Mendelian Genetics


Uploaded by bozemanbiology on 30.07.2011

Transcript:



Hi. It's Mr. Andersen and welcome to Biology Essentials Video 29. This is on
Mendelian Genetics and it's called Mendelian Genetics. It's named after Gregor Mendel.
In biology there are two famous names, Darwin who was famous and controversial in his own
time and Gregor Mendel who died in obscurity. But both of them made huge advances in the
field of biology. And Gregor Mendel in the area of genetics. If they could have gotten
together those two theories, when they finally came together as modern synthesis, it was
really powerful. Unfortunately he dies in obscurity. But this is what he did. He was
crossing pea plants. And so he can take one pea plant and you use a paint brush to transfer
pollen from one to another so you know who are the parents. He would then create offspring
as a result of that. Now peas are great because they have a number of different characteristics
and more importantly you can make a lot of them really quickly. So in a pod each of these
peas is an actual a new organism. So you can plant those and see how they grow. And so
he figured out a lot of genetics as a result of that. And so genetics in this podcast I'll
talk about are Mendelian or simple genetics. He identified the gene and he came up with
two laws. The law of segregation and independent assortment. I'll talk about those. We'll also
have some problems. So I'll have some practice problems you can try and I'll show you how
to work those out. And we'll finish with genetic disorders. The example I'll talk about is
Huntington's disease. And with genetic testing it opens up this whole idea of ethics and
privacy. Now what won't I be talking about? I won't be talking about linked genes. In
other words if genes are ever on the same chromosome or on a sex chromosome or caused
by multi genes, it's really complex, and so I'll talk about those in the next podcast
on advanced genetics. But know this, that simple Mendelian genetics, the rules are simple
rules of math, rules of probability. And if you understand those then how to do the first
cross that I show you, you should do well in most any of the crosses you get in genetics.
And so this is what Mendel did. He crossed purple flowers with white flowers and he got
purple flowers. Now a few things that you should know. The first cross in any genetic
cross is called the P cross or the parental cross and then the offspring of that are called
the F1 or filial one or the offspring of that cross. And so at the time of Mendel, everybody
believed in this idea of blending. That if you crossed two parents, so mom and dad, their
kids look a lot like them and so it's maybe a blending of all some, they didn't know that
they were genes, something inside of them. And so when you cross purple with white and
got purple flowers that made total sense back then. This kind of fits in thus blending.
But what he did next was he crossed these purple flowers with themselves and what he
got was a 3 to 1 ratio of purple to white and that white returned just as crystal white
as that first white was to begin with. And so what he said was that there's a character
or trait that's passed through here, but we now identify that as a gene that's carried
distinctly through each of those generations and then shows up again. Now if you know anything
about genetics you know that simple Mendelian genetics, it looks like purple is dominant
and white is recessive but we know that because of the work of Mendel. And so this is what,
the second cross is the one that was puzzling and this is what he eventually figured out.
So these offspring right here were hybrid for the trait. And so if we look at the parents,
the parents would be big P big P, that represents one purple flower. Little p little p represents
the white flower and so each of these would be big P little p. And so you can use a Punnet
square. A Punnett square you put the pollen or the male here the female right here and
each of these genes get a certain column. And so this would be one parent, big P little
p and so you simply write that across. So we've got big P and little p here. We've got
big P and little p here, so this represents the two different parents crossing with each
other. And then we simply figure out what comes. So here's big P big P. So we get a
big P from 1, a big P from the other, so that'd be big P big P. Since big P is dominant we
get a purple flower. On the next one I'm going to take a big P from here a little p from
here and so that would be still purple because this one is a dominant allele or a dominant
gene. Down here we get a big P from here and a little p from here and so that's purple.
And the reason we get white flowers is that you get a little p from both of the parents.
And so the neat thing about a Punnett square, and that's what this is, is it not only allows
you to quickly do the probability but it shows you the percent we should get in those offspring.
In other words three of them should be purple and one of the should be white. So we should
have a 3 to 1 ratio. And if you ever get a cross like this in a problem for example a
cross like this, if you're not sure what the offspring are going to be, just do a simple
Punnett square where these two alleles or versions of the gene are going to be on the
top and these two are going to be on the side. And we'll do some practice problems in just
second. And so what are Mendel's Laws? Well the two things that he figured out are the
law one, Mendel's Law One. It's called the Law of Segregation and Mendel's Law Two is
the Law of Independent Assortment. And so let's start with law one. And so if we ever,
and I've got a coin here, because the actual way you get genes are almost like a coin flip.
So if you think about a coin and it has heads on one side and tails on the other, when you
flip a coin what are the odds that you're going to get head or tails, well it's a one
in two probability that you'll get heads. Same thing with genes. And so if this is that
F1 generation and this shouldn't be B it should be P, so we say this is big P little p, what
are the odds that the offspring are going to get a big P? Well it's a 1 in 2. What are
the odds that they're going to get a little p, it's one in two. And so that separation
of those two alleles is called segregation. And so this idea of segregation says that
there's a 50% chance that you're going to get either of these genes. And so that's segregation.
They separate, and it's just random chance. The next on is the Law of Independent Assortment.
The Law of Independent Assortment says that this gene, the gene that causes for example
hitchhiker's thumb, which is where your thumb actually bends back and the gene that causes
an attached earlobe, so right here I've got a free earlobe. Those two traits don't effect
each other. In other words they sort independently. And so we can work problems without mixing
these two together. They're going to not influence one another. Now sometimes we'll find for
example that some things do travel together. So you'll notice that people who have red
hair also have freckles. And that's because those two genes are actually found on the
same chromosome and so they seem to travel together. And so we're not going to deal with
linked genes. Again we'll do that later. And so independent assortment means that traits
don't effect each other. And so what I'm going to do next is I'm going to leave this for
a second. These are 6 problems that I'll work through, but if you want to work these you
could pause the video at this point and then you could come back and start the video again
and see me work through each of these. So I'll pause. Alright. So let me go through
these. Question 1. A coin is flipped four times, comes up heads each time. What is the
probability that the next coin flip will come up heads? Well everything that's happened
in the past can't influence anything that's going to come in the future and so it's a
one-half probability that you'll get heads. In other words you could have 10 kids. They
could all be boys. What are the odds that they next one is going to be a girl? It's
still a 1 in 2 probability. Let's look at the next one. And so we've got some things
up here. Round peas are going to be big R and wrinkled peas are going to be little r.
Yellow will be big Y and green is going to be little y. Generally whatever is the dominant
trait, we give that the capital letter. In this case round gets the big R and yellow
gets the big Y. And so question number 2. Classify the following as heterozygous or
homozygous. Heterozygous means you have different genes or different alleles. Homozygous means
you have the same. And so this first one, big R big R would be homozygous dominant.
They're the same. This would be heterozygous and we also sometimes refer to that as hybrid.
The next one would be homozygous recessive. And the next one would be homo, excuse me,
heterozygous yellow, homozygous for the round. So it's going to be heterozygous and then
homozygous dominant on the next one. So that tells you the alleles that you have. Let's
look at number three. What's the phenotype of the following? Well this right here is
going to be the genotype. In other words big Y little y is going to be the genes that you
have. What's going to be the phenotype? Well that's physically what you look like. And
so for the first one, this one right here, even though it's genotype is big Y little
y or it's heterozygous for that, it's phenotype would be yellow. So this is going to be yellow.
This one is going to be round. This one is going to be green, and this one here is going
to be yellow round. Phenotype is physically what you look like. Let's look at the next
one, number four. What's the probability of this cross? So we have two rounds seeds producing
wrinkled seeds. Well, like I said before if you ever get one of these it's a simple monohybrid
cross I would always do a Punnett square. So we'd put big R and little r one side of
my Punnett square. Big R little r on the other side of my Punnett square, so what are the
odds that I'm going to get wrinkled seeds? Well, there's a little r here a little r here.
And so there would be a 1 in 4 probability that we'd have wrinkled seeds. Because this
one's going to be round, this one's going to be round, this one's going to be round
as well. And so again if you ever get a simple cross like that do a Punnett square. Let's
look at the next one. What's the probability that this cross would produce green seeds.
Well I do the same thing again. Big Y little y crossed with little y little y. And we're
looking for green seeds. Green seeds remember are going to be little y little y. And so
I could get it here. I could get it here an so that is a 2 in 4 or a 1 in 2 probability
we're going to get green seeds from that. And so even though you might think that you're
super smart, do a Punnett square. You're never going to miss the problem then. Now this is
a problem we'll sometimes get on the AP bio test as well. If these parents are crossed
together, what are the odds that you'd get that? Well to do this one you'd have to actually
set up a pretty intense Punnett square. And so if you get one like this don't do a 4 by
4 Punnett square. Just work each of them individually. And so what do I mean by that? Well let's
start on the Rs. What are the odds that if you produce this and that you can get that.
So let's do our Rs first. Big R little r crossed with big R big R, what are the odds that we're
going to get big R little r? Well neither of these. But this is a big R little r. This
is a big R little r. And so the odds of these two parents producing these offspring is going
to be a 1 in 2 probability. So I'm going to write that right over here underneath, the
1 in 2 probability. Now let's work the Ys. So let me get a different color. So if we
do the Ys, what are the odds that these two parents are going to produce that offspring?
Well let's do those together. So here are the parents, big Y little y crossed with big
Y little y. So this is going to be big Y big Y, little y little y, big Y little y, big
Y little y. So what are the odds that we're going to produce again big Y little y, well
it's a 2 in 4 or a 1 in 2 probability. So I'm going to write 1 in 2 here. Okay. So now
instead of doing this huge unwieldy four by four Punnett square, what I've done is I've
almost got there. Because the odds of getting this are 1 in 2 and the odds of getting this
are 1 in 2 so what are the odds of getting both of those? I simply multiply those together
and it's a 1 in 4. In other words what are the odds of rolling or flipping the coin and
getting heads? One half. What are the odds of flipping two heads in a row? It's a half
times a half or a fourth. And so you can solve problems like this just using what's called
the Law of Multiplication. These two things have to happen. So I hope you did well on
those problems. Last thing I want to talk about is disease. And a nasty disease is called
Huntington's Disease. It's named after the person who identified it in the 1800s but
essentially what you get is degeneration of the nerve fibers in this portion of your brain.
And so what happens is eventually you start to get uncontrollable shakes, you can't really
walk. Eventually you die as a result of that. Now the problem with Huntington's Disease
is you don't know you have it until you're middle age. So I could have Huntington's Disease
right now. I'm going to die as a result of this disease, but I don't know it. And so
I've already had kids. I've already passed the genes on. A famous person who had Huntington's
Disease is this guy. His name is Woody Guthrie. You probably know him. He wrote the song "This
Land is Your Land, This Land is My Land". And he died as a result of having Huntington's
Disease. Now it's a dominant trait. In other words, if you are this you get Huntington's
Disease. If you are this, you don't. And so let's look at a pedigree. A pedigree shows
you how a disease can be passed down through organisms. And so on a pedigree a square's
always going to be a male. Circle's going to be a female and if you ever have a horizontal
line between them it means that they had offspring. And so this is the grandparents in this case
and they had a boy. And then the next kid they had was a boy. And then they had a girl
and then they had another girl. And you can see that this girl for example had her own
family, but you can trace the disease through it. In other words since this parent right
here is big, let's use a different color, big H little h and this one is little h little
h, this big H was actually transferred to the son. It was not transferred to this son.
It was not transferred to this daughter, put it was transferred to this daughter over here.
And so the odds of passing it on are 1 in 2 and you can see that 1 in 2 of their kids
had that. And if it's a dominant disease like this, lot's of times we'll see it in generation
after generation after generation. But you've reproduced already by that time. And so it's
almost too late. Now where does this become an ethics issue? Well, we now have a test
for Huntington's Disease. And so Woody Guthrie had a number of kids. One of those is named
Arlo Guthrie who is also a famous folk singer. And so Arlo Guthrie may have the Huntington's
gene, he has a one in two probability of getting it. We now have a test that can figure out
if you have that gene, but it'll influence your life in the future. And so would you
want to know that you're going to get a disease that will cause a nasty death as a result
of that? There's not a lot of treatment for Huntington's Disease, or not? And would your
insurance company want to know that as well? And so again the genetics behind simple Mendelian
genetics are fairly simple but it open up all these moral issues. And I don't have an
answer for any of those questions, but it's something we're going to have to deal with
in the future. And so that's genetics, Mendelian genetics and I hope that's helpful.