Introduction to Genetics and Evolution is a college-level class being offered simultaneously to new students at Duke University. The course gives interested people a very basic overview of some principles behind these very fundamental areas of biology. We often hear about new "genome sequences," commercial kits that can tell you about your ancestry (including pre-human) from your DNA or disease predispositions, debates about the truth of evolution, why animals behave the way they do, and how people found "genetic evidence for natural selection." This course provides the basic biology you need to understand all of these issues better, tries to clarify some misconceptions, and tries to prepare students for future, more advanced coursework in Biology (and especially evolutionary genetics). No prior coursework is assumed.
Contribution of Genes vs. Environment I (G, S)
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Introduction to Genetics and Evolution
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De la lección
Heritability and Population Growth
This module begins the transition to evolutionary genetics by looking at the relative contributions of genetics and environment to traits, and also introduces how population growth is studied.
Conoce a los instructores
Dr. Mohamed NoorEarl D. McLean Professor and Chair,
Biology
Hello.
And welcome back to Introduction to Genetics and Evolution.
We're at a transition point in the class now.
We've already gone over a lot of basic transmission genetics.
We've talked about things like recombination rate.
We've talked about genetic mapping, things like that.
What you'll start to see now, over the next couple of videos, is,
we'll still be talking about topics related to genetics, but
we'll gradually be getting significantly more evolutionary.
And we'll talk about some things having to do with evolution,
starting with today's talk.
Today we'll be looking at the different contributions
from genetics versus the environment, two various traits.
Basically, how do we tell if each of the two contribute to a particular trait?
How do we tell that some trait has a genetic contribution
versus an environmental one?
Well, the media often portrays this genetics versus environment, or
nature versus nurture, as they like to say it, as some sort of raging debate.
In fact, I've had people on airplanes ask me, where do you stand on this debate?
There is no stance.
That's just ridiculous.
It's nearly always somewhat of both.
But we do want to able to actually identify that there is a genetic component
or an environmental component because there are a small number of traits
that only have one.
Let me start with this example.
This is a picture from my backyard.
These are some corn plants that I was growing right next to our deck there,
as you can see.
Now what I did here to make it a little bit easier to see is I put these white
bars, as you can see here, just depicting the size of the corn plant next to it.
So you see I have some that are about this tall, some that are about this tall.
And right next to the deck I have some significantly shorter one.
So why is that?
Why do you think that there's this difference?
It's not just one corn plant,
there's a bunch of them that are all about the same height, a bunch of them there
are all about the same height, a bunch of them there are all around the same height.
Do you think I split up the seeds and put all the tall corn over here,
the medium corn over here, and the short corn over here?
Probably not.
Do you think I watered them differently?
No, they were all pretty much watered the same.
Well, if you think about it for a second, the answer is kind of obvious.
These corn plants out here were getting a lot more sunshine,
whereas these corn plants were getting a lot less.
Cuz as you can imagine, as the sun was going over,
all of them were still getting sun, like it's way over here.
These ones are still getting sun, but
the ones up against the deck are not getting any sun.
So, there clearly is some sort environmental component to this corn but
how do we quantify this, and more fundamentally, why do people even care?
Why do people want to quantify whether there's a genetic versus
environmental component?
Well there's a lot of reasons, as you'll see later in the class you'll
see that we want to be able to predict evolutionary change.
Right?
And evolutionary change requires that there's some sort of genetic component.
You can't have natural selection or evolution by natural selection,
unless there is some hereditary basis to the traits that are being selected.
Okay?
This is often also used in animal and crop breeding.
So let's say for example you're a farmer and you want to breed for
a corn that grows tall.
Now let's say that there is basically no genetic component to it.
Do you want to spend a lot of energy trying to select when there's in fact
going to be no response at all?
Well let's say there's no environmental component.
Do you want to spend a lot more resources for excellent fertilizer and
things like that when it's not going to matter?
It's all going to grow to the same height anyway.
No!
Basically, you want to know this sort of information so
you can spend the least amount of effort or energy possible for the best yield.
This is true for both animal breeding and crop breeding.
You want to know if the environment matters, and
you want to know if it helps to pick parents that have particular traits.
This is also very important in the context of medicine.
Let's say for example you're a doctor and somebody comes to you and says, my mom and
dad had peptic ulcers.
Am I likely to get them?
Basically is there some sort of genetic predisposition to peptic ulcer.
You want to be able to have an answer to these things and
this is where this kind of work is very useful.
So, let me emphasize again, does not appear dichotomy.
This is very rarely an either or question, but there's usually some of both.
However, one may contribute way more than the other.
So, something like, for example, eye color.
Yes, there's probably some small environmental component to that, but
there's a very strong genetic component to eye color.
That I could lie in the sun staring at the sun forever and
my eyes would not turn blue.
[LAUGH] Something like that.
Similarly, HIV status.
Yes, there are some genotypes that are slightly
predisposed to being less likely to getting infected with HIV.
But there's obviously a huge environmental component that your behavior is
very likely to affect how likely you are to get HIV.
So let's dive into this.
First, do genetics contribute?
The way people usually identify some sort of genetic contribution,
and I mentioned this in the beginning of the genetics section,
is there's some sort of parent-offspring resemblance.
That this could indicate that genetics are important because
people who are related tend to look more similar to each other than to just
somebody at random from the population.
However, this is not a perfect measure.
And part of the reason for that is something we'll come back to a little bit
later on in this set of videos, but not this video in particular.
It's that parents and offspring often share a part of their environment too.
So how do we separate these two things?
How do we distinguish?
Well I'm gonna show you a couple of different means.
In the first part here I'll talk about resemblance between relatives.
And I'll use examples of identical versus fraternal twins or
monozygotic versus dizygotic twins as an example.
I'll use that just as an illustration and then we'll come to the more meaty parts
which is common garden experiments or reciprocal transplant experiments.
And those use the same principles as the sort of twin examples.
You'll see this in just a moment.
The idea here is that we want to separate genetics from environment.
We want to keep one side constant and vary the other and see if there is an effect.
For keeping the environment constant, we vary the genetics and
we see an effect when there must be some genetic contribution.
If we keep the genetics constant, we vary the environment and we see some sort of
effect on the phenotype then there must be an environment component.
That's the basic principle.
So, looking at the two types of twins I mentioned,
monozygotic which are identical.
These are ones that came from a single sperm and egg that then separated out and
they shared a single placenta.
These are genetically exactly the same, okay.
Dizygotic, or fraternal twins,
are actually no different from any other brother and sister.
They are separate sperm and egg fertilized.
You can never have identical twins that were different sexes because you can't
have different sexes that have exactly the same genotype.
Famous example of some twins are the Olsen twins here.
They by the way are fraternal rather than identical, just so you know.
Now we could leverage this difference between monozygotic and dizygotic twins.
We predict, if there's a genetic component to some trait, so
let's say you're looking at height.
If there's a genetic component to the trait the monozygotic twins should be
more similar to each other in that trait, such as height, than dizygotic twins.
On average, if you look across monozygotic twins they should be very, very similar.
Dizygotic might be a little bit more different,
because the dizygotic twins don't have exactly the same genotype.
They have some different genes.
Whereas the monozygotic have exactly the same genes.
And we're assuming these twins are basically reared in the same environment,
or close enough, There is no particular difference.
Like if you were having kids, if you have monozygotic twins versus dizygotic
twins you probably don't treat them any differently.
You treat them the same because they're your kids.
The way we measure this, the way we measure how similar or
different you are is using correlations.
So, using correlations and traces, we do this.
This gives you some measure of how well traits match between groups.
I know some of you may have had a statistics background and
some of you may not.
So, for those of you who already know some statistics, I apologize for this review.
Correlations are given a numeric score, which is often labeled in r.
And that r ranges from -1 to 0 to positive 1.
And what this does is it indicates how closely one predicts the other.
So let's say for example you have a perfect positive match.
That would be an r of 1.
So an example of this would be your height in centimeters to your height in inches.
One perfectly predicts the other.
You have this absolutely perfectly linear correlation between the two.
In contrast, let's look at one that's not quite so strong.
Here's one.
Let's say you compared your height to your brother's height.
You may see something more like an r of 0.7, where on
average if you have a tall brother you are likely to be tall, on average, if you have
a short brother you're likely to be short, but it's not this perfect correlation.
There's some individuals who have a tall brother but
are a little bit on the short side, and things like that.
So it's not perfectly predicted.
And again, in this case, some of this may be from genetics and
some of this may be from shared environment.
That's why you do see some sort of shared association here.
Now let's go a little bit further.
Let's imagine there's no correlation.
You may see this in the context of your height to your neighbor's height,
your next door neighbor's height.
You know it's quite possible that you'll have some very short neighbors.
You may have some very tall neighbors.
But there's no reason to expect that their height is any more or
less like your height.
So this would be a case where the correlation is 0.
And you may be wondering, well I've only done this range from 0 to 1,
what about the negative part?
Well negative is when you have literally a negative prediction such
as daily high temperature to morning rainfall in the month of April.
That you may have a lot of rainfall associated with low temperature, but
as rainfall goes down your associate you get a higher and higher temperature.
Okay, so this would be an example of a negative correlation or
a negative 0.5 in this particular example.
So I hope that gives you an illustration of what I mean when I say correlations.
So let's come back and use this.
The prediction, as I've said before,
is that if there's a genetic component to a trait monozygotic twins
should have a stronger correlation in that trait than dizygotic twins, right?
The monozygotic are the ones that are genetically identical.
Right? And dizygotic ones
have some different genes.
We're assuming that the environment is about the same in both.
So what do we see with the data?
Well if we look at correlations which ones we often see this difference
between monozygotic twins and dizygotic twins.
Here are a couple of examples.
One is for IQ, or the supposed intelligence coefficient.
The monozygotic twins tend to score very similarly on these tests.
The correlation is 0.85.
Whereas dizygotic twins have a correlation of 0.42.
This suggests there is a genetic component to what is measured here as IQ.
Same sort of thing for some diseases.
When you look at gastroesophageal reflux disease monozygotic twins have a 0.29,
dizygotic twins have a 0.13.
Again we see this higher association when they are genetically the same
than when they are genetically similar, but
not identical which means that there is some sort of genetic component.
There may be an environmental component.
I should stress this.
None of proved the absence of an environmental component.
All it shows is the presence of a genetic component.
I should stress that.
Now, since this didn't prove the absence of an environmental component,
how do we look at that?
Well, what we can do there is keep the genetic relations constant, but
vary the environment.
What you do is you look at monozygotic twins reared together
versus monozygotic twins that were reared apart.
And your prediction is that monozygotic twins
reared together should have a higher correlation in these traits than
monozygotic twins reared apart if there's an environmental component.
The reason is, those monozygotic twins reared together have exactly the same
genes and the same environment, or very, very, very similar environment.
Whereas monozygotic twins reared apart have exactly the same genes but
somewhat of a different environment.
Now this has been done quite a bit, especially in the context of psychology,
but in other areas as well.
So here's some results.
Looking at body mass index,
or BMI, monozygotic twins reared together have a correlation of 0.74.
In contrast monozygotic twins reared apart have a correlation of 0.70.
This suggests there may some environmental component, but
it is surprisingly very small.
Very small, surprisingly small environmental component.
You make this is unexpected for something like body mass index.
But in fact, for this particular study, that's what they found.
In contrast, for verbal ability monozygotic twins reared together
had a quotient of 0.76, whereas monozygotic twins reared apart had 0.51.
So this suggests a very strong environmental component to verbal ability.
Now we can again do the same thing with monozygotic versus dizygotic.
So looking at this, this showed a strong environmental component.
This also shows a strong genetic component.
Because again, the monozygotic together versus dizygotic together,
together it means the environment's the same.
Mono versus di is changing the genetics.
In this case, we see a pretty large genetic component and
a moderately large environmentally component for the same trait.
Again, seeing this difference doesn't necessarily mean that there's,
it doesn't mean the absence of a genetics component,
it just means the presence of an environmental one.
Okay?
So again, what we try to do is we try to control for environment, vary genetics and
see if there's an effect.
Or control for genetics and vary the environment and
see if there's [INAUDIBLE].
Well, the same principles are done when you're looking at animals or plants and
things like that.
And not involving mono and dizygotic twins.
And, in fact, these have been done quite some time.
Let me show you some classic work.
This is from the 1930s, work done by Clausen, Keck, and Hiesey.
And here were interested in these differences among
plant forms identified from different elevations.
So they got some plant forms, as you can see on this elevational gradient,
from Timberline, which is very high in the Sierra Nevada of California, from Mather,
which is moderately high in elevation, and from Stanford,
which has a very low elevation, just above sea level.
And what they found is that the plants, these are the same species,
looked very different in these different forms.
That at high elevation they tended be kind of thin, sparse, not particularly bushy.
Here's a color photograph of the high elevation form.
As you got to lower elevations, you got this much bigger, bushier form down there.
Is it genetics, or is it environment?
Well, they pioneered a set of work that was extraordinarily influential.
So, one thing they tried were these common garden experiments.
These had actually been done by other researchers in the past.
But they applied this to these particular plants in California.
Essentially what they do for
this is they grow plants obtained from different places in a single environment.
So the idea here is they're trying to keep the environment constant.
By getting these plants from different places,
these likely have different genes, right?
Because they're coming from different places,
they likely have a different genetic makeup.
So if the form difference between, say, the high elevation plants and
the low elevation plants, well it's all environmental, right?
If you grow them in the same place, they should all look the same.
In contrast, if this form difference between the tall and
short plants was all genetic, then the plants should look like quite different.
Even if it's partially genetic, they should look quite different.
This is very similar to the concept of mono versus dizygotic twin studies because
again, we're using unrelated plants, so they have different genes, but
looking at them in a constant environment.
Well, what did they see?
In fact, they found there was a form difference apparent even when grown in
the same environment.
So these are ones from the species potentilla gracilis and grown in one spot
in Mathers, so this is that intermediate immediate elevation spot.
They found quite a big difference in form.
This suggests this form difference does have a genetic component.
It does not prove there was no environmental component.
It only proves there is a genetic component.
Okay? Well how do we do the other half of this?
Well, the other thing you can try is what's referred to as a reciprocal
transplant experiment.
In this case, you grow plant forms obtained from a single environment.
So since they're from the same place we're assuming the genetics is fairly constant.
And you grow them in several different environments.
So you are taking all of these ones that have the same form and
you are growing them in different environments.
So maybe you take it from Mather and grow it in Timberline, in Mather, and
in Standford, for example.
If some of the form difference is environmental then
they should in fact look different.
In fact, if the form difference was truly genetic,
then all the plants would still look exactly the same, right?
Cuz they're all derived from that same place, correct?
This is the same concept as these twins reared together versus apart.
That we're using the same genetic makeup and
we're varying the environment in which the plants are grown.
And in fact, with this they did still see an effect.
The form difference for
the same type grown in different environments was still apparent.
This is showing potentilla glandulosa, and this is grown at Stanford, Mather,
and Timberline.
And you know what again?
The Timberline ones are quite small.
The Stanford ones are much bigger and bushier.
This is just this form difference does have environmental component.
Now, everything I've shown you so far is how to tell is there a genetic component,
yes or no.
Is there an environmental component, yes or no.
And remember,
just because we say yes to one does not mean you're saying no to the other,
because quite often, as you saw with these examples, there is some of both.
Now what you probably want to do is to quantify
how much of the variation that you see is genetic versus environmental.
Well that we'll come to in the next unit.
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