All language subtitles for 67031x_MH_Mapping_Linkage_02_Map_Distance_Equation_-en

MICHAEL HEMANN: All right, so what are we looking at here.

Well, what do we mean when we talk about a crossover?

So let's think about these two chromosomes.

And so we can draw out these chromosomes have a centromere.

And so here we have w plus and m minus, w plus m minus.

So this is one homolog, a replicated homolog.

And then we have another chromosome

that is w minus m plus, w minus m plus.

So as we talked about in meiosis I,

you have pairing of homologs, and here

you actually have recombination between these two homologs.

So this is meiosis I. In meiosis I, you have crossing over.

And so a crossover could be a crossover between these two

alleles here, between single chromosomes--

between sister chromatids and homologs that result

in, essentially, a mixing of maternal and paternal genomes.

So here are the top allele is still w plus m minus,

but the bottom allele is now mixed.

So we have part of it, we have w plus here,

but the m minus has actually moved down.

So you have w minus m minus, we have w minus m plus,

and on the top here, we have w plus m plus.

So we've reassorted our alleles here a bit,

and so the result is during the second meiotic division

in meiosis II, you get four different alleles.

And of these four different alleles,

you have two that are parental.

The one at the top is a parental allele, the one at the bottom

is a parental allele, and the ones in the middle

are recombinant alleles.

So what does this actually look like?

So here is a picture of cells that are in meiosis I.

And what you have are these synaptonemal complexes.

These synaptonemal complexes are essentially these two homologs

that are tightly paired.

So I've drawn them as four distinct strands

that are spatially separated.

In reality, each one of these lines

represents all four strands of DNA, all four pieces of DNA

that are really tightly associated with one another.

So you have this really tight interaction

and you have recombination that is occurring, essentially,

on every chromosome.

And in fact, recombination is actually essential for meiosis.

If you do not have strand exchanges,

if you don't have breaks in DNA and interaction,

physical interaction between the strands of DNA,

they don't properly synapse and you don't actually

have proper meiosis or chromosome segregation.

So you need recombination.

So recombination, in addition to independent segregation

of chromosomes, are how we mix our genomes,

how we mix our maternal and paternal genomes

so that we inherit something that

is an aggregate, a combination of both of those genomes,

that, when it becomes us, they become

our maternal or paternal genomes that we

pass on to our children.

But this is a process that occurs on every chromosome.

On average, it occurs once per chromosome arm.

So if you have two copies of 23 chromosomes,

that's occurring 46 times for each chromosome on average.

It can occur more than once, but on average

once per chromosome arm.

And this is sort of regardless of species and regardless

of chromosome size.

All right, so what do we take from this?

Well, from this crossover frequency,

we can come up with a term that we call genetic distance.

Genetic distance essentially is the number

of crossovers, more or less, that you have

per meiosis in a given region.

So a genetic distance, or map distance,

and the unit that we use here is centimorgans,

after TH Morgan, the fly geneticist,

is equal to 100 times the number of crossover

gametes over the number of total gametes.

All we're asking here is what is the frequency

in a given interval of recombination per meiosis.

Again, it is a genetic distance.

And there is a correlation between genetic distance

and physical distance base pairs that we will

talk about in a couple slides.