Most students understand Mendelian genetics and could do a simple Punnett square like

this, but in this video we're going to dig more deeply and see that those genes are actually

carried on chromosomes and packaged into gametes. And so it's built on the work of Thomas Hunt

Morgan. And so we're going to go through a monohybrid and dihybrid cross. Not only the

Punnett square but let's dig into how the chromosomes are actually delivering those

genes. And so in this monohybrid cross, this first Mendelian cross, he crossed remember

yellow with green and got just yellow seeds. And then he selfed yellow with themselves

and found that 3 to 1 ratio. So if we were to add letters to that that represent the

genes, the parents are going to be homozygous dominant and recessive. And the F1 generation,

they are going to be heterozygous. And then we produce a Punnett square that shows us

all the alternatives that we could get from that F1 cross. And so what do these two sides

really represent? This big Y represents the odds of delivering that big Y allele or that

dominant allele. And this is the odds of representing that little allele or that recessive allele.

And so there's a 50 percent probability. And so these are really the odds of producing

gametes that have the dominant gene or the recessive gene. But remember you don't just

produce two gametes, you produce four. And so we're going to dig into that in just a

little bit. These would be the gametes from the other offspring. Or the other parent.

And then these are going to be the four possible offspring that we could get. Now remember

those genes are actually delivered on chromosomes. And so if we look at it at the level of Thomas

Hunt Morgan, and in this model I just have 2 chromosomes here. We find that that dominant

gene is actually found on a chromosome. And the recessive gene is found on a homologous

chromosome. In other words it's the same length. Now where did these chromosomes come from?

They came from the P generation. And so in the P generation you could imagine that parent

would have 4 chromosomes. They would all be red. And each of their chromosomes too are

going to have those dominant Y genes. And likewise for the other parent. And so now

we have to figure out what possible gametes could come from this. And so we have to dig

into meiosis. Remember meiosis starts with duplication of the DNA and then two divisions.

And so let's duplicate the DNA and see what happens. We've got two that have the big Y

and then two that have the little y. And so you could imagine we're going to make 4 gametes.

And you can almost visualize those. So each of these vertically is going to make a gamete.

And so how many possibilities do you see so far? I just see the big Y, here's another

big Y. Or the little y. Now remember during meiosis they're going to independently orient

themselves at the metaphase plate. So they could line up like that. Or they could line

up like that. Or they could line up like that. Or they could line up like that. There are

four arrangements. But it doesn't really matter how they are orienting themselves. We're still

seeing those 4 possible gametes. But two of them are going to be exactly the same. Each

sex cell is either going to get a big Y or a it's going to get a little y. And so let's

see those two divisions in meiosis. So there's one division. And there's another division.

So remember you produce four cells in meiosis. But you can see that two of those are going

to be the exact same genes, the exact same chromosomes. And so we really only have two

alternatives. Big Y or little y. Now how does fertilization work? Remember we're going to

have those gametes of the other parent. And then they're going to combine. And since there

are two possibilities on each of the parent we could only have four possibilities of the

offspring. Let's kind of visualize those. We could get a big Y from each of the parent.

So that would be one possible zygote. And that's going to be a yellow seed. We could

arrange it like this. Get a dominant from one and recessive from the other. Or a dominant

from this one and a recessive from the other. Those are each going to be yellow as well.

Or we could get two recessive genes or recessive chromosomes. And so how many possibilities

do we get? We get a 3 to 1. Now the Punnett square worked great. We didn't learn anything

new really by looking at the level of the chromosome. But that's about to change. Now

let's look at a dihybrid cross. And so in this cross Mendel crossed yellow round with

green wrinkled and he found in the F1 generation that they were all yellow round. So yellow

and round seemed to be dominant. And so if we were to add letters to that these would

be homozygous yellow yellow round round versus green green wrinkled wrinkled. And so what's

going to be our F1 generation? They're each giving one of each of those letters so there

going to be hybrid for both. But now Mendel is presented with this problem. And so do

those genes travel together? Are they dependent on one another? Or are they independent of

one another? Because those two alternatives could produce different possibilities. You

could get a Punnett square that looked like this where we only have two possible phenotypes.

Or we could have 4 possible phenotypes. And tons of different ratios. And so you probably

know what's the right answer. But let's go through the Morgan way. Look at the chromosomes

and figure out what's going on with those genes. And so if we go back to the F1 generation,

and let's just put the yellow and the green on our respective chromosomes, now the question

is begging, is the R gene, is that round gene going to be found on the same chromosome as

the yellow or is it going to be found on a different chromosome? And so let's just play

out those two possibilities. Let's just say that the round is found on the same chromosome.

So let's put it right here. Again, you're getting the round and the wrinkled gene from

that one parent, the dominant parent we could say. And then the two recessives from the

recessive parent. And so let's go through all the possibilities that we could get. And

so now we've got interphase where we copy the chromosomes. And now we have to figure

out all of those different orientations. It could like up like this. This would be a gamete

that's going to be round and yellow. And this would be a gamete that's going to be green

and wrinkled. And so let's figure out if they orient themselves in a different way. Does

that change the different gametes that we could get? So they could line up like that.

That's not really changing the possibilities. They could like up like that. It's not really

changing anything. You can see that the round is going with the yellow. And the wrinkled

is going with the green. They could orient themselves like that as well. And so really

we only get two possible gametes in this case. And so if we were to go through meiosis, let's

grab one like that. This would be one parent. This would be the other parent. So how many

possibilities could we get? Well since this parent only has two gametes and this does

as well, there's really only four boxes in our Punnett square. So you could have an individual

made from round and yellow. So dominant for both traits. You could get a dominant from

this side and a recessive from the other. Or vice versa. Or we could get both of those

recessive genes and it would look like that. So if we were to do this one alternate reality,

how many possible phenotypes would we get? These are all going to be round yellow except

one-fourth of them are going to be wrinkled and green. And when Mendel did that cross,

that didn't happen to be. And so we now know that those are going to be found on different

chromosomes. And so the round and the yellow are found on different chromosomes. So let's

go through meiosis and figure out how we're getting all these different possibilities.

And so again during interphase they're going to copy all of their DNA. So we've duplicated

the chromosomes. And so they could line up like this. And so what kind of a gamete would

this be? It's going to be a round with a yellow. And a wrinkled with a green. But if it lines

up like that, nothing changes. We've still got round with yellow and wrinkled with green.

But let's say it lines up like that. Now what do we have? You can visualize now we've got

a wrinkled with a yellow. And we're also getting a round with a green. And so how many gamete

possibilities do we have here? Four possibilities. This again, three and four, are going to be

the exact same thing. And so now there are four possible gametes. And so if we were to

play that out, using just a Punnett square we'd have to put all those possibilities on

one side. All those possibilities on another. And we'd find that all these individuals are

going to be round and yellow. These three individuals are going to be round and green.

These three are going to be wrinkled and yellow. And only one of those 16 possibilities are

going to be wrinkled and green. Now Mendel was able to figure this out not understanding

what's going on with chromosomes. And so it took decades before scientists looked at this

and found a flaw in his model. And let me show you that. So in this Thomas Hunt Morgan

cross we're dealing now with fruit flies rather than peas. But in this cross it's a simple

test cross. So what he has is a wild fruit fly. But it's hybrid. So what does that mean?

It has one of the wild genes, and this is for normal wings with a vestigial gene, which

would normally create these small little wings. And then it's also hybrid for coloration.

So it's got the normal coloration. And then it has one of the genes for this black coloration.

Likewise this other parent over here is going to be recessive for both. And so when Thomas

Hunt Morgan, because he understood Mendelian genetics, set up his Punnett square he set

it up like this. And so what are going to be the possible gametes that this fruit fly

could give? Well it could give VG+ and it also could give B+. It could give that. It

could also give each of the recessive genes. So it could give that recessive vestigial

wing gene and the black gene. It could also then give a combination of those two. So there

are four possibilities that we could get from that one parent. Since this one is recessive

for both, it really can only give one gamete. And so Morgan knew that there would only be

four possibilities and we should get 25% of each of these. And so if you get these gametes

you should get a hybrid for each and so you're going to look exactly like this parent. You're

going to be wild type for both. If you're this, you're going to get all recessive genes

and so you're going to look like this parent. And so we call those parental phenotypes,

because they look just like the parent. This one here looks like that parent. This one

looks like that parent right here. You could also get what are called recombinants. And

so this one right one right here is going to have the normal wings but it's going to

have that black coloration. And this one over here is going to have normal coloration but

it's going to have vestigial wings. And so Morgan knew that it would be 50% parental

types, 50% recombinants. But when he actually did the cross that's not the number he got.

What he got was 17% recombinant, 83%. So Medel was wrong or his model didn't go deeply enough.

And so in the next video on genetic recombination and gene mapping I'm going to answer that

and show you not only did Morgan figure out now the genes are carried on chromosomes.

But the importance of crossing over producing those recombinants. And so that's chromosomal

inheritance. It's understanding genetics not at the level of Mendel, but also at the level

of Morgan. And I hope that was helpful.