All language subtitles for 37031x_MH_DNA_Markers_05_Marker_Gene_Distance_v1-en

MICHAEL HEMANN: OK, so this idea of having a heterozygous parent

with the condition is something that we can't always--

we don't always have in humans because we can't control

the inheritance pattern.

We can't require one parent to have

a particular genetic composition.

But in flies we can do this, and in other organisms

we can essentially create that heterozygote

with the phenotype.

And so say we have a phenotype.

In this case, the phenotype is wg minus, which is wingless.

No wings, an easy phenotype to see in a fly.

And so let's cross a female that is a true breeding

homozygote that is wingless so it has this phenotype

and is homozygous for this allele,

and is homozygous for the A version of the marker.

And we'll cross with a male that is wingless plus

and has the B version of that marker.

So both true breeding homozygotes.

And we create from this an F1, and the F1 is female,

is heterozygous for both things.

So it has one allele of this recessive wingless gene

and is heterozygous for the markers.

So we've essentially created this female

through crossing that's going to be informative

as she passes these alleles on to the next generation.

The meiosis that she has are going

to be able to tell us whether this mutant allele or this wing

allele is segregating or not segregating

with a particular allele of this marker with A or B.

So we can do a test across here with a male

that is homozygous recessive for wingless and has the A allele.

And we can look at the gamete genotypes.

So here we're just going to look at what the female is

passing on, the alleles that she can create during her meiosis.

So she can create wingless minus A, wingless plus B.

These are both parental.

They look like the ones that she already has

or ones that she inherited from her parents.

Or we can have recombinants or crossovers,

and those would be wingless plus A and wingless minus B.

And say we get some numbers here, 45, 40, 7, 8.

So this is essentially precisely what

we'd done before looking at two different phenotypes,

but here we have a marker that we're

looking at distance between the marker and the gene.

So how many recombinants do we have here total?

We have 15.

And so we have 100 times 15 over 100,

which equals a total of 15 centimorgans distance.

Would it be possible to determine

the alleles of the gametes without performing a cross?

No.

I mean, unless you could isolate the gametes themselves

that they're generating, and that turns out

to be a very difficult thing to do.

But in essence, you have to do the crosses.

So here we've done this cross, and we've actually

identified a relationship between a marker, just

this arbitrary piece of DNA that's not really

doing anything but serving as a signpost in the genome,

and some gene of interest.

We're able to place this gene next to a particular marker

of interest.

And if we did this through the entire genome,

if we interrogated every marker, we'll

see that for most markers, wingless

would be entirely unlinked.

If it's on a different chromosome

it would appear to be totally unlinked.

And we can develop a relationship then

with markers on the same chromosome, just how

far it is away.

Is it 40 centimorgans away?

Is it 5 centimorgans away?

Is it one centimorgan away?

So it's completely linked so we can actually,

again, place this in a precise location on the genome

or in the genome.