids I drew them in the same color,
because they are the same. And for that matter, these
are also sister chromatids. These two yellow chromatids
are also sister chromatids. And I'm just gonna review a bit to give us some context. So normally in the cell,
the paternal chromosome would just look something like that, with the centromere middle. And the maternal chromosome would also look something like that. But, during myosis or
mitosis, when they replicate, this chromosome will turn into that. So it's gonna duplicate itself and each one of those strands
is a sister chromatid. And so the maternal chromosome turns into something that looks like that. But, this is still
considered one chromosome. And this is considered one chromosome. And that can be a little bit confusing, because how could this be one chromosome and this also be one chromosome? If this X has double the
material as the original strand? And the answer to that question is because we count chromosomes by the amount of centromeres that are present. So this has one centromere,
it's considered one chromosome. This X has one centromere, so it's considered one chromosome. However, they're not quite the same. This, on top, is just one chromatid. And the bottom chromosome has
ended up with two chromatids. So try not to get confused, even though we're calling this one chromosome, it's really made up of two
identical sister chromatids. But anyway, that was
a bit of a digression. Let's focus again on our
homologous chromosomes. So I picked a couple of
genes, that I just put on, and we're gonna focus on
three genes in particular. And to make this a bit
more real or relevant, let's just say that the
green genes represent I don't know, maybe complexion. Maybe like a dark complexion
versus a lighter complexion. And on the maternal chromosome,
I also drew it in green, to remind us that these
are homologous chromosomes, they're homologous
alleles, in other words, these are both alleles
that code for complexion, but different shades of green, because they're probably
different versions. So these green genes
also code for complexion. And let's say that this
darker purple, or magenta, gene codes for, let's say hair color. So that means that the lighter purple on the maternal chromosome
also code for hair color. And then let's say that the
blue genes code for eye color. So we have dark blue on
the paternal chromosome and light blue on the maternal chromosome. And let's then focus on
the sister chromatids where genetic recombination occurs. So these two strands are going to swap genetic information between them. Maybe like a chunk on bottom will swap, maybe something in middle will swap, maybe something on top will swap. Actually, genetic
recombination also occurs between sister chromatids. However, sister chromatids are identical so it would be of no consequence. Anyway, so let's look
at the two chromatids where genetic recombination is happening. The two that I circled. Let's take a closer look at those. So here are the two
chromatids that are going to exchange genetic information
and undergo recombination. And I want to ask you a question. So I'm gonna give you two choices. And I want you to try to figure out which one is more likely. So, for the first choice, let's look at the purple and green genes. And the question is, what's the what's more likely? Is it more likely that
the purple and green genes undergo recombination? And when I say that, I
mean that the genes that are originally on one chromosome separate. So I mean to say that, for example, this purple gene will get
separated from that gene, and this lighter purple gene will get separated from that gene. So is it more likely that the purple and green genes recombine? Or the way I like to
view it, get separated. Or, is it more likely that the blue and purple genes recombine or get separated? And the way to think about this is, well look at the distance between them. So the distance between
the purple and green genes is something like that. So in order for recombination to occur with respect to the
purple and green genes, it would have to happen
somewhere over here. Or on the other side, it doesn't matter, it doesn't make a difference. So you have this whole distance, whereas if you wanted
recombination to occur with respect to the purple and blue genes, you have a very smaller area to work with. It would have to happen somewhere here. So somewhere along this
line of the chromosome. So, that means that it's much more likely for recombination to happen with respect to the purple and green genes. That is more likely because you have this entire
distance to work with. If these two chromatids
swap genetic information anywhere along this
stretch of the chromosome, so the green and purple genes
will separate and recombine. And the blue and purple
genes are less likely to recombine because
recombination would have to occur only in this little
sliver of chromosome and that's just smaller than the other part of the chromosome
that we were looking at. So, we just learned a
very important concept and that is that the
further apart two genes are, the more likely it is that they will recombine. I'm not actually not sure
if that's the proper way to use the word, but I
just use it that way. So I'm gonna put it in quotes. And again, when I say
recombine, I mean that two chromosomes that were
originally on the same, sorry, two genes that were originally on the same chromosome get separated. And then the next thing we learned is that the closer two genes are to each other, I'll abbreviate each other, just e.o., the less likely it is that they will recombine. And now i'm gonna introduce
you to some terms. The centimorgan is the unit of measurement that we use to measure
distance on a chromosome. And another way to say
centimorgan is a genetic map unit. Or, m.u. which stands for map unit. And I'll give you the official
definition of a centimorgan, because I think it's important
and it kind of ties in distance to what it has
to do with recombination. And so a centimorgan is the distance between genes, I'm just gonna
abbreviate between like that, for which one product of myosis in 100 is recombinant. And to put that in simpler terms, it means that if two genes
are one centimorgan apart, it means that one out
of one hundred times, or one percent of the
time that myosis happens, those two genes will be recombinant, or separate, or recombine. And we'll actually do
an example of this to illustrate what this means. So here we have our two chromatids again and let's just say that
the distance between the purple and green genes is 25 map units. Remember, a map unit is the
same thing as a centimorgan. And let's say that the distance between the blue and purple
genes is six map units. So this is clearly not
drawn to scale very well, but it's just an estimate. So let's first focus on
the purple and green genes. So if they're 25 map units apart, so remember if two genes
are one map unit apart that means that one percent
of the time they'll recombine, so these are 25 map units apart, so that means that 25% of
the time that myosis happens, recombination will occur with respect to the purple and green genes. So let's see what that looks like. So we're gonna see that in
this spot, right over here, the chromatids just swap information, So let's draw what that would look like. So let's draw our paternal chromosome, or at least part of it. And then we'll draw our, actually I'm gonna draw
that a little bit higher, because we need more room. So here's our paternal chromosome. And then our maternal chromosome. Or chromatid. And then, since they kind of swapped in this spot, so I'm gonna draw yellow, right over here. That's the part that came
from the maternal chromosome. And then I'm gonna draw orange over here. That's the part that came
from the paternal chromosome. And now let's fill in our genes. So the blue genes will
just stay where they were. So, we just leave them put. And the same goes for the purple genes. But then, we have that piece
of maternal chromosome. So we get that hunter
green gene over here. And then we get the lime
green gene right over here. So this is what I mean by recombination being a separation of genes. So this purple gene and
that lime green gene were on the same chromosome before, but they got separated. They're not on the same
chromosome any more. And the same applies to these two genes. Now let's look at the
blue and purple genes. So they're six map units apart. So that means that 6% of the
time that myosis happens, the purple and blue
genes will get separated. So, we're gonna see that
upwards of this spot, the chromatids swap information. So, let's draw that. So here we'll draw a part of our paternal. that's the paternal chromosome
and the maternal one. And they swapped somewhere like over here. So let's fill that in. So that's the part of
the maternal chromosome that lands up, sorry, yeah, that's the part of the maternal chromosome that ends up on the paternal one. And the orange over here. And now let's fill in our genes. So lets's first fill in the
ones that just stay put. So we have that lime green gene, then we have that hunter green gene. And our purples also stay put. But the blue genes swap chromosomes. So we have our dark blue gene over here and our light blue gene over here. And again, take note of
how they swap places, or how the separate. So these two genes were together on the same chromosome before, but not anymore. And the same for these two genes. So let's just tie this back into the bigger concept going on. If we were to do a statistical analysis of how often certain
recombinations happen, that can help us map out
the genes on a chromosome.