All language subtitles for 777031x_MH_Mapping_Linkage_03_Map_Distance_XLinked_v-en

MICHAEL HEMANN: So let's actually put in some units

here.

So we have a cross, and the cross

is a cross between a female that's

w plus m minus and w minus m plus.

And here I've introduced a new nomenclature.

This line here, this bar indicates

that these alleles are in the same chromosome

and they're potentially linked with one another.

And so we're going to cross with a male,

and the male is w plus m plus over y right.

The advantage of looking at the X chromosome

here is that looking at the results in males,

we can see all of the phenotypes immediately, because they only

have one X chromosome.

So the phenotypes are essentially

equal to the genotypes in the males.

So we can have different kinds of males.

The males can be w plus m minus, w minus m plus, w plus m

plus w minus, m minus.

We can come up with some numbers.

This is just a random experiment.

So say we get 23, 33, six, and seven.

Now during this cross, the females

are less interesting, because just by looking at them,

we can't tell their genotype, because everything

is on top of this wild-type allele that's

inherited from the father.

So all of these phenotypes are essentially

going to be wild type, so w plus m plus phenotypes

when we look at females.

So in this case, how many crossover gametes do we have?

Right, 13.

We have 13 crossover gametes.

These two, these now w plus m plus and w minus m minus,

they're not there in the mother there.

So they had to come via some crossover event.

So we're going to say the number of centimorgans

equals 100 times 13, which is the number of crossover

gametes divided by the number of total gametes, which

is 23 plus 33 plus six plus seven equals 69, which

equals 19 centimorgans.

So these are 19 centimorgans across.

So this is randomly generated data, experimental data.

And more or less, we can assume a Poisson distribution.

And so in a Poisson distribution,

if you have a mean of n, then the standard deviation

is the square root of n.

And so this is something that we can think about

in terms of what is the distance, plus or minus

the standard deviation, by saying this

is essentially 100 times 13 plus or minus

the square root of 13 over 69, which

equals 19 plus or minus 5.2 centimorgans.

So this is roughly the distance between the wingless

and the white-eyed genes on the X chromosome.