Genetics


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.

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