Hi. It's Mr. Andersen and welcome to biology essentials video 33. This is on
genotypes and phenotypes and how changes in genotype can affect changes in the phenotype.
You should know what both of those are. Genotypes are going to be the letters of the DNA. Or
the genes inside you. Phenotype are going to be the physical characteristics that you
have. And so an analogy could be the letters that we put in a book is like the genotype.
But the story would be like phenotype. And so I've written a sentence right here which
probably means nothing to you. Let's turn to the next page of this text. And we'll see
that we now can see the word the, but still doesn't really mean anything to you. But now
let's turn to the third page and we can actually see a story starting to emerge. "The fat cat
ate the rat." And so the letters would be like the genotypes. But the phenotype is going
to be the story. And so when you look at me what you see is phenotype. You see that I
have blues. You see that my nose looks like this. But what you don't see are the letters
behind that. Now if you're really paying attention on those first two pages, you actually could
have seen the message there. So let's turn back for just a second. And so this is that
first one. And it doesn't make sense when you look at it. And that's because the whole
thing has been frame shifted. In other words if I were to put a letter in right here. The
. . . you'll also see the fat cat ate the rat shows up as well. And so this is a mutation.
If we ever have a change in here, so this would be a deletion. So we deleted a letter,
it shifts everything over one. Or if I were to go to the next one. What kind of a mutation
do we have here? Well, now we've got the but now we've added this letter E. And so we've
inserted that. And so if I get rid of that I can still see the fat cat ate the rat start
to emerge as well. And so changes in the letter have huge implications when it comes to the
story. And likewise, in us, changes to the DNA can affect our phenotypes or physically
what we look like. Those are played against our environment. And that's called natural
selection. And so let me summarize what I'm going to talk about. Genotypes and phenotypes
and mostly how changes in the genotype can affect phenotype. Now what are genotypes made
up of? They're simply made up of DNA. And so how do we change the DNA? Or how do we
get errors in the DNA? Well there's essentially two ways that we can do that. The major source
of errors comes from damage to the DNA or mistakes in DNA replication. We call those
things mutations. They're played against the environment. And some of them are actually
beneficial. Some of them are harmful. And some of them are none of the above. They're
neutral. There's other ways that we can get errors in our DNA as well. We can have errors
in the cell cycle. So when we're actually making copies of cells or when we're making
sex cells we can have errors in that. That can eventually lead to new phenotypes which
can be good. I'll talk about bread wheat and can be bad. I'll talk about a mule. And it
can also lead to human disorders. A famous one that probably you're familiar with is
Down Syndrome. And so these are all simply changes in the DNA. But those eventually cause
changes in the phenotypes. And so let me give you an example of that. One of the most famous
ones that we always talk about as biology teachers is sickle cell anemia. And so if
you have sickle cell anemia, this would be a regular red blood cell like that. If you
have sickle cell anemia you have a red blood cell that looks like that or this sickle kind
of appearance. Now it leads to a lot of other problems within your body. But it's simply
a change in one letter. And how could a change in one letter in the DNA cause this huge change
in the blood cell itself? Well let's think about DNA for just a second. Let me actually
clear this off and get a better color. And so DNA has two strands remember. This top
strand would actually be called an nonsense strand or the non-reading strand. And then
this bottom one is going to be called the sense or the reading strand. And so if you
look right here, there's a G on one side of the DNA. That means there's a C on the other
side, or there's a complimentary strand. And so those three letters will code in messenger
RNA. So this would be transcription right here. Code for these three letters over here.
Which codes for this one amino acid. And so when you're making hemoglobin inside blood,
that's how we get this first valine is going to be this first amino acid inside hemoglobin.
And if we trace down, as long as the letters stay the same, we're going to have the same
amino acids. But if you have sickle cell anemia, you have a mistake right here. You have a
mistake in this first amino acid. It ends up changing the chemistry of that amino acid.
Therefore it changes the whole structure of the protein and can lead to this sickle cell.
And the interesting thing is that that's passed on. Because that's what you pass on as your
DNA to your kids, generation after generation. And so that's been passed on and selected
for or against. And that's how changes in the letters of the DNA, any kind of mistake
in this letter, leads eventually to changes in the amino acids. Changes in the phenotypes.
So how do these mutations come to be? Well we make mistakes. And so when we're going
to copy one strand of DNA and make two. So during the cell cycle, when we want a cell
to divide, DNA polymerase will make a really perfect copy of that DNA. But occasionally
it will make mistakes. And so maybe there's a T over on this side instead of making a
T it actually makes a C on this side. And so we can get mistakes just from copying,
or DNA replication in DNA polymerase. We can also get mutations through radiation. Be it
like gamma radiation or UV radiation. That can cause changes in the letters as well.
And we can also get it in chemicals. And so we find if something is a carcinogen, for
example it's a cancer causing agent, good example would be like diesel fumes is a carcinogen,
but how does it make cells cancerous? It's affecting their DNA. And it's making them
so they keep reproducing out of control. And so these are all ways that we can get mutations.
And there are other ways we can get mutations as well. But these mutations are then played
against their environment. And we could group those mutations into three different types.
If you've got a neutral mutation that means we've changed the DNA, but it doesn't effect
the organism. How could that be? Well remember in eukaryotes a lot of our DNA actually doesn't
contain genes. It's areas between the genes. And so neutral mutations are not going to
effect the protein or they're going to effect the protein so little that it really doesn't
effect the phenotype. Now the reason I put a clock down here is that we can actually
use these neutral mutations and we can make what's called a biological or an evolutionary
clock. Because mutations occur at a specific rate. And so we can look at how many mutations
have arisen. We can actually see how long ago it was that the two organisms, for example
shared a common ancestor. We can also have mutations that are then beneficial. In other
words they help an organism. One of my favorite types of beneficial mutations, we really started
to study just fairly recently, and that is mutation in the, so this would be in the helper
T cell, it's in a protein found in helper T cells. And so it's a deletion. So that's
going to be when we're missing a section of that gene. And if you are normal your CCR5.
But if you have this mutation you have delta 32 mutation or a change in that gene. Why
is it beneficial? Well if you have this gene, or better yet two copies of this gene, you
can't be infected by HIV or human immunodeficiency virus. In other words there's something different
in the structure of that helper T cell so this virus can't gain entry. What's interesting
about this is it's most prevalent in Europeans. We don't find it at all in Africans or Asians
for the most part. And so that suggests to us that it was selected. And so when scientists
started looking they found that this offered protection against either bubonic plague or
small pox. And now is providing protection for descendants of the people who first came
up with this mutation. And so it helped them a long time ago. And it helps them today.
So that would be a beneficial. However, most of the mutations are going to be harmful.
Harmful to that individual. And if it's in the germ cells, harmful to generations that
come after that. And I wrote 70% and 7%. And what that means is that if the mutation occurs
within a gene, 70% of the time that change within the gene is going to cause a harmful
mutation. If it's outside the gene, only 7% of the time it is. An example of one would
be PKU. PKU is an inherited disease. It's a recessive disease, so you have to get two
copies of it. But if you have phenylketonuria and they test all babies for this, you can't
break down or you can't do well with a diet that's rich in phenylalanine. And so they
have to accommodate for that. They change your diet. And so it's a harmful mutation.
And this was discovered just in the last century. And so when we weren't testing for this, babies
that were born, their diet actually was killing them. A normal diet. So that would be an example
of a harmful mutation. We can also have mistakes not only in the copying or duplication of
DNA, but in the cell cycle. And so what I've drawn right here is meiosis. Meiosis is how
we make sperm and egg. How do we go from a diploid cell eventually to a haploid cell?
So you'd think of this making sperm or making eggs. We can also have mistakes in here. And
a lot of these mistakes will actually occur during meiosis one. These chromosomes will
fail to separate. And so eventually what you get on this side are, instead of having ones
that are haploid, you'll actually have some that could even be 4n or 3n or 2n on this
side. Usually when we have sperm or egg that have the wrong number of chromosomes that
leads to a zygote that just doesn't go anywhere. Or it's not viable. But sometimes it can lead
to a new species. And so bread wheat, or the wheat that we use to make bread, some of that
wheat is actually tetraploid or hexaploid. They've got extra sets of chromosomes. And
that can actually lead to vigor. In plants, a large number of plants are actually made
through mistakes in the DNA. Mistakes in the cell cycle. It can also lead to sterility.
And so you probably know that a mule is when you have a female donkey and a male horse.
And they're sterile. And the reason why is that a male horse is going to have 64 chromosomes.
And donkey is going to have 62 chromosomes. And so a mule is going to have 63 or an odd
number of chromosomes. It's hard for them to make a viable sperm or egg. I think there
has been one case of a female, a few cases of female mules actually giving birth to foals.
But it's really, really rare. And so that can lead to sterility. It can also, these
changes in meiosis can lead to disorders. A famous example of that is the Down Syndrome.
And so this would be a karyotype or a picture of all the chromosomes in an individual. In
this case it would be a female because there are two X chromosomes. But it you look at
here, with an individual who has Down Syndrome, you'll notice that they actually had three
21st chromosomes. Now what does that lead to? A lot of the characteristics of a person
who has Down Syndrome, development is going to be slow. Sometimes they'll have a problem
with speech. And the reason why is that a lot of those things are linked to proteins
that are on the 21st chromosome. It's not a disorder. If you know somebody who has Down
Syndrome, they live a really normal life and they can be some of the best people you ever
meet. But mistakes in this can eventually lead to genetic problems. On this map right
here I'm showing how natural selection is actually played out. And so at the beginning
remember I was talking about hemoglobin and how sickle cell anemia can lead to awful things.
The neat thing that it can lead to is also the ability to not get malaria. And so this
map on the left side is where malarial mosquitoes are found. On the right side it shows the
incidence of sickle cell anemia. And so we're going to find a higher rate of sickle cell
anemia in especially sub-Saharan Africa. And the reason why is that it created sickle cell
anemia which was bad, but the beneficial mutation has allowed them not to get malaria, which
is a huge selective pressure in Africa. And so that's genotypes. That's phenotypes. And
I hope that's helpful.