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

Â This is one of my favorite parts of the course.

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

Â And that is basic single gene inheritance.

Â In the last video, that you saw.

Â We talked about the process of mitosis,

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

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

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

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

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

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

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

Â Boom!

Â Fertilization happens and

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

Â That is really cool.

Â So, let's talk about that briefly.

Â 1:02

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

Â Mendelian inheritance, Gregor Mendel was a monk who actually

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

Â He lived from 1822 to 1884 in Austria.

Â What he worked with were pea plants.

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

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

Â seed color being yellow versus green.

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

Â There were a lot of traits he looked at.

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

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

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

Â It is not intermediate but would actually just be round.

Â So from this he identified some simple rules of inheritance.

Â 1:58

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

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

Â true breeding yellow pods and cross them together.

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

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

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

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

Â Typically, but

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

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

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

Â What gametes can it give?

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

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

Â 2:52

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

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

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

Â little y that came from mom.

Â All they have to give are little y's.

Â The F1s are the offspring of these two things.

Â The F1 is offspring from a cross.

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

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

Â So they are big Y and little Y.

Â Nonetheless, they are all green.

Â 3:22

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

Â Right.

Â The masking of the yellow color by the green copy.

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

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

Â in there.

Â What will this plant over here give off?

Â Well, he has two different gene copies.

Â He has a big Y and a little y.

Â So he can give either of these to his kids.

Â 3:54

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

Â Heterozygous meaning having two different alleles at

Â a particular gene that we're looking at.

Â In this case being the pod color gene.

Â All right, so that have both.

Â They're heterozygous.

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

Â to follow patterns of inheritance.

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

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

Â So here's your boy big Y little y.

Â Here's your girl big Y little y.

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

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

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

Â from the girl.

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

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

Â 4:43

Well we know, that is green.

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

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

Â 4:51

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

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

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

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

Â that in fact is actually the ratio that Mendel saw.

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

Â 152 yellow peas from this cross.

Â That's very similar to the expected number.

Â So that's great.

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

Â let's go over that briefly.

Â 5:29

So Mendel's First Law has three postulates.

Â First that we have unit factors in pairs.

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

Â This is assuming you're a diploid.

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

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

Â 5:48

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

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

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

Â a white plant come together and make a pink plant.

Â But sometimes you do see this where

Â one form completely masks how the other form may look.

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

Â and the offspring looks just as green as the dad.

Â 6:13

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

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

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

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

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

Â So let me get your view of problems to try,

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

Â 6:37

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

Â Okay. Now let's say

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

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

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

Â So you cross the tall and short strains together, and

Â this case you get strains that are intermediate in height.

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

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

Â Well let's work through this together.

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

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

Â tall and little t for short.

Â 7:18

Okay.

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

Â Now what would these intermediate strains be.

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

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

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

Â Big T, little t.

Â 7:50

We could get big T, little t.

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

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

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

Â 8:05

So, where do we see it with this?

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

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

Â 8:45

So this is starting similarly.

Â You're a farmer working on corn.

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

Â short strains together and you get intermediate height corn.

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

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

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

Â What will you find?

Â 9:16

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

Â this is just like the previous example.

Â However, rather than breeding the intermediates together,

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

Â So this is a little bit different than last time.

Â So here's your pure breeding tall corn.

Â We'll just say that's the female.

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

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

Â 9:39

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

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

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

Â So you can see half the individuals are TT.

Â 9:52

You can add T from Mom and t from Dad.

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

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

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

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

Â Now, I hope this illustrates something to you,

Â especially contrasting to the pea example.

Â And what I hope this illustrates is that dominance

Â does not matter in how offspring will.

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

Â it does not matter in how gametes will pair.

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

Â we use the same sort of Punnett square, for

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

Â Because dominance only effects how things look.

Â It does not effect how pairing happens, okay.

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

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

Â And there are known mutations in the FGFR2 gene that

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

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

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

Â 11:09

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

Â you have about a 15% risk of getting cancer.

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

Â Cancer, okay?

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

Â You're, you're an attractive female.

Â You meet somebody you know, sitting in a cafe.

Â Have a nice chat with them.

Â And let's pretend this is in the future.

Â Let's say it's ten years from now.

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

Â And you use the DNA scanner on their coffee cup.

Â And you say, oh my goodness!

Â He is heterozygous for the FGFR mutation.

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

Â you have the lowest probability of breast cancer.

Â But your potential hubby is a heterozygote.

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

Â you can see where everything was.

Â What is the probability that your daughters would get

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

Â Basically, how much does it increase it,

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

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

Â little f You are big F big F.

Â 12:25

So we're just looking at your daughters.

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

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

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

Â 12:37

Half your daughters will have this mutation,

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

Â So there's a slight increase, but

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

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

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

Â 13:00

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

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

Â It's averaging all four of those numbers together.

Â So really the guy is only increasing your odds of

Â having daughters with breast cancer by about 1.5%.

Â 13:21

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

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

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

Â That's just a fictitious example and

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

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

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

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

Â an albino dad and they have eight kids.

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

Â 14:05

Welcome back.

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

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

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

Â they have eight kids, four of which are albino.

Â I said albinism is inherited as recessive in humans.

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

Â The dad necessarily has to be homozygous for the recessive.

Â So let's call it A.

Â So dad has to be little a, little a.

Â Now mom is non-albino, so

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

Â Right.

Â Now what if mom was AA?

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

Â So none of them would actually be albino.

Â 14:50

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

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

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

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

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

Â Basically, what is the underlying genetic components?

Â What alleles do they have within them and what combination?

Â