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.
Origin of Genetic Variation (G)
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Introduction to Genetics and Evolution
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Genetics III
This module delves even more deeply into the complexities of the genetics underlying traits,the origin of genetic variation, and how "complex" traits (ones controlled by multiple genes) are studied genetically.
Meet the Instructors
Dr. Mohamed NoorEarl D. McLean Professor and Chair,
Biology
Hello, and welcome back to Introduction to Genetics in Evolution.
So far we've studied the inheritance of very simple genetic traits.
Traits whose variation is caused by maybe two alleles at a single locus.
And we've looked at how to map that genetic grid.
What I'd like to do now is get into some slightly more complicated path.
Let's look at how traits really are.
Most traits are not, most variation in most traits is not usually caused by
a variation at a single gene with two alleles.
So we'll first talk about mutations as the source of genetic mutation.
And then we'll go into what the genetic basis is of normal traits and
what are some complications rather than the overly simple model we used so far.
So, let's get started.
You go back somewhere between three and a half and
four billion years ago, we had what we believed to be the first origin of life.
And we don't know exactly what that life looked like or
exactly the conditions were, but we can imagine that maybe there was something
along the lines of single haploid genome.
I say haploid, I mean having one copy of the genome, in a single primitive cell.
Now genome there is probably an exaggeration.
It probably wasn't anything nearly as complicated as most of
the genomes you see today.
But it would have been some small piece.
Now the question I'd like to pose to you is what would you, what would have to be
true for you to define this thing as alive, this original cell as alive.
How would you decide it was alive rather than inanimate?
Well the most likely answer people would say is it replicates, it reproduces.
That it makes another copy of itself so this one makes these two other daughters,
and what it makes will be dictated by its gene.
Now let's imagine hypothetically that the genetic code that was found in that
original cell never, ever changed over the three and a half billion years of time.
What would life look like on the planet today?
The answer is if it never changed, and if its genetic code is
dictating what it can make, basically life would look like this today.
There'd be a lot of copies of that simple,
haploid genome inside cells all around the planet.
That's obviously not what we see today.
So there obviously has been introduction of some sort of new
variation into the diversity we see on the planet today.
Now where does this variation come from?
Well this is the theme for today's lecture.
Where does this variation come from?
What is the source of these changes in the genetic code?
Well, the answer is mutations.
Mutations are often defined in textbooks as a changed DNA sequence.
But very importantly, I'd like to emphasize they are the ultimate source
of all genetic variation on the planet.
Because again, if we have a single origin of life, which we do assume, single origin
of life, common ancestry of all existing life, then all the diversity we see has
to come from iterations of mutation from that original self-replicating form, okay.
So, what is a mutation or what is a mutant?
I mean you hear this word often, both in common parlance and in the movies,
things like that.
Well, a mutant may be what you call your brother when he's being very annoying.
Get off me you mutant!
I heard that many times.
Those of you who are fans of science fiction, a mutant in the comic world is
an individual who possesses a genetic trait called an X-gene that allows
them to naturally develop superhuman powers and abilities.
So this is, there's a picture here form the comic X-Men.
You may have seen the movies from that.
Probably not the definition we'll be using here in this class, as you'd guess.
You make think of a mutant as
someone who has an extreme disfiguration such as extra fingers.
So, this individual right here has polydactyly.
So if you count, they actually have six fingers.
This actually something that does come up.
This is a mutation that does crop up periodically.
It's a little harsh to refer to the person as a mutant because they have this.
It is a mutation which arises.
Now honestly, there's a little bit of truth to all those definitions.
Though less so for obviously the X-Men version.
[LAUGH] But again, the ultimate source of all variation is mutation.
It is any sort of change in the genetic code, okay.
So you can think of it this way, that mutations happen and they happen a lot.
They happen far more frequently than you may guess,
and you'll see in a subsequent video, we'll talk about mutation rates.
Now one of the most common types that we'll focus on for this, is an error
replication or a meiosis that leads to a change in a base or a nucleotide.
So imagine you have this stretch of sequence in a mom, okay?
And let's say the mom is homozygous with this.
So she has two copies of this stretch of sequence that starts with ATG.
Now let's say, in one of her eggs that makes the kid, there was this base change,
going from A to G.
So this is actually what's in the kid.
The kid then has a mutation.
He or she has a nucleotide variant that was not found in either of the parents.
So that is by definition a mutation.
Now importantly, let me emphasize what a mutation is not.
So mutations do not happen when you need them, right?
They don't happen preferentially when you need them.
You can imagine something like, oh it'd really be great to have six fingers.
[SOUND] No, no, no.
That doesn't happen at all.
They are random with respect to need, okay?
Now many mutations are bad.
In fact, probably, you could argue that most mutations even are bad.
Or at the very least, a large fraction.
Why is that the case?
If you think about it, our genetic code is not completely, but
largely fairly streamlined.
So if you introduce a random change, it's likely to be something that's bad.
It could be something, but doesn't really matter.
But it's likely to be something that's bad.
The analogy I often like to use is imagine that you have a car, right.
And this car has come together over a very long period of time.
And you take a hammer from way across the room and throw it towards the car.
What are the odds, that hammer will make the car better in some way?
Probably not zero, but it's extremely low.
In contrast, what are the odds the hammer will either do nothing, or
nothing substantive, or it will actually be bad for the car?
Probably pretty high.
So, on average,
we tend to assume that most mutations are either bad, or they don't matter.
I used one example here of one that doesn't matter.
Maybe you're familiar here with the triplet code.
So again, genes tend to code for protein.
They have a triplet code associated with it.
And very often,
that third base in the code does not change which protein's produced.
So, if you have as part of this code, AAA versus AAG,
you would actually put the same amino acid into that protein.
There would actually be no change in the final protein.
So again, most mutations tend to fall into one of these two categories,
either the bad or the don't matter.
Now rarely, but it does happen, a mutation may be good, advantageous.
Now again, it doesn't arise when you need it, but it may just happen to be good.
And it may not necessarily even be good at the time when it first arises.
But it might end up being something that is useful in a subsequent generation.
So thinking about that, let's imagine that a mutation has happened,
and then you vary it down so it exists in the population.
So here's our first generation.
Everybody has the same sequence, okay?
And we have this one mutation like the one we talked about previously which
has arisen in one egg cell.
Now as you'll notice,
we now have the birth of a SNP, a single nucleotide polymorphism.
Because here all the sequences were exactly the same.
But here we have one site which is polymorphic.
It has two different variants there.
Now let me ask you a question.
Is it more important that this transmitted meiotically or mitotically?
Remember we talked about meiosis and mitosis way back an the beginning.
Well the answer is definitely meiotic because let's say for
example, this new mutation starts in this part of your hand,
it spreads to the rest of your hand, it never gonna get passed down to offspring.
It's much more important that it gets transmitted meiotically so
that it gets onto further generations and gets into generation two.
So what happens then?
What happens then is this new variant may be lethal,
it may cause immediate death of the embryo.
That's a possibility and in the previous slide,
I show you one individual was heterozygous for it.
Maybe that individual will tie at the age of.
A couple of months in utero.
It could cause some other sort of loss of fitness.
It could be, you notice it doesn't immediately die, but they have extreme,
gastrointestinal distress that makes it so they're not able to thrive as well.
It's a possibility.
These are definitely in the bad category, right?
Now it could be that this mutation cause no change in fitness.
That this mutation is just the same as
the parent in terms of overall probability of passing on the genes.
So this will be what's referred to as the neutral category.
And finally, maybe it causes super fertility.
Maybe it makes it that this person is extremely attractive, or
that when they do mate with an individual, they're much more likely to fertilize.
This would definitely be a good category.
This would much more be like X-men version category.
So, let's follow what happens after that.
So again, we left off last time with generation two.
Here's this new mutation.
Well It may spread, it may get lost.
It's possible that this individual would leave no offspring.
Let's imagine this, the case when I said this individual died at a very young age.
Or could just die for a random reason.
Maybe this individual gets hit by a car at the age of ten.
It could happen that this just never gets transmitted.
That's one possibility.
But, let's assume it does get transmitted.
Well then some individuals will get it.
This individual gets it here, this individual gets it here, and in this case
we even start getting some homozygotes because this individual gets it and
passes it on and has a kid with this one.
So, again we have an individual here who's homozygous.
We have a few others.
This one seems to be spreading in this particular case.
So it looks like maybe this particular mutation was advantageous or
possibly neutral, but certainly not very bad.
So this is a kind of thing that can happen with new mutations.
We have this new variation.
It arises.
It may just get immediately lost because it's bad.
It may get immediately lost if it's neutral.
It may even get immediately lost if it's good.
Let's say for example, this individual had super fertility.
But again, got hit by a car.
That could happen.
But, for a lot of mutations, they will arise, you have the birth of a new snip.
They'll be there in population for some time.
Now, over time many rounds of this mutation, not just that individual one
which generate a lot of variation in natural population.
Again, as I emphasize,
a lot of mutations don't necessarily have an effect on fertility or life span.
But they may still effect a phenotype.
They may still effect how an organism look, right?
Let's say for example there's a mutation that's slightly lengthened your earlobe.
Maybe that wouldn't effect your probability of having kids whatsoever
passing on your genes.
But, it still could get passed on, and in that sense,
it is still neutral, even though it has an effect.
Now, I'm gonna come back to this example in the next video.
But I just want to introduce it.
There's lots of genetic variation in things like, for example, human height.
Now some of that of course, has to do with nutrition rather than genetics.
But where does that variation come from?
Is that actually all just from mutations at one gene?
Well, take a look at the next video and see what you think.
Thank you for joining.
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