Hi. It's Mr. Andersen and in this video I'm going to walk you through the
sodaria cross. Sodaria fimicola is a fungi that you're going to find in the feces of
herbivores. But it you're also going to find it in the AP Biology classroom. Because it's
easy to grow and it's easy to look at a mutant strain and how meiosis occurs. And so what
you're going to do is a cross between a dark or a wild type. Wild type remember is what
they normally look like. And then a tan type of fungi. And as these grow together the spores
are going to fuse and we're going to be able to see meiosis occur. And so let's look at
meiosis again to remember what that is. What are we doing in meiosis? Remember we're starting
with a diploid nuclei. So we're going to have two chromosomes in this case. It'll undergo
DNA replication. So this would be the centromere. And here are going to be the two sister chromatids.
We're then undergoing meiosis I and then meiosis II. At the end of meiosis what do we have?
We have four cells. And those cells are going to be haploid. Remember they're going to have
one half of the genetic information that's going to be found in that diploid parent cell.
And so in us these would become sperm and egg and eventually we have fertilization.
But it's going to be a little bit different in these fungi. They're going to be haploid
in general. Their cells are all going to be haploid. And what happens is one haploid cell
is going to fuse with another haploid cell and then we're going to create the next generation.
We're still going to have meiosis and fertilization. It's going to give us recombination again,
but it occurs kind of if a different, the life cycle occurs a little bit different than
in us. And so how does this occur? Well we're going to have one cell that's formed by the
fusion of a spore from the wild dark type. And one from the tan type. And so what they're
going to deliver is a chromosome from each. We now have two chromosomes. We have a diploid
cell. That's going to undergo DNA replication. And so now this would be our centromere here
and this is going to be our two sister chromatids. Now let's say no crossing over occurs. And
now we're going to go just through regular meiosis. Those are going to split in half.
This would be meiosis I. And then we're going to have meiosis II. And now we end up with
these four cells that each are haploid in relation to that fuse cell. But something
weird happens here in sodaria. And what's going to happen is we're going to have another
mitotic division. So each of these chromosomes are going to be replicated. And so we're going
to have four cells. And therefore four spores that are going to be of the tan type. And
then four that are going to be of this dark mutant type. And so if you ever get a four
to four ratio of spores, no crossing over occurred. But it could have occurred in a
different way. It could have occurred with the dark on the top and the tan on the bottom.
And what would be get? We'd still get a four to four spore ratio of the dark to the tan.
And so kind of remember these. If you ever have four to four, no crossing over occurred.
Now let's look at how crossing over could occur. So we began with those that are copied.
But we have remember crossing over occurring right here. And so where you're crossing over
we're going to take parts of one and switch that with another. And so as we do that, we
now have a recombinant chromosome. It's a mix between the tan and the dark. How does
this play out as we go through meiosis I and meiosis II and finally mitosis? What you are
get are two spores that are going to tan. Two dark. Two tan. Two dark. Now it doesn't
have to occur specifically in that way. We could have the recombined chromosomes end
up on the end, so up here and down here. What's that going to create? Still a two to two to
two to two. Or it could occur like this where we have one of the recombinants on the inside
and on the outside of this other one. And we get a two to four to two. Or it could occur
like this. And so these are going to be four ways that recombination or crossing over can
create these new, and remember these are called asci, which are going to be these eight spores
found within the parathecium of the sodaria. Okay. And so how does this lab lay out? Well
let me show you. But before that let me talk about the importance of crossing over. Remember
Thomas Hunt Morgan and his student Alfred Sturtevant were the people who figured out
the importance of this. So as those chromosomes cross over they literally are creating new
chromosomes that haven't been built before. And that gives us recombination. If two genes
happen to be on the same chromosome, then they would tend to travel through time together.
They would be linked genes. But crossing over is a way for us to get variation between that.
And so what we can look at is the frequency of cross over and that tells us how far those
genes are going to be apart on the same chromosome. Now in this case we're not looking at in sodaria,
we're not looking at two genes. We're looking at one gene. But we're looking at the cross
over. And so we're looking to measure the distance between the gene and the centromere
which is going to be that connection between the two chromosomes. And so this is how the
data is presented. What you would do is take these parathecium. You put them underneath
a glass slide. And you push down with it. And you kind of get this explosion of these
asci and you can start to count them. And so you may want to pause the video here. How
many on these asci are going to be parental or no crossing over? And how many of them
are going to be crossing over? So you may want to count those out. Because that's a
skill you should be able to pick up. So I'll pause for a little bit. And so what you should
have found is that there are eight ascus that show no crossing over. And there are going
to be 10 that show crossing over. And so how do we know again the no crossing over? It's
that 4 to 4. And then the yes crossing over are going to have a different combination.
Sometimes 2 to 2 to 2 to 2. And sometimes 2 to 4 to 2. Why is that important? Well now
we can start to plug in our data. And so if we look at this chart. This is from the AP
Biology's Lab Book, we start with a number of asci that are showing no crossing over.
And how many was that? Remember we said 8. How many are showing crossing over? 10. And
so how many total asci did we count? 18. Now this is not a big enough number. You should
count at least 50 of these to get good data. But let's just use the data right here. The
next thing we're going to figure out is the percent of cross over. Now it's interesting.
Since we undergo that mitosis in sodaria, we have to take that crossing over number
and we have to divide it by two. And so how do we figure out our percent of cross over?
What we're going to do is take the total showing cross over. Which is 10. Divide by the total
number, so that's 10 over 18, and I get around 57%. After I've done that I divide that in
2 and I get around 28%. And so that's going to be not only our percent cross over divided
by two, but that's going to be our gene distance. Or that's going to be the map units from the
gene to the centromere. And so again that crossing over is a random event. And so if
we can look at how often this is occurring, we can figure out how far that gene is away
from the center of the chromosome. What's the accepted value? In the lab book it says
it's around 26. And so this is not correct, and that's because not only did we have a
small sample size, but it's data that I just made up, so we could do an animation of it.
And so that's the sodaria cross. You may want to go out there, google sodaria cross and
you'll find all these images and you could try to figure out the frequency of cross over
by yourself. Remember divide by 2 and that's going to give you your gene to centromere
distance. And I hope that was helpful.