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MICHELLE: Hi, everyone.

It's Michelle here again.

And welcome back to our final segment on meiotic chromosome segregation.

Today, we're going to discuss a very important topic--

recombination.

Let's jump into it.

We're going to consider a diploid cell.

Here's the nucleus and 2n equals 2.

So we're looking at one chromosome.

And you got one copy from mom, here in red, and one copy from dad is in blue.

And that connects back to other tutorials.

I'm already showing the first stage of meiosis here, which is when the

chromosomes replicate.

And these are now the sister chromatids.

And after replication occurs, homologous chromosomes pair, and they

undergo recombination, which is also sometimes called crossing over.

So recombination links chromosomes like this.

The site of the crossover, which is right here, is random.

Crossovers occur along the length of the chromosome, but they don't occur

in the same place in every meiosis.

During meiosis, at least one recombination event will happen

between each pair of homologous chromosomes.

So in humans, many recombination events will occur along each

chromosome.

But for simplicity, we're just going to consider one crossover event.

Recombination must occur in order for meiotic chromosome segregation to

occur properly.

And recombination has important consequences for how alleles on the

same chromosomes segregate.

So the consequence of the crossover is that this part of the red chromosome

arm becomes connected to this part of the blue chromosome arm.

And this part of the blue chromosome is now connected to this bit of the

red chromosome arm.

So now I'd like to go back to the example that we used in the previous

tutorial, where we looked at the D and E alleles.

So again, this individual is going to be heterozygous.

And I'm going to write down and the gametes that the mom contributed and

the dad contributed.

And you can refer back to the previous tutorial to see how we got there.

I'm going to place, now, these alleles on the chromosome, as we did

previously.

OK, so now again we're looking at the alleles of the D and E genes.

Pause the video and take a minute to trace these alleles through all the

stages of meiosis and figure out the genotypes of the gametes

that you'll get out.

Great.

Here we go.

So in the first step of

meiosis, chromosomes replicate, and sister chromatids

are pretty much identical.

Now these homologous chromosomes are going to pair and

undergo crossing over.

The red chromosomes still has the big D alleles and the little e alleles.

The blue chromosomes still have a little d and the big E.

OK, now we're ready to progress into meiosis 1.

And this is a bit different than you've seen before.

This is the result of the crossover.

So now this is what you get out of meiosis 1.

So now the big D and a little d are matched.

We haven't seen that before.

And now let's watch sisters segregate in meiosis 2.

Here we have a big D, little e.

Here we have a little d, and a little e, big D, big E, little d, big E. So

here are the genotypes of our gametes.

And there are some very important things I'd like you to notice.

So first of all, if we look at the maternal combination of alleles, we

see that, likewise, the paternal is over here.

But now we have gametes that are little d, little e, and big D, big E.

And these are different than both configurations of

the parental alleles.

And therefore, the little d, little e, big D, big E combination are known as

recombinants.

And this is very important.

So let's now look at another example of recombination.

And I'm going to show that here.

So again, we're going to deal with the D and E alleles.

And I'm going to place them quickly on the diagram.

And I'm also going to place the products of replication.

Just like this.

And now, we're going to imagine that the crossover actually happened

somewhere else.

So crossovers occur on every chromosome, but their placement is

random in each meiosis.

So in a population of gametes from the same individual, you are going to get

different meiotic products because crossovers are going to occur in

different places.

So now I'd like you to reconsider the alleles of the D and E genes, given

that the crossover is in this other location.

What are the genotypes of the gametes this time?

Excellent.

Let's go through it.

So if the crossover occurs on the other arm, remember, the low sides

don't change.

So there we have a little d and a little e, and a big E and a big E. So

now here's meiosis 1.

And we care about the D and the E genes for this example.

Recombination on the other chromosome arm is going to greatly affect how the

alleles on the other chromosome arm segregate.

But for the D and the E alleles they are going to stick together since

there wasn't a crossover between them.

OK, so let's look at the product of meiosis 2, shown here.

You get only big D little e or little d, big E out.

This looks a lot like the previous tutorial.

Remember again that mom contributed big D little e.

And dad contributed little d big E. So these are maternal genotypes and these

are paternal genotypes.

We got no recombinants with respect to the D and E you genes when the

crossover occurred on the other chromosome arm.

So I want to make some points.

Remember that, when we're doing crosses, we're looking at populations

of individuals, where many cells are going through meiosis.

And when we count frequencies of recombinants, we're looking at that

population.

The closer together that genes are, the less likely it will be that a

recombination event will occur between them.

And if genes are far apart on the same chromosome, a recombination event will

certainly occur, and two, or three, or four more events are likely to occur.

So on genes that are close together, at some low frequency, a crossover

will form between them.

And you'll get recombinant gametes.

But much more likely, a crossover will not occur between them.

And in that case, the genes will remain linked, and you'll only see

parental genotypes.

So you'll see a small percentage of recombinants in the population.

Great.

So when we started this discussion, we knew that all genes found on the same

chromosome are not linked, but are diagram did not match that knowledge.

So now we have a diagram that can explain why genes that are close

together on the same chromosome are linked, and genes that are far away on

the same chromosome are unlinked.

Great work everyone.

Today we've covered the very important concept of recombination and how it

functions in meiosis.

And in this series of tutorials, we've gone through so many concepts

fundamental to genetics.

Great work, see you soon.