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Hello, and welcome back to Introduction to Genetics and Evolution.

This is one of my favorite parts of the course.

This is what people typically think of, when they think of basic genetics.

And that is basic single gene inheritance.

In the last video, that you saw.

We talked about the process of mitosis,

which starts with a diploid cell, having two copies of all genetic information.

And forming new, two new diploid cells that are genetically identical to each

other and genetically identical to the parent cell from which they came.

Now meiosis which is what we'll be focusing on here,

you start with a diploid cell again, having two copies of genetic information,

and you make a haploid gamete, has only half the genetic information.

Now, this haploid gamete comes together with another haploid gamete.

Boom!

Fertilization happens and

you have a new diploid cell that is different from either of the two parents.

That is really cool.

So, let's talk about that briefly.

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Meiosis and fertilization are what are needed for, what's referred to as,

Mendelian inheritance, Gregor Mendel was a monk who actually

described this process very elegantly, and identified how this actually worked.

He lived from 1822 to 1884 in Austria.

What he worked with were pea plants.

He identified a number of traits that he was studying with these.

As you can see here, seed form being round versus wrinkled,

seed color being yellow versus green.

Pod form, inflated versus restricted, etc., etc.

There were a lot of traits he looked at.

What he found was that some forms he referred to as, quote, dominant.

And dominant meant that if you bring together a round form and a wrinkled form

and cross them together, the offspring would actually just be round.

It is not intermediate but would actually just be round.

So from this he identified some simple rules of inheritance.

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Now we can understand dominance and recessivity from these simple forms.

So let's use in this example true breeding green pods and

true breeding yellow pods and cross them together.

Okay, now when I say true breeding, I mean all of their offspring

when they are bred with others that look like them, look exactly the same.

Now what we'll do in this case, is we'll identify the genetic factors that

they have, or the alleles that they have using a letter.

Typically, but

not always, the capital letter is associated with the dominant form.

So in this case, the true breeding green pods are dominant, so it is big Y, big Y.

Again, it has one copy that came from its dad, one copy that came from its mom.

What gametes can it give?

It's only got big Y, so its gamete would have to be big Y,

with respect to the form of this pod, with respect to it being green.

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These are guys right here, the yellow pods there was referred to as recessive and

I'll show you why that is in just a second.

They are little y little y, again they have little y that came from dad,

little y that came from mom.

All they have to give are little y's.

The F1s are the offspring of these two things.

The F1 is offspring from a cross.

Well they'll get the big Y from this person, or

from this plant, and the little y from this plant.

So they are big Y and little Y.

Nonetheless, they are all green.

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So in this case, we have a masking of the yellow color by the green copy.

Right.

The masking of the yellow color by the green copy.

So big Y, the green one, is definitely dominant.

Little y is definitely recessive because you don't see it when you have one of each

in there.

What will this plant over here give off?

Well, he has two different gene copies.

He has a big Y and a little y.

So he can give either of these to his kids.

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So this big Y and little y are referred to as heterozygous.

Heterozygous meaning having two different alleles at

a particular gene that we're looking at.

In this case being the pod color gene.

All right, so that have both.

They're heterozygous.

So we can use what's referred to as a Punnett square

to follow patterns of inheritance.

Now it's equally likely for it to give one copy as the other copy.

So we'll take a boy that's big Y little y.

So here's your boy big Y little y.

Here's your girl big Y little y.

Now half of the gametes from the boys will be big Y.

Half the gametes from that same boy would be little y.

Half the gametes from the girl will be big Y, half the gametes will be little y,

from the girl.

So the possible combinations, the boy might give this big Y,

the girl might give this big Y, and we may get a big Y, big Y individual.

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Well we know, that is green.

The boy may give big Y, the girl may give little y,

well we know when you're heterozygous you're still green.

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The boy may give little y and the girl may big y, still green.

And only in this case, when the boy gives little y and

the girl gives a little y, do we get a yellow pod plant from this.

So the ratio we should get is three green to one yellow And

that in fact is actually the ratio that Mendel saw.

So here are some actual numbers from Mendel, he saw 428 green and

152 yellow peas from this cross.

That's very similar to the expected number.

So that's great.

Now Mendel put out various laws and these laws have postulates within them, so

let's go over that briefly.

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So Mendel's First Law has three postulates.

First that we have unit factors in pairs.

So basically every gene, you have two copies of it.

This is assuming you're a diploid.

You get one allele from your mom, and you get one allele from your dad.

Similarly, if you're a boy like me, you will give one allele to your kid,

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your spouse will give one allele to the same kid as well.

There's also this feature of dominance or recessivity, you don't always see this,

it is possible sometimes to have, say a red plant and

a white plant come together and make a pink plant.

But sometimes you do see this where

one form completely masks how the other form may look.

So you could have a green and a yellow pod, as you saw,

and the offspring looks just as green as the dad.

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And finally, we have the equal segregation of gametes, all right?

So you have these two factors, the big Y and little y, and you're

equally likely to transmit either the big Y or the little y to one of your kids.

You could transmit the big Y sometimes, the little y sometimes,

you don't have any control on it, but it's a 50 50 shot.

So let me get your view of problems to try,

first I'll do one with you and then I'll give you one just to try on your own.

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Let's imagine you were a farmer working on corn.

Okay. Now let's say

that you have pure breeding tall and short strains of corn, and

you've heard that this is caused by a single gene.

This, by the way, is very very unlikely, but let's just pretend it's the case.

So you cross the tall and short strains together, and

this case you get strains that are intermediate in height.

So in this example we don't actually have dominance like in the previous example.

So you cross these intermediate height parts together what would you see?

Well let's work through this together.

So again we started with pure breeding tall and short strain.

Let's imagine the alleles we're working with are big T for

tall and little t for short.

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Okay.

So let's measure these are the two forms we're working with.

Now what would these intermediate strains be.

Well since we said they were crossing a pure breeding tall which was big T, big T.

So pure breed short was gonna be little t, little t.

The intermediate ones could presumably be big T, little t.

Big T, little t.

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We could get big T, little t.

We could get big T, little t or we could get little t, little t.

In this case, this is little t from dad, big T from mom.

Or, in this case, this is little t from dad, little t from mom, etc.

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So, where do we see it with this?

Well, in this case, we have one quarter here would be big T,

big T, which we will identify that as being tall.

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So this is starting similarly.

You're a farmer working on corn.

If you're pure-breeding tall and short strains together, you cross the tall and

short strains together and you get intermediate height corn.

This is, again, very similar to the previous problem.

You cross the intermediate height corn to the tall corn now.

So you're crossing your relay corn to pure-breeding tall corn.

What will you find?

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You started off in this case you had, you had your intermediate corn, so

this is just like the previous example.

However, rather than breeding the intermediates together,

you're breeding intermediate corn with pure breeding tall corn.

So this is a little bit different than last time.

So here's your pure breeding tall corn.

We'll just say that's the female.

It doesn't really matter which one's which.

And we'll say your intermediate was the male down here.

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Well in this case, there's actually fewer possible offspring.

So you can T from here, T from here and you get TT.

Here you get T form here, T from there, TT again.

So you can see half the individuals are TT.

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You can add T from Mom and t from Dad.

And again, big T from mom and little t from dad.

So in this case, unlike in the previous example, you would get one half

tall corn and you would get one half, two fourths, intermediate sized corn.

You would not, in this case, see any short corn coming out from here.

Now, I hope this illustrates something to you,

especially contrasting to the pea example.

And what I hope this illustrates is that dominance

does not matter in how offspring will.

No, I'm sorry, dominance matters in how things will look, but

it does not matter in how gametes will pair.

You know what, we could use the same kind of Punnett square,

we use the same sort of Punnett square, for

things which exhibit dominance as things which do not exhibit dominance.

Because dominance only effects how things look.

It does not effect how pairing happens, okay.

So let's try out a medical example in this case.

So about 12% of women, on average, get breast cancer.

And there are known mutations in the FGFR2 gene that

are associated with an increased risk of breast cancer, okay.

So let's call this case the non-mutant form, FF,

and let's say that they have the default 12% risk of cancer.

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If you're heterozygous, if you're Ff, you have a 20% higher chance, so

you have about a 15% risk of getting cancer.

If you ff, this is the worst one, you have a 19% risk of getting cancer.

Cancer, okay?

Now in this fictitious example let's say that, that you're going out someplace.

You're, you're an attractive female.

You meet somebody you know, sitting in a cafe.

Have a nice chat with them.

And let's pretend this is in the future.

Let's say it's ten years from now.

They leave, you pull out your little iPhone, and you go, boop!

And you use the DNA scanner on their coffee cup.

And you say, oh my goodness!

He is heterozygous for the FGFR mutation.

[LAUGH] So let's assume that you are homozygous big F, so

you have the lowest probability of breast cancer.

But your potential hubby is a heterozygote.

He is big F, little f, and again I put here all the probabilities, so

you can see where everything was.

What is the probability that your daughters would get

breast cancer if you were to have kids with this potential hubby?

Basically, how much does it increase it,

relative to if you had kids with somebody who is just like you?

Well, we can do this very easily, so your potential hubby which is big F,

little f You are big F big F.

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So we're just looking at your daughters.

We're not worrying at all about your sons in this case.

Well, half your daughters will be big F, big F.

So half of your daughters will be just the same as you.

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Half your daughters will have this mutation,

cuz it'll have the thing coming from your new hubby.

So there's a slight increase, but

these ones have a 12% chance of getting breast cancer based on this gene alone.

Of course we're not considering everything else, like lifestyle and other genes.

These ones have a 15% chance of getting breast cancer.

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So on average, instead of having a 12% chance of getting breast cancer,

your daughters would have about a 13.5% chance of getting breast cancer.

It's averaging all four of those numbers together.

So really the guy is only increasing your odds of

having daughters with breast cancer by about 1.5%.

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So, you can decide if that's important enough,

or not in terms of what you want to keep this hubby.

And we'll see if this sort of iPhone app ever comes about in the future.

That's just a fictitious example and

a humorous one just to give you an idea of what's happening here.

Now, we can do the same type of cross with unknowns and actually infer the parents.

So albinism, being an albino, is inherited as a recessive in humans.

Now, what if you have a case of a non-albino mom and

an albino dad and they have eight kids.

I'm gonna say four of these eight kids are albino.

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Welcome back.

I hope that one wasn't too difficult for you,

but let's go ahead and work this one out.

So, we have a non-albino mom and an albino dad, and

they have eight kids, four of which are albino.

I said albinism is inherited as recessive in humans.

Because it's recessive, since the dad is albino.

The dad necessarily has to be homozygous for the recessive.

So let's call it A.

So dad has to be little a, little a.

Now mom is non-albino, so

she could be big A, big A or big A, little a.

Right.

Now what if mom was AA?

If mom was AA and dad was aa, all the kids would be Aa.

So none of them would actually be albino.

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So given that we know that four of the kids are albino, we can cross this out,

and we can say, in fact, mom must be Aa.

So in this case, from looking at just the phenotype,

from looking at how the individuals look, that's why you go by phenotype, and

from looking at what happens with the kids, we can actually infer the genotype.

Basically, what is the underlying genetic components?

What alleles do they have within them and what combination?