Hi. It's Mr. Andersen and in this podcast I'm going to talk about this
guy, Gregor Mendel, who we credit for really coming up with the simple ideas behind genetics.
He worked in a monastery, grew pea plants, studied their characters or their characteristics
and then he would breed them. Create a bunch of new pea plants. That's what each of these
seeds in a pea pod is going to be. He would grow them and then see how those characteristics
played out. And he came up with simple, we call it, Mendelian genetics. Now sadly, nobody
really paid attention in his lifetime. And his work was rediscovered in the early 1900s.
But we still give him credit for these ideas. And so I'm going to assume that you know some
basic terms in genetics. And so let me flash these terms up here. And if there's some you
don't understand, make sure you take a look a those. Okay. When I did this in class there
were a few stumpers. Some kids were confused by the difference between a monohybrid and
a dihybrid cross. Monohybrid cross would be something like this. If we're doing purple
pea plants. If I'm crossing that with that. Since I'm just studying one trait or one gene
we call that a monohybrid cross. A dihybrid cross would look something like this, big
P little p big Y little Y. So if I were to do a cross like this, this would be a dihybrid
cross. What's another one that was confusing? The independent assortment basically means
that two different genes for example, flower color and whether or not the seeds are wrinkled,
those aren't going to effect each other. In other words they assort independently and
that's going to take place in meiosis 1 as the homologous chromosomes separate. And then
another term that some people are confused with is segregation. This is one of his big
laws. It basically means that assuming that it's a diploid organism, if you are big P
little p, there's a 50% chance that you're going to give a sex cell a big P and a 50%
it's going to give it a little p. But most of the people in class kind of understood
the other ones. Ah, here's another one that was somewhat confusing. That's a test cross.
Basically if we have something that is big P little p or big P big P, let's say this
is a purple flower and this is a purple flower, in other words I know the phenotypes of the
two, how do I figure out what the genotypes are? Well basically if you do a test cross,
that means you're going to cross it with one that is homozygous recessive. You can imagine
if I cross one that's homozygous recess with this one, half of them are going to be purple.
And if I cross it with this one 100% of them are going to be purple. And so I can figure
out what the genotype of these two are. And so those are some terms that you probably
want to become familiar with before we dig a little bit deeper into genetics. But before
Mendelian genetics, most people believed in this kind of a blending idea of genetics.
That kids look a lot like their parents. And so there was something inside parents that
kind of blended together to make children. But the didn't really know what that was.
And so the big discovery of Gregor Mendel was this. He took purple flowers. Now these
purple flowers were true breeding. What that meant is he crossed the purple flowers with
themselves over and over and over and over again. And so what he had was purple flowers.
And these ones likewise were white and generation after generation they only produced white
offspring. So we knew that we would call those pure. Pure true breeding purple and white
flowers. He crossed those together. And so in the parental cross, he crossed purple with
white and the F1 generation he basically got all purple flowers. Now if you think about
blended inheritance, this makes sense. If you take purple paint and mix it with white
paint, it totally makes sense that you're going to get purple flowers. And so if he
would have stopped here, he wouldn't have learned anything. But what he did then was
took these ones and bred them with themselves and what he found was this characteristic
3 to 1 purple to white ratio. And this would be the phenotype ratio. And that white flower
had come back. It had skipped a generation but it was as white as that original white
flower. And so that told him that something was being passed from generation to generation
to generation. And the three to one ratio gave him some hints as to how that was actually
passed. If we were to write out how this works, and this is how he figured it out, well the
purple here would be big P big P, homozygous dominant. The whites are going to be little
p little p. And so if you think about if we cross these two, all of this generation is
going to be big P little p. And then if I cross this with itself, we could do a punnett
square but basically one out of four is going to be big P big P. We're going to have two
of them be big P little p. And then we're going to have one of them be little p little
p. But since purple is dominant, if you have one purple that means you're going to be dominant.
That's why these three right here ended up being purple. And this one ended up being
white. And so that's pretty much what we had formulated. And it's held to this day. So
he basically talked about two Mendel's Laws or two laws. And basically they are the law
of segregation. The Law of Segregation means that each organism is going to have two genes
for each trait. So this one is going to have a gene for purple and another gene for purple.
In this case it's heterozygous so it's got one big P one little p. And so the law of
segregation is just like flipping a coin. Basically what you're doing is flipping a
coin on each of these genes. There's a 50% chance that the gamete or the next generation
is going to get the big P. And there's a 50% chance that it's going to get the little p.
And so the law of segregation isn't scary. Basically it means everything is a coin flip.
And there's a coin flip on each gene. Next thing he discovered was the idea of independent
assortment. And so basically once you've discovered genes, the big thing we have to figure out
then are genes tied together? In other words, this one, and please remember these letters
because they'll come up in just a second. This is a yellow pod. And then this would
be a green pod. We'd normally use the capital letter to represent the dominant trait. In
this case it's yellow. And then the recessive letter is going to be the green. And then
this one would be round seeds and this would be wrinkled seeds. Or wrinkled pods we could
say. And so basically this is going to be the dominant and this is the recessive. And
so what he wanted to study, and this is what he studied in his traits is, are these two
linked? Are they tied together? Or do they assort independently? And independent assortment
basically says that the yellow versus green is not going to effect the round versus wrinkled.
In other words these two genes segregate. These two genes are separate. Now we'll find
that once we get into a little bit of chromosomal genetics sometimes they will be linked together.
But independent assortment means the genes don't effect each other. The yellow versus
green and the round versus wrinkled sort independently. So what do you use to solve these? Well a
simple way to do it is just doing simple probability and using a punnett square. And so if you're
not sure what a punnett square is, basically let's say that we have big P little p here.
Since we have the law of segregation, half of the gametes or half of the sex cells are
going to get the big P and half are going to get the little p. We now know that's called
segregation. And so really what the two sides on the top are going to represent are the
different gametes that we have. Likewise, let's say we're copying this with little p
little p. Then this is going to segregate as well. So these are going to be the two
different Ps that you can get. And then what does the middle represent? Well the middle
represents fertilization. These are all the possibilities that you could get when these
two alleles that separated come back together again. And so let's try some of these kind
of in your head. So you could stop if you want to, but let's say we take this, purple
with white. So if you where to do a punnett square for that how many different genotypes
am I going to get? And the right answer should be one. Big P little p. And how many physical
phenotypes am I going to get? Just one. Let's try the next one. Kind of in your head if
we do this cross. How many genotypes would I get? Well I'm going to get three genotypes.
Big P big P, big P little p, little p little p. How many phenotypes am I going to get?
Well I'm only going to get two physical phenotypes. So I'm only going to get purple and white.
Let's try the next one. Let's say here. How many different genotypes would I get? Right
answer I'm going to get 2 different genotypes. How many phenotypes? Just one. They're all
going to be yellow. Or this one when I'm mixing round with round, how many genotypes will
I get? 2. How many phenotypes am I going to get? I'm just going to get one. They're all
going to be round. And so if you're struggling with a punnett squares you may want to always,
on a monohybrid cross like this, work it out. When in doubt, work it out. So now let's go
through some sample problems. So I'll read it and then quickly come up with an answer
and I'll tell you what the right answer is. So a coin is flipped four times. Comes up
heads each time. What's the probability the next coin flip will come up heads? Right answer
should be 1 in 2 or half of the time. So basically what I'm getting at here is all these earlier
events aren't going to effect the next event. Let's go a little easier. Classify the following
as heterozygous or homozygous. Right answer should be, this would be homozygous. This
would be heterozygous. This would be homozygous. And this is going to be heterozygous for the
first trait, homozygous for the second. We could get a little more specific. This would
be homozygous dominant. This would be homozygous recessive. This would be heterozygous and
sometimes we call that hybrid. Let's go to the next one. What is the phenotype of the
following? Phenotype remember is physically what do they look like. I would call this
one yellow. This one round. This one green. And this one's going to be yellow and round.
Let's go to another one. What's the probability of this cross, big R little R, big R little
r producing wrinkled seeds? Right answer is going to be 1/4 or 1 in four probability or
25% chance. And that's going to be each of them. It's a one half probability on each
of the little rs. Let's go to another one. What's the probability of these producing
green seeds? I would say 1 in 2. These ones are always going to give the recessive trait.
This one's going to give the recessive trait half of the time. So it's going to be a 1
in 2 probability. And now let's go to the last one. What's the probability that this
parent and this parent would produce this? Take a second. See if you could do that one
in your head. Okay. If you can't do it in your head that fast, you probably don't know
the law of multiplication. And it's really simple. Don't try to do a punnett square.
If it's ever a dihybrid cross the way you solve it is this. Let's start on the rs. And
so if we have this Rr and this RR what are the odds that we're going to produce this
Rr? Well if you're confused you could always do a little punnett square. So I could write
it out like this. Big R, little r crossed with big R big R. So that's going to be big
R big R, big R big R, big R little r, big R little r. So the odds that we're going to
produce this, you can see, is a 2 in 4 or a 1/2 probability. So you just write 1/2 underneath
this one. What are the odds with this parent and that parent that we're going to produce
that? Right answer is going to be, you could do a punnett square but you should be able
to do this one in your head. A 1/2. And since we have to get both of these. So we have to
get this and that then we multiply the two together. And so the right answer would be
1 in 4. Now if you were to do this as a punnett square and a dihybrid cross, it's going to
take you minutes and minutes to just set it up. And you're probably going to screw up
how to do the meiosis. And so always do the law of multiplication if you have a dihybrid
cross. And this goes all the way out to a trihybrid. All of those are simple, easy to
do with multiplication. And I put together a video on law of multiplication and addition.
How it can be applied to genetics. But that's Mendelian genetics. In the next video I'm
going to talk more about advanced or non-Mendelian genetics. But for now, I hope that's helpful.