All language subtitles for 97031x_PR_Laws_Inheritance_07_Determining_Expected_-en

PETER REDDIEN: All right, so now one way of figuring out

what to expect in the F2 is we could make a big matrix here

where we have sperm up here and eggs here.

And we know there's four classes of each of these.

So we could make this matrix here.

We could write out the gamete genotypes and each

of these compartments and then list all the expected

genotypes that would emerge.

So there's 16 possible combinations here.

We could do that and then add up the classes that are the same

and get frequencies for this.

This way of doing it is what's called a Punnett square.

Many of you have probably done these before,

is a totally valid way to do it, but I recommend just getting

used to working with frequencies and expected outcomes

as when things get more complicated,

it gets just a little easier to work

with without having to draw out every possible combination.

You could just calculate it.

And I'll just go through that now.

All right, so if we think about then our classes here,

I'm going to say that not every possible scenario is

one we need to think about because for a number of classes

we will see different genotypes with the same phenotype,

as we just went through.

So we don't need to write out every different genotype

in order to predict our phenotype frequencies.

So let me just write out what I think

are the most relevant genotype classes

for the different hypotheses we have.

So we could have a scenario where

we inherited a pair of wild type allele

from one parent with anything else

and a shibire wild type allele with anything else.

What do I mean by that?

This would be shibire wild type, shibire wild type,

or shibire wild type, shibire ts.

Because the trait is recessive, either of these classes

will have the same impact on phenotype.

So that's why I'm just sort of considering them together.

So what would the frequency of--

expected frequency of this genotype be?

Well, this is-- you could just use the sum rule of probability

here, which states for the probability of mutually

exclusive events A and B happening--

so A or B happening, then the probability is--

of A or B happening is probability of A

plus the probability of B. So the probability of getting this

is the probability of getting this plus the probability

of getting this.

So the probability of shibire wild type anything

equals the probability of this plus the probability of that.

So what's the probability of getting this?

What's that?

1/4.

What's the probability of getting this?

One half.

This is basically just what we did here.

3/4 of our individuals had at least one pair

of wild type deal in that cross, same idea here.

All right, so then the probability of this class

is going to be equal to 3/4 times 3/4.

Because the probability of getting this

is 3/4, probability of getting this is 3/4.

So to get this and this is 3/4 times 3/4.

Makes sense?

So 9/16.

Again, you could draw all this out in a square

and convince yourself if that was at all unclear.

So what other classes do we care about

for thinking about these hypotheses?

Well, we could look at the case in which we have para,

let's say, wild type with anything and shibire

ts, shibire ts.

So what would the probability of this class be?

Anybody?

Yeah.

STUDENT: 3/16.

PETER REDDIEN: 3/16.

So the probability of this one is 3/4.

The probability of this event would be 1/4.

3/16.

So as you go through a few of these, you'll see why this--

if you get used to doing it this way,

it gets really fast and easy compared to drawing everything

out and adding it all up.

If you add more genes, you could see

how it could get pretty complicated pretty quick.

So now we'll do para ts, para ts, shibire wild type,

and anything.

Same calculation is above, 3/16.

Final class.

para ts, para ts, shibire ts, shibire ts.

Expected frequency?

Anybody?

1/16.

So we have here a 9 to 3 to 3 to 1 ratio of these

predicted classes of genotypes and there are a lot of variants

on the expected outcomes of phenotypes

that show the variance on this 9 to 3 to 3 to 1.

There can be a 9 to 6 to 1, things like that,

in terms of phenotype classes, or maybe

all of these have different phenotypes and so on.

So you could think through some examples

where you have different phenotypes, like red, and blue,

or things like that.

And what you you'd expect the different phenotype classes

to be.

But so they're just variants of adding up these 9 to 3

to 3 to 1s.

So that's a common thing in genetics.