It's Mr. Andersen and welcome to biology essentials video number 30. This
is on advanced or non-Mendelian genetics. I thought I'd start by tying together some
of the things we've talked about this year already. This is a phylogenetic tree of giraffes
in Africa. And two that I've pictured here are the reticulated giraffe and the Rothschild's
giraffe. And what we find is that even though they look very similar, they actually lie
on two different branches of the phylogenetic tree. And how do we know that? How do we know
which giraffe is related to which giraffe? Well we don't use simple genetics. We use
something called mitochondrial DNA. Mitochondrial DNA is DNA found within the mitochondria.
It's passed from mother to mother. And it's found in all offspring. And so it allows us
to trace evolutionary history. But it's non-Mendelian. In other words it's genetics that lie outside
of what we would sometimes refer to as simple genetics. And so in this podcast I'm going
to talk about some of the problems with Mendelian genetics. Not problems per se, but things
that we didn't address in the Mendelian genetics podcast. First one is the idea of linked genes
or genes that are found on the same chromosome. We'll then talk about multiples genes. And
this is actually one of the most important things in all of genetics. In other words,
most things are not, like a hitchhikers thumb, most things are not caused by just one simple
gene. Next we'll talk about how sex chromosomes can affect inheritance. Example I'll give
you is color blindness. And then finally we'll talk about non-nuclear transfer. In other
words DNA that's not even found in the nucleus. Found in the mitochondria or the chloroplast
and how that's passed maternally. And so let's start with Mendel. Mendel studied seven things.
He studied the stem length of plants, seed color, pod shape, seed shape, flower color,
flower position, pod color. In other words he studied seven different characteristics.
And he was able to come up with his idea of independent assortment. What is independent
assortment? Well essentially it means that all of these things assort independently.
All of these genes assort independently. In other words one can not affect another. Now
what's interesting about the seven that he chose is that all seven of them are found
on different chromosomes. Now if he were to study most characteristics he'd find that
the genes would actually be found on the same chromosome. If they're found on the same chromosome
he never could have discovered this idea of independent assortment. And maybe he did find
characteristics that were actually on the same chromosome and then decided not to include
that data. We can't be for sure. But we do know that a number of things do travel together.
For example red hair and freckles seem to go together. Why is that? Well they're found
on the same exact chromosome. And they're found very close to each other. And so they're
going to travel together. And also during meiosis, crossing over is not going to be
able to split them apart. And so a lot of the time when we're doing genetics problems,
if they're linked that means the genes are on the same chromosome. And then our simple
punnett squares just aren't going to work. Next I want to talk about multiple genes.
But I want to start with an example of multiple genes. If you've ever played Monopoly what
you'll find is that's it's really rare to get double ones, snake eyes or box cars, double
sixes. But you keep getting seven all the time. And the reason why is that we could
lay out the different dice on a punnett square. And so this is one die. You could roll a 1
through 6. This is the other die. You could roll 1 through 6. And so if we put out all
the possibilities that you could get, you'll find that certain things show up more. In
other words the odds of rolling a seven, you could actually get a seven in six different
ways. And so if you roll two dice, the odds of getting a seven are 6 in 36 or 1 in 6.
Whereas the odds of rolling snake eyes or box cars are actually 1 in 36. And so most
genes aren't transferred like this. You usually have one or two types. But we do have polygenic
inheritance. And so let me give you any example of that. Height in a person, and if you're
like me when you were growing up you really wanted to know how tall am I going to become.
And your doctor can't tell you. And the reason why is height is caused by a number
of different genes. And so in this simulation I'm going to say that it's caused simply by
three different types of genes. And you get two different alleles of that gene. And that
determines how high you are going to be. And so let's say you get zero height genes you're
going to be short. And let's say you get all six of the height genes you're going to be
really really tall. And so we have this continuum of one to six, zero to six genes. In us it's
actually thirteen different genes, a bunch of different varieties. It would be way more
complex. And I don't have the patience to make a simulation like that. And so let's
say we take a boy and a girl who are average. A boy and a girl who each have three height
genes. And let's say they grow up. They have kids. And let's figure out what their kids
are going to be like. And so what do I mean by 3 height genes? Well this one is going
to be for height genes A, B and C. He has one, two, three of the tall genes. And she
has three of the tall genes as well. Now if he were big A, big A, big B, big B, big C,
big C he'd be really tall. But he's not because he just has one of each of those. So let's
say that in the future they have kids. And we do the punnett square for this. Well this
boy right here could produce eight different varieties. In other words he could have a
big A, a big B and a big C in one of his sperm. But he could also have a big A, a big B and
a little C. And I could keep doing this, but there are eight different possibilities. I've
represented those across the top. And likewise since she is hybrid for each of those traits,
there's eight possibilities on this side. And so to kind of lay this out, if they happen
to give all their high genes to their offspring, then they would have six height genes and
they'd be really, really tall. But let's kind of step through the probabilities here. And
so if they were to have 64 kids, we would expect that only one of those would be super
short. And only one of those would be little A little A little B little B little C little
C. And so there's a 1 in 64 chance of producing a really small child. A 6 in 64 of getting
1 height gene. A 15 in 64 and now we're getting into big numbers. In other words there is
a 20 in 64 probability of producing an offspring that has three of those height genes. There's
a lot of different areas we could get that. And then it drops off to 15, 6 and then 1.
And so the odds of having, with these two averaged height individuals having a really
tall or a really short offspring are going to be really really low. But if we lay all
of these out what we get is this bell shaped curve. And if you were to look at height for
example in your classroom, you'd find that your height is assorted according to a bell
shaped curve. That implies that it is multi genes or polygenic inheritance. For example,
I'm pretty average height. When I go to buy shoes I'm a size 11. It's really hard to find
a size 11 in the sale aisle. And it's rare that they have my size. And the reason why
is that I'm really average. And so the shoe companies have to make a lot of those. It
may be really hard to find shoes at the other end as well. But if we ever see this bell
shaped curve, which could be anything, height, eye color, skin color, intelligence. If we
ever see this bell shaped curve it means that it's not caused by one gene, it's cause by
polygenic inheritance. And doing punnett squares gets really out of control when you're trying
to do polygenic inheritance. Next I want to talk about sex-linkage and the person who
really came after Mendel and discovered that everything he had discovered wasn't all of
it was Thomas Hunt Morgan. And he crossed fruit flies. In this cross he crossed females
with dark eyes with males with light eyes. And he got all offspring with the dark eyes.
So that make sense. But in the next cross he crossed these females with males and what
he found is that all of the female offspring had dark eyes, but half of the males actually
had this light mutation. And so he saw a difference between the gender. And so that puzzled him
a little bit. He also discovered linked genes. But what he figured out was sex linkage. There
are certain genes that are actually found on the sex chromosome of the X and sometimes
the Y chromosome. And so that gives us sex linkage. Let me give you an example of this
in humans. In humans, here are three different pictures, each of these have a two digit number.
And so pause for a moment and make sure you figure out what each of these are. So let
me flip over those letters. And it should have read 37, 49 and 56. Now I'm color blind.
And I'm color blind this type. In other words I could see 37. I could see the 56 and kind
of trace that. But I have no idea where the 49 is because I can't see certain shades of
green. Or I have a decrease sensitivity to this green. And that's because I'm color blind.
And I'm missing a gene on my X chromosome. And so how does that work? Well I got it from
my mom. And so my mom is a carrier of that. Remember your mom has two X chromosomes.
Your dad has an X and a Y. And so my mom is a carrier of that color blindness. Now if
I test her, she's not going to be color blind. That's because she has another gene on the
other X chromosome that's totally good. And so if we show what are all of the offspring
that we could get from this. Well this would be a carrier who's a girl. This would be a
normal girl. And actually my sister is a carrier as well. Because some of my nephews are color
blind. And so if we go to the next one, this would be me. And this would be another boy.
Okay. And so I'm color blind. That means that I got the gene from my mom. What did I get
from my dad? Just a Y chromosome. And the gene happens not to be located on there. So
color blindness is an example of that. What, if we look at my sister, is she colorblind?
No. Because she got a good one as well. And so in males roughly 1 out of every 10 males
are color blind. But to be a girl and color blind, it's an odds of 1 in 10 times 1 in
10 or 1 in 100 probability. So it's rare that I find kids in my classroom, girls, that are
colorblind. But very common to be boys. And that's because girls have this extra X chromosome.
It's like a back up copy. If something goes wrong they have that extra. Hemophilia is
an example of that. It's a sex linked chromosome, sex link gene. We find it way more commonly
in males than we do in females. Now I'm still talking about DNA that's found in the nucleus.
But there's also what's called non-nuclear inheritance. In other words, the mitochondria
inside you have their own DNA. So they have their own, I don't know where the DNA is in
the picture. Let's just make it. They have their own DNA because we think they came from
bacteria. And so that is passed, not within the nucleus, but it's passed actually within
the mitochondria as they divide. Plants, the DNA in plants as well is non-nuclear. And
so it's not from the nucleus of the plant. It's actually found in the chloroplasts itself.
And so as mitochondria make copy after copy after copy that DNA isn't inside our nucleus.
It's actually found inside the mitochondria. And so you might think to yourself, well how
did I even get my mitochondria? Those mitochondria were in that egg of your mother. And so the
DNA is passed from egg to child to egg to child. It's passed maternally, so passed through
females down through generation after generation. The neat thing about it is there's no crossing
over. There's no meiosis. And so we can look at changes within that mitochondrial DNA.
We can see mutations and then we can trace evolutionary history. Just like in the giraffes
that I talked about at the beginning. And so that's non-Mendelian genetics. Not super
advanced, but it's hard to do simple probability in simple punnett squares using genetics like
this. And I hope that's helpful.