- [Instructor] This is the
last lecture for the segment
where we're looking at genetics.
We're looking at inheritance of genetics
passing on from one generation
to the next, and the like.
After this lecture, the
rest of the semester,
the last few weeks that we have,
we'll be primarily focused
on evolution and ecology.
So, this one's gonna take
a little while, though.
As such, too, your quiz is a
lot longer than other quizzes,
because there's some problem solving
that you're gonna have to do in this one.
Namely, working with Punnett
Squares and the like.
So let's review our definitions
on some of these concepts
so that we can understand
how it is that these can,
how we can predict with a
high degree of certainty
the outcome of when you
have two individuals.
Even though we pass on genetics randomly,
we can still make predictions
on the probabilities
of what things are gonna be passed on,
because they follow certain laws.
So there's laws of inheritance
that were discovered
actually 200 years ago
that still hold true today,
even before they understood what DNA was,
and what genes were, and whatnot.
So I'll give you a
little background history
in a little bit of that.
For now, let's just review.
So chromosomes, remember
our eukaryotic organisms
are these discrete segments of DNA
that are wrapped around proteins.
And we have 46 of them, other
organisms have more or less.
Again, it doesn't come
down to how many you have,
it comes down to how much of those
is actually composed of genes,
and how those genes are
transcribed and translated,
and spliced and all that kind
of stuff we've talked about.
So chromosomes have segments within them
that are all the information
necessary for the cell
to be able to make a protein.
So remember that a gene
is that sequence of DNA,
those nucleotides that
encode a polypeptide chain.
Now, remember also that only 2%
of our genome actually encodes genes.
So of the 46 chromosomes, only 2% of it
is actually made of these
segments that encode proteins
that the cell uses to do
everything that it does.
Now, we brought up this term last time,
and this is what's gonna
prevail in this lecture,
as far as understanding
inheritance is alleles.
Now we've talked about mutations,
and how when you get something
like a point mutation
or a frame chip mutation,
that changes the sequence
of the nucleotide
by adding one, removing
one, substituting one.
Ultimately, that change in
the sequence of nucleotides
can change the overall protein,
and its function and its role.
A lot of times it just
kind of messes it up.
But there are other times where it doesn't
and you get some type of new function.
They're few and far
between when that occurs,
but that's occurred over
a long period of time.
We call these alleles.
So if you look at
chromosomes, like these two,
and you look at specific
locations on homologous pairs,
you'll find that you have
the same type of gene
in the exact same locations
on the homologous chromosomes.
And that's really the definition
of homologous chromosomes.
They're chromosomes with
the same size, same shape,
same amount of DNA, and
the same types of genes.
So if you had a gene for hair color here,
you've got a gene for hair color there.
If you've got a gene for eye color there,
you've got a gene for eye color there.
So these are very specific proteins.
But due to the fact that
you've inherited these
from separate parents, and
the nucleic acid sequence
might have mutated
somewhere in their past,
the alleles might be different.
Now they don't always
have to be different.
Sometimes two parents
pass on the same allele,
but that's the nature of alleles.
Alleles is what version
of that gene do you have?
And you're gonna see
several different examples
of that today when we look at traits
that are being inherited,
such as blood type,
and hair color, and things of that sort.
Now, let me forewarn you,
a lot of times people
will ask questions about their parents,
or their friend's parents
that might reveal things
that you might not want revealed.
So make sure you (sighs)
know what you're asking,
and so that you know what
the answer's gonna be.
'Cause they've had people ask,
"Well both of my parents are blood type O,
and I'm blood type A,"
and then it gets awkward.
(students chuckle)
Because,
there are circumstances.
So there are, that's a possibility
that you're the biological
offspring and we'll show why,
but just be aware of that.
All right, now.
Alleles, this is what we're primarily
gonna talk about in inheritance.
Not all alleles, now since we
have homologous chromosomes,
which means we have two every
gene, we have two alleles.
Now the alleles might be the same,
the alleles might be different,
it all depends upon what your mother
and father passed on to you.
Now the effect that these
alleles have on one another,
comes down to certain
terminology that we use
such as dominant and recessive.
So every person has two of every gene.
You can see that here from
the homologous chromosomes
is you get one allele from your mother
and one allele from your father.
If the alleles exert a dominate influence
then we call it a dominant allele.
So what is a dominant allele?
Well, a dominant allele is one where
it is expressed no matter what.
So it's expressed, let's
say like brown hair
is a dominant trait, because
it's a dominant allele.
Whereas, things like let's
say eye color, blue eyes,
is a recessive trait, brown
eyes is a dominant trait.
So the alleles that make
your eyes brown are dominant.
So if you have a one
allele for brown eyes,
now I'm simplifying eye colors,
not down to just one gene,
but I'm simplifying it
for the sake of examples.
If you have one allele
which is for brown eyes,
and one allele for blue eyes,
your eyes are going to take on
the dominant allele's expression,
in which case you would have brown eyes.
Which is why we say brown
eyes is a dominant trait
and blue eyes is a recessive trait.
And we know that there
are many different types
of eye color, and I can
talk a little bit later
more about how that's done,
but for now we'll keep it simple.
Now recessive, recessive
traits can only be manifest
if the person has two recessive alleles.
Because if even one of
them is a dominant allele
then it supersedes the recessive allele.
So the recessive allele always gets masked
in the presence of a dominant allele.
Now, here's one thing
you have to understand.
Recessive doesn't mean least frequent.
In fact, some recessive
traits like blood type O
and blue eyes are actually more common
than the dominant traits.
So in Europe, blue eyes
is the most common,
even though it's a recessive trait.
Here in the US, blood type O
is the most common blood type
even though it's a recessive trait.
So recessive doesn't mean infrequent.
It just means that if the
only way to get that trait
is if both of your alleles are recessive.
Now that brings that up
to a way of describing
the make up of our alleles.
Since we have two of
each, we have terminology
which ultimately defines the difference
between what we call a
genotype and phenotype.
If you think of genotype,
geno as the genes,
the genotype is essentially
asking the question,
what are your two alleles?
Which ones did you get from your parents?
If your alleles are the same,
you got the same allele from each parent,
we call that homozygous.
However, since there are two
different types of alleles,
dominant types and recessive types,
you have to quantify what
type of homozygous it is.
So if it's homozygous dominant,
that means that you have
two dominant alleles,
and that's homozygous dominant.
If you're homozygous recessive,
it means your alleles are recessive,
and you have two recessive alleles.
So you have to say
whether you're homozygous
dominant or recessive.
You can't just say, "Oh, I'm homozygous."
Because that can mean
two different things.
Now heterozygous that
only means one thing,
it means the alleles are different.
You might think, well why didn't you say,
one allele is dominant, one's recessive?
You'll see later on it's not as simple
as everything being dominant or recessive.
So it's easier just to define now
as saying heterozygous means
the alleles are different.
And that's it, you could just say,
"Oh, I'm heterozygous."
That means that your
alleles are different.
Now, the phenotype, this
is what's being expressed.
Now even though it says
outward expression,
that doesn't mean all of your
traits are on the outside.
For example, your blood type,
that is not an outside look, you know,
that's what's going on inside your body.
But your blood type is a phenotype.
And we'll talk about blood type today.
So phenotype just means,
what is the manifestation
of the combination of your two alleles.
If you're homozygous dominant
then you'll typically
have a dominant phenotype.
If you're homozygous recessive
then you'll have a recessive phenotype.
And if you're a heterozygous,
well if one of them is
dominant, one's recessive,
then you'll have a dominant phenotype.
So the phenotype is
essentially when you put
these two alleles together,
what's the outcome?
All right, so here's an example.
This works for plants, this
works for people, and whatnot.
We're gonna talk a little
bit about both today,
because a lot of the
experiments that were done
to figure out how inheritance works
were done on plants first.
And then we apply that to people, as well.
Now let's say that there's
a gene that ultimately
can make you, determines your height.
Now one version of the
gene makes you tall,
and the other version of
the gene makes you short.
Well the tall version
is a dominant allele.
But notice the genotype
for these two individuals
who have the tall phenotype are different.
This person is homozygous dominant,
this person is heterozygous.
But here's the thing, you
wouldn't be able to tell
by looking at them,
which person was which.
So having two dominant alleles
versus one dominant allele
doesn't make the person doubly tall.
They both have the same phenotype.
So make sure you understand that
that two dominant alleles
versus one dominant allele,
the phenotype will be the same.
There will be no difference
between two individuals.
And the same thing is true when we look
at things like blood type.
You could see two people
with blood type A,
but you wouldn't know whether
one person's homozygous,
or another person's
heterozygous, you couldn't tell,
because their blood type
is exactly the same.
Over here, the short allele only shows up,
or the short phenotype only shows up
when both of the alleles are recessive.
Because if you have a recessive allele,
but the other one's dominant,
you're gonna have the dominant phenotype.
So make sure you know this terminology,
it's gonna come up in
most of your scenarios
where we talk about the person's genotype
or their phenotype.
So again, genotype is
what are the alleles?
Phenotype is what combination
do those alleles cause to happen?
What's your blood type? Are you tall?
Do you have brown hair, blue eyes?
Whatever the case may
be, that's the phenotype.
Now, a little background history
before we get into the laws.
These were discovered centuries ago
by a monk who had a lot of
time on his hand (chuckles)
gardening and whatnot.
And he would breed plants
over, and over and over,
and actually take statistical data.
He was the one that actually
coined the phrase alleles.
This is before they
understood what DNA was,
that there even was something like DNA,
or even something like genes.
So that's why he called them alleles.
Now we still call them alleles today,
but we understand them
in modern vernacular
than he understood it back in the day.
But the monk was Gregor Mendel,
he's really kind of the
father of inheritance,
and genetics and whatnot,
because of all these
experiments that he did.
In fact, a lot of this
information was lost for awhile.
Someone else did these
experiments later on,
discovered it, but then
they found his experiments,
they're like, "Oh well, he gets the kudos,
because he did it so long ago."
Now a lot of his of experiments were done
with the pea plants
that he cross pollinated
and bred with one another.
Now before I show you a couple
of the experiments that he did,
let's talk about the laws
that he came up with, there are two.
Now remember in our progression
of the scientific method
these laws ultimately give
us such a good understanding
of that concept, that
we can make predictions.
And that's really what
inheritance is all about.
How is it when you go into the doctor,
and he sees your genetics and he says,
"Ah, you have a wonderful chance of having
a child with cystic fibrosis,"
how can he make that prediction?
Well it's based upon
these laws of inheritance
that are universal for any organism
that undergoes sexual reproduction.
So let's talk the first
law, the Law of Segregation.
This is where we're gonna spend most
of our time this lecture
in terms of application.
The second law is important,
but we're not really gonna deal
too much with it in terms of
making predictions and whatnot.
So Law of Segregation.
You've actually learned
this from the last chapter
when we talked about myosis
and how alleles ultimately
get separated from one other
during the myotic process
where homologous chromosomes
are separated during anaphase one.
So that's really all the
Law of Segregation is,
is that we have two alleles.
But every time we undergo
sexual reproduction
we only pass on one of
those two, every time.
So you may start off,
you may be heterozygous,
and every time you have an offspring
it's essentially a 50/50
of which one you pass on.
You can either pass on that one,
or you can either pass on that one,
but you don't pass on both.
Okay, so that's the Law of Segregation.
Why is this applied to myosis?
Because each of these alleles are on
a different homologous chromosome.
And during anaphase one in myosis
when homologous chromosomes separate,
the respective alleles also separate.
And you don't inherit
two from the same parent.
So that's the Law of Segregation
that ultimately each parent
only passes on one allele.
So that's manifested here.
Now obviously if a pair is
homozygous the law still applies,
they only pass on one of
their dominant alleles,
but as far as statistics go
it won't make any difference,
they're gonna pass on the
dominant allele either way.
And the same thing's true if
someone's homozygous recessive.
They'll pass on the recessive
allele no matter what.
So statistically that's
all it could pass on.
It's when you have a heterozygous parent
that it really comes
down to a 50/50 chance
half of the gametes are gonna
have the dominant allele,
half of the gametes are gonna
have the recessive allele.
When you combine these two together,
then you have certain outcomes that result
from the fertilization process.
That is the Law of Segregation.
Now, there's a way in which we apply this,
and this is what about seven
out of your 10 questions
are gonna be like, is the Punnett Square.
The Punnett Square, let me
make sure you right this down
'cause it's key, the
Punnett Square is the same
as the Law of Segregation.
The Punnett Square is the application
of the Law of Segregation.
So what you're seeing here is the genotype
of the two parents.
Well this is how we
calculate the statistics
is we split up the two
alleles from each parent,
and then we combine them in this square,
called Punnett Square, and that gives us
the probabilities of the
outcomes that result from that.
So, here's what Gregor Mendel did.
He took some pea plants and he was looking
at various phenotypes, like
what color were the pea pods,
or the peas in the pod,
or how tall were the peas,
or were they round versus
wrinkled, or whatnot.
And there were very
quantifiable female types.
Well, he found that some plants were
what he called true breeding
plants for certain traits,
meaning every time he crossed two plants
that had yellow peas, they
always had yellow peas.
And every time he crossed
certain green pea plants
that had green pea pods
they always were green.
Well the reason for that is because each
of those parent plants were
either homozygous dominant
or homozygous recessive.
For example, if you
cross two yellow plants,
or two pea plants that have yellow seeds,
there's no other outcome but yellow seeds.
That's how he initially saw
that these were true breeding.
Same thing true for the lower case y,
which is why we denote recessive.
Is that when you cross green
plants with one another,
or plants that produce green pea pods,
the only outcome was green peas.
But what happened when he
crossed that true breeding yellow
with the true breeding green?
Well, the end result was he
still only got yellow seeds,
however, there was something
different about them.
Whoops, I want this to be a
little y, (marker thudding)
and this to be big Y.
When he crossed these plants
that only produced yellow
peas versus green peas,
they were all still
yellow, now why is that?
Why do you think from
what you can see here?
- [Student] Yellow was the dominant trait.
- [Instructor] Good, yellow
was the dominant allele,
and no matter what
you're always gonna have
a dominant allele with
the recessive allele.
This is the second generation,
this is what he discovered.
When he took the offspring of those plants
and bred them together, all of a sudden
the green peas started showing up again.
And this is the reason why.
He called them alleles and he knew that
they were being passed
on from the parents.
Again now we understand them as variations
of the same type of gene, but here's why.
So here we have a heterozygous parent,
here's the other heterozygous alleles,
and when you put them together
in the Pundett Square,
you have a 25% chance of
being homozygous dominant,
50% chance of being heterozygous,
and 25% chance of homozygous recessive.
And that's what he saw in his outcomes,
is every time he would
cross pollinate these
and he would count the pea pods,
and it was a lot of peas, it
was thousands upon thousands,
he got this three to one ratio.
He got 75% of the seeds being yellow,
and 25% of the seeds being green.
Now notice when he counted them he
was counting the phenotype.
Notice too, that the
genotypes here are different
between these, yet their
phenotype is exactly the same.
And that's what we talked
about with the dominant allele.
It doesn't matter whether
it's two dominant alleles,
or one dominant allele, the
phenotype is exactly the same.
So here's how you do a Punnett Square.
When you have an offspring
from two parents,
you essentially put the alleles
on the two sides of the square.
Now it doesn't matter male alleles here,
male alleles there, female
alleles, it doesn't matter.
But just choose a side.
So in this situation here we
have the male probabilities,
because he's heterozygous, he's gonna have
a big Y and a little y.
And because the female's
also heterozygous,
she's gonna have a big Y and a little y.
So here's how you put them together,
you just take this big Y
and put it down this column,
this little y and put it down that column.
And this little y, put
it down that column.
This big Y will go down
this column (marker thuds)
and this little y will
go down this column.
Now for the sake of simplicity,
we usually put the dominant allele first,
but you get the same outcome.
It doesn't matter whether
you write it like this,
or whether you write it like that,
it's the same genotype.
Okay, so don't think that these
two are different genotypes.
Which is why you'll typically
see me do something like
this, even when I'm putting these across,
you'll see me put it off to the side,
because I know a dominant
allele's gonna come in there.
That just makes it easier to read.
Here's where we get probabilities.
One in four chance of
being homozygous dominant,
which is equal to 25%,
this is where your math comes
into play, really simple.
Two out of four chance, which is 50%.
Now on the test I'm not
gonna do one out of four,
or two out of four, I'm
gonna do percentages,
so it would be 25%, 50%, 25% and whatnot.
So this is what Gregor Mendel did
with these true breeding plants,
and then when he crossed them again
that was the previous slide,
here's another example
of a heterozygous parent
with a homozygous recessive parent,
and he ended up getting 50/50.
He ended up getting half
of the seed being green,
and half of the seeds being yellow.
So those are some different things.
We're gonna do a lot of Punnett Squares
as we go through here.
Now, we're not gonna do any Punnet Squares
with the Law of Independent Assortment,
because in reality the Law
of Independent Assortment
doesn't really make predictions.
But it does talk about one
of the natures of inheritance
that must be considered, okay?
So you've seen that word
before, independent assortment,
where have you seen it?
Or I told you about it before.
You should know it for this week's quiz.
(student speaks faintly off microphone)
You're getting close, but not quite.
(marker thudding)
What happens during metaphase one?
(marker thudding)
What line up along the middle?
- [Student] Chromosomes.
- [Instructor] Homologous
chromosome line up on the middle,
remember we talked about this
as a recombination event,
independent assortment?
It's one of your three
recombination events,
crossing over, independent
assortment and fertilization.
So why is this a law?
What is this law about?
Well, the law essentially
states the following thing.
The inheritance of one
gene does not influence
the inheritance of another gene.
If I look at you and say,
"Oh you got brown hair,
so you're most likely blood type A."
No, they have nothing
to do with each other.
Because due to the fact
that chromosomes can line up
however they want along this middle,
that's the Law of Independent Assortment.
That genes do not
influence the inheritance
of other genes when
they're being passed on.
So I can't look at you and
know what you're blood type
is based upon your hair color,
or based upon your height or
anything else of that matter.
There's no correlation between the two.
Now we can show this by
doing a really difficult kind
of square, but you're not gonna
be doing things like this.
I found that it's just
simpler to just show you
the complexity of it and then pass it by.
But I do want to illustrate
this process really quickly.
Remember this happens
during metaphase one.
So here's an example of the
peas that Gregor Mendel did.
And this is what he found when he
was doing these experiments,
is he was looking at
lots of different traits.
And when he looked at the
seeds he found that some
are yellow, some are green.
He also found that
there was another trait,
the seeds were either round,
or they were wrinkled.
So the yellow, remember
the yellow or green,
we dominant with the Y gene,
the yellow being the dominant,
the green being the recessive.
Over here we've got the wrinkled or round.
Now he found that round was a dominant,
so the big R symbolized the dominant,
and wrinkled was recessive,
so the little r represents a recessive.
Well here's one scenario.
If the homologous pair line up like this
and then the homologous
chromosomes separate
from one another, then
the outcome is this.
The seeds that inherit these alleles
would have the yellow round,
and the seeds that inherit these alleles
will have the green wrinkle.
But that's not the only
way that it can recombine,
here's the alternative where these flip.
Now the seeds that inherit these alleles
will be round and green,
and the ones that inherit
these alleles will be wrinkled and yellow.
So you can see that the color of the seeds
doesn't influence whether
or not they're gonna
be round or wrinkled.
The two genes are sorted
independently of each other.
So that's the Law of
Independent Assortment.
And so he showed that by doing
these more complex scenarios,
but you would be able to
find if you analyze this
that there was no difference
in the inheritance of one or the other.
Now there is an exception to
the Law of Independent Assortment.
And that is if you're
looking at two traits
that are influenced by two genes,
and the genes are on the same chromosome.
That's the only exception
to this law, why?
Because it doesn't matter
then where they flip
in relation to each other.
Because the genes are
on the same chromosome,
they'll flip together,
because they're on the same chromosome.
So the inheritance of genes that are on
the same chromosome is the
exception to this rule.
Now we're not gonna do any
exceptions to these rules,
but there is a way of
recombining these alleles still.
Do you know how?
- [Student] Crossing over.
- [Instructor] Crossing
over, you could still
get crossing over, and that
will still mix and match.
Now it's not as probable,
but it does happen.
And so you do get recombination.
And in fact, they've
shown where the genes are
on the chromosomes by the percentage
and probability of
crossing over between them.
And they find that the more
they're inherited together,
the closer they are on the chromosome.
So this was before we
were able to sequence
all the DNA and know
exactly where they're at.
Now we know exactly where they're at.
So, (sighs) most of
the time when people do
the Law of Segregation where
they're making predictions
about the outcome of their
child, they don't care,
they're not worried about what
hair color the child will be.
They're not worried about
what their eye color will be,
or typically not what their blood type is.
They're more worried about
if the parents genetics
recombine in such a way
where they'll inherit
a genetic problem, a genetic disorder,
like Huntington's disease
and sickle cell anemia
and cystic fibrosis, and Tay-Sachs,
these are the concerns.
So when we look at the family pedigrees
this is there genetic
counseling comes into play.
Because if you know you
have Huntington's disease
and you go and get tested and they say,
"Oh yeah, you're heterozygous,"
Huntington's disease is a dominant allele.
Which means that if you're heterozygous,
you have a 50/50 chance of
passing that on to your child.
And this is where genetic
counseling comes into play.
And this is true for any
type of genetic disorder
where people are informed of
the statistical probability
of having a child.
They may look at both
parents' genetics and say,
"You have a 75% chance of
this child having this.
And then allow them to make that decision
on whether or not they
want to have any children.
So let's look at some genetic disorders.
Most genetic disorders
are not this simple.
But we're going to use the simple ones.
So there are a lot of ones that
are caused primarily by a single gene.
But not all genetic disorders
are caused by a single gene.
For example, you may say,
"Oh, what are the chances
that I'm gonna have a heart attack?"
well there are many things
that lead to a heart attack.
It's much easier to say,
"What are the chances
that I will have something
like sickle cell anemia?"
That's easier, because
it's down to one gene.
What are the chances I'll
have Huntington's disease?
That's easy, 'cause it's down to one gene.
So most of the genetic
disorders that we use this for
are very quantifiable, very predictable,
because they're single gene mutations.
They're very studied,
they're very well studied.
But there are many others
that we can make predictions,
but they're more complex for this class
than we need to do.
Now, just like there are
traits which are dominant
and traits which are recessive,
diseases also follow the same rules.
Some diseases are dominant,
and other diseases are recessive.
Now we're also gonna show
that there's a difference
on which genes are inherited.
Some mutations, because
most of our chromosomes
are non-sex chromosomes,
these are the ones
that we look at quite often,
are what we call autosomal disorders.
Remember autosomal chromosomes
are the non-sex chromosomes,
but we're gonna look at both.
We're gonna look at
disorders that are found
on the autosomal chromosomes,
and disorders found on
the sex chromosomes.
When we say disorder
on the sex chromosomes,
we usually call it sex linked disorders.
So autosomal disorders doesn't matter what
the sex of the individual is.
Sex linked disorders does,
it does matter whether you're
a male or female, because it
influences us differently,
depending upon what we
inherit, and you'll see why.
Now, you're not gonna have
to memorize any of these,
nor explain what the outcome is,
but here are several quantifiable examples
of genetic disorders that we look at.
Let me just go through a couple of them.
Achondroplasia, this is actually
a dominant disorder that causes dwarfism.
In fact, 95% of the
time dwarfism is caused
by this mutation, by this problem
right here Achondroplasia.
There's Huntington's disease,
that's also a dominant disorder.
Marfan syndrome, this
one actually has many,
many, many different symptoms,
most of them being abnormalities
in the skeletal system.
But there can be other
abnormalities, as well,
depending upon some of the dynamics.
Polydactyly, extra digits in
your fingers or your toes.
Now, these are dominant disorders.
So let's look at a
scenario that you may have.
Look at Huntington's disease,
let's see why this is so problematic.
Now one of the reasons
why Huntington's disease
has not been weeded out
of the human genome,
is because it doesn't show up
until after reproductive success.
So it's not something that prevents you
from having children,
it's something that basically
affects you later on
in your life after you've
passed the genes on.
And that's why a lot of times people,
if it runs in the family, will get tested
so that they know whether or
not they want to have children
and have that probability
of passing it on.
In this scenario, the normal
phenotype is recessive.
To have a normal brain function
and not any problems with this,
then it's recessive to the
actual problematic allele
which causes Huntington's disease.
So Huntington's disease can be caused
by either a homozygous
dominant, or a heterozygous.
Most of the time it's
heterozygous, most of the time.
But that doesn't preclude the possibility
that somebody's homozygous
for this dominant allele.
Again, it doesn't give
someone a higher probability
of having Huntington's disease
by having two dominant alleles
versus one dominant allele.
They'll have Huntington's
disease one way or the other.
You don't get it sooner, you
don't get it more severe.
It doesn't affect whether
or not, just you have it.
You're gonna see throughout a
lot of these questions, too,
when I say someone has a normal phenotype,
'cause I've been called
out on this before,
they're like, "What's really normal?"
Normal means you don't
have the disease, okay?
So if I say normal phenotype,
it means they don't
have Huntington's disease,
they don't have polydactyly,
they don't have cystic fibrosis,
they don't have sickle cell anemia, okay.
So just we'll make that clear.
Now, let's do a scenario.
Let's say that, let's start very simple.
Let's say neither parent
has Huntington's disease
what is the probability
that they'll pass it on?
(marker thudding)
If neither parent has Huntington's disease
what's the probability that
their children will have it.
- [Students] Zero.
- [Instructor] Zero, okay?
So you can see that
people that don't have it,
don't have a history of it, or whatnot,
it doesn't just all of a sudden pop up,
okay, it doesn't show up.
All right, now, let's say one parent
has Huntington's disease
and they are heterozygous.
That's a qualifier that
I have to give, why?
What is if I say the
parent has the phenotype
Huntington's disease, but I
don't tell you anything more?
Why is that a problem?
Why do I have to tell you
what their genotype is?
'Cause it's a dominant
disorder, what's the scenario?
(marker thudding)
Either one of these genotypes
causes Huntington's disease.
So if I say, oh, a parent
has Huntington's disease
and leave it at, you don't
know which one it is.
So that's the scenario
with dominant disorders,
you have to say, "Oh, and
they're homozygous dominant,
oh, and they're heterozygous."
So you'll see that in
some of these questions
where I'll qualify, "Oh, their
genotype is heterozygous."
Okay, so let's do that.
One parent has Huntington's
disease and is heterozygous,
the other parent does not
have Huntington's disease,
they have the normal phenotype.
So the parent with Huntington's
disease is heterozygous,
what are their alleles?
(students murmurs answers)
Big H, they're heterozygous,
good, little h,
remember the alleles are different.
The parent who's normal, doesn't
have Huntington's disease
what are their alleles?
(students murmur answer)
Good, little h, little h, because if even
they have one dominant allele,
they're going to have
Huntington's disease.
Okay, big H down this column,
little h, down this column,
little h across that column,
little h across that column.
What are the chances that they'll have
a child with Huntington's disease?
- [Students] 50%.
- [Instructor] Good, okay.
Now, let's say both parents
have Huntington's disease
and both parents are heterozygous.
What are the chances
that they'll have a child
that's normal, no Huntington's disease.
So let's look at their scenarios.
Both parents have Huntington's disease
and both parents are heterozygous,
what are their alleles.
- [Students] Big H, little h.
- [Instructor] Big H, little h, and?
- [Students] Big H, little h.
- [Instructor] Big H, little h, okay?
(marker thudding)
Big H across there,
little h across there,
big H down that column,
little h down that column.
What are the chances they'll have a child
with a normal phenotype?
- [Students] 100%.
- [Instructor] These are the statistics
that parents want to know.
These are the things that genetic CALRs
essentially show them.
You have a 25% chance of the child
not having Huntington's disease.
All right, so those
are dominant disorders.
Whether it's Huntington's,
whether it's Achondroplasia,
whether it's polydactyly,
the principle is the same.
One dominant allele causes
the dominant disorder.
Now that's not the situation
for autosomal recessive disorders.
Like recessive traits you
need two recessive alleles
to have that disorder.
So these are ones that not are as common,
because you'll have to
have two recessive alleles.
Now when I say not as common, like I said,
people can carry this,
but a lot of times some
of these diseases are fatal.
For example, if a child
has Tay-Sachs disease,
they die about the age of three.
And in the past, individuals
with cystic fibrosis
died fairly early.
Most of the modern medicine
though is allowing people
to live fairly normal
lives with cystic fibrosis.
But there's still problems with diseases
like sickle cell anemia, which
is also a recessive disorder.
So that's why they're not as frequent,
is because when you get
some of these diseases,
they actually kill you before
you are able to reproduce,
and therefore don't
have a high probability
of being passed on.
But they're still
maintained in populations.
And we'll explain why in the
next lecture in evolution.
Because there is an advantage sometimes
of carrying these diseases,
believe it or not.
So autosomal recessive.
Now, in this scenario there's a new word
you're gonna have to
learn, it's called carrier.
Carrier is synonymous with heterozygous.
(marker thudding)
Because a carrier is
an individual that has
a dominant allele, oh let's not use H,
let's use T for Tay-Sachs disease.
A carrier is an individual
that has an dominant allele,
and because this disease is recessive,
that makes them have a normal phenotype.
But they carry the recessive allele
that would cause the disease.
So their phenotype is normal,
but they do carry that
recessive allele with them
that they could pass on, that's
why they're called carriers.
So carriers, this only applies
to recessive disorders,
not to dominant disorders.
So if you're looking
at a dominant disorder
like Huntington's
disease, we wouldn't call
this individual a carrier,
because they will have
Huntington's disease.
They do not have a normal phenotype.
So that's why it doesn't
apply to dominant disorders,
it only applies to recessive disorders.
So carrier means heterozygous.
Now, in order to have the disease you must
have both recessive alleles.
So let's do cystic fibrosis.
Let's say two parents
have a normal phenotype,
they don't have cystic fibrosis.
But they are both carriers
for cystic fibrosis.
What are the chances that they'll have
a child with cystic fibrosis?
So let's look at their alleles.
What are the alleles of each parent?
They're both carriers, which means what?
(students murmur answer)
Heterozygous, and they
have a normal phenotype,
so what are their two alleles?
- [Students] Cs.
- [Instructor] Big C, little
c, 'cause this is normal,
this is normal, that's cystic fibrosis.
You have to have the two recessive alleles
to have that phenotype of cystic fibrosis.
That's the difference
between recessive disorders
and dominant disorders, okay?
So what's the other pair?
- [Students] Big C, little c.
- [Instructor] Big C, little
c, because they're a carrier.
All right, so big C here, little c here,
big C here, little c there.
What are the chances that they'll have
a child with cystic fibrosis.
- [Students] 25%.
- [Instructor] 25%, okay?
That's where that
statistic comes into play.
All right, let's do another one.
Let's say one parent has cystic fibrosis,
and the other parent is a
carrier of cystic fibrosis.
Okay, so the parent that
has cystic fibrosis,
what's their genotype?
- [Students] Little c, little c,
and what about the parent who's a carrier?
- [Students] Big C, little c.
- [Instructor] What are the
chances that now they'll
have a child with cystic fibrosis?
- [Student's] 50%.
- [Instructor] 50%, okay?
What if one parent has cystic fibrosis
and the other parent doesn't
and that parent is not a carrier?
What are the chances they'll have
a child with cystic fibrosis?
(marker thudding)
Zero, 0% chance that they'll have
a child with cystic fibrosis.
So if one parent has cystic fibrosis,
the other parent doesn't
and is not a carrier,
meaning they're homozygous dominant,
'cause that's what carrier means,
then there's no chance for them
to have children with cystic fibrosis.
But it's 100% chance
that all their children
will have the allele, will
have the recessive allele.
Sex-linked is different,
because now we're dealing
with non-homologous
chromosomes for the males.
Men, you get the short end of the stick
when it comes to diseases
on the sex chromosomes.
Because instead of having two
alleles, we only have one.
Now when we look at sex-linked inheritance
we don't look at the Y chromosome, why?
Number one, there's
only 100 genes on here,
and mutations in this gene
typically cause sterility.
So they don't get passed on
from one generation to the next.
So we really don't look at the inheritance
of mutations on the Y chromosome,
because any mutations
here cause major problems.
But the X chromosome has
over 1,000 genes on it,
and some if them have nothing to do
with the sex development
of the male and female,
so they don't cause sterility.
One or two examples, and these are the two
that I'll be using for
your quiz and whatnot,
color blindness and hemophilia.
These are sex-linked.
This is why men are more prone
to color blindness than females are.
So let me explain how this works.
Now color blindness and hemophilia
are recessive disorders.
So we're only gonna deal with recessive
sex-linked inheritance.
Yes, there are dominant alleles,
but for my class I'm just gonna deal
with the recessive for the sake of it.
Why do men get more affected
by recessive alleles than women do?
Well here's the thing,
women have two alleles
for every gene on the X chromosome,
because their chromosomes are homologous.
They have homologous chromosomes.
Men on the other hand,
we only have one allele
for all of our X chromosome genes.
Which means that we either
inherit the dominant allele,
or the recessive allele, but not both,
we don't have two alleles, we've got one.
Women on the other hand have two alleles.
So it follows the same rules
as the other recessive disorders.
Color blindness being
a recessive disorder,
women have to actually
inherit two recessive alleles
to be colorblind.
Men, you get that one recessive allele,
there's no other allele
on the Y chromosome
to mask that you're color blind.
So that's the nature of
sex-linked inheritance.
Hemophilia's the same thing,
men are more prone to it,
because it's on the X chromosome.
One recessive allele and you've got it.
So let's look at a scenario.
Now, people tend to get confused
on the sex-linked inheritance
for one main reason,
they forget men only have one allele.
So here's a trick to make sure
that you don't make that mistake.
Draw the sex chromosomes first.
When you do that, then
you remember, oh yeah,
there's no allele on the Y chromosome.
So when doing the Punnett
Square you'll still look
at the probabilities, but you'll see that
it shows the outcome for the males,
and it shows the outcome for the female.
'Cause now it matters what
the sex of the individual is
and the statistics that arise from that.
So what I typically do is
if a woman is color blind,
then I put the little
c right next to the X.
If she's not color blind,
but she's a carrier,
then it'll look like this.
And if she's not color blind
and she's not a carrier
then it'll look like that.
Men, we have an X and a Y,
so we either have the dominant allele,
we're not color blind,
or we have the recessive
allele and we are color blind.
So let's look at the first scenario.
Let's say a mother is color blind,
and the father is not color blind.
Now here's the thing that
also people get tripped up on.
All I have to say is the
father's not color blind,
and a lot of times people will be like,
"Well, I don't know if
he's homozygous dominant
or heterozygous," he's neither.
Because that doesn't apply
anymore to the sex-linked.
We either have the dominant
allele or the recessive allele.
So if men are not color
blind what's their allele?
- [Student] Big C.
- [Instructor] It's the
big C, the dominant.
If women are color blind,
since this is a recessive
disorder, what are her alleles?
(students murmur answer)
Little c, little c, all
right, let's put it together.
(marker thudding)
You might want to make
your Punnett Squares
a little bigger, too.
(marker thudding)
What are the probabilities that
the daughters will be color blind?
- [Student] Zero.
- [Instructor] Zero, because
though they are all carriers
they can't be color blind,
because they need two recessive alleles.
What are the chances his
sons will be color blind?
100%, 100% chance that the
son's will be color blind.
So if you mother's color blind,
then the males are gonna be color blind,
that's just the nature of it.
'Cause it doesn't matter which
X chromosome she passes on,
you're going to get the recessive allele
that causes color blindness,
and there is no other allele to mask that,
there's no dominant allele, or
any other allele to do that.
That's why males are more susceptible
to a lot of these recessive
sex-linked disorders
than females are.
Do you have a question?
- [Student] Sorry, my mom is color blind,
and my brother got it, as well.
So would that make my grandpa color blind?
- [Instructor] In order for
a woman to be color blind,
the father has to be color blind, yes.
And the mother of that,
your mother, so to speak,
would at least have to be a
carrier for that to occur.
So both parents don't
have to be color blind,
but the father has to be color blind,
and the mother at least
has to be a carrier,
and then that combines
and causes color blindness
in women, so yeah.
So once a woman is color blind,
all of her sons will be color blind,
that's just the nature of it.
All right, now those are three out
of your seven Pundett Square questions
that you're gonna have.
So there's still quite a bit to go through
as we go through here.
We're not gonna do any pedigree charts,
but this just illustrates some
of the dynamics of inheritance.
For example, we were
looking at achondroplasia
which is a dominant disorder.
Individuals which have achondroplasia
it doesn't skip generations.
If it doesn't get passed
on then it's not going
to continue to be passed from
one generation to the next.
But if it gets passed
on, because it works in
a dominant fashion, then it
can continue to be passed on.
Recessive traits on the other hand,
recessive disorders like
albinism in this case,
can actually pop up
where you're like, wait,
this generation and this
generation didn't have it.
Now the have means carrier,
but they don't actually
express the recessive disorder,
in this case albinism.
So here is where you
can see how individuals
which are both carriers,
they can have offspring where
because of the recombination,
it's a one in four chance,
but that's still pretty likely
that they inherit the
two recessive alleles,
and therefore, have albinism.
So this is where you could
have a couple generations
where you don't see the disease,
and then all of a sudden it pops up,
because it's a recessive disorder.
Here is a example of a sex-linked,
the color blindness as I mentioned,
where neither parent are color blind,
but here the woman is a carrier.
So when she has a son she
really has a 50/50 chance
of whether or not she
passes on the X chromosome
with the normal allele,
or the X chromosome with the
recessive color blind allele.
And in this case the son's normal,
but in this case the son
inherited the recessive allele,
and therefore is color blind.
Over here they had a
daughter and she happened
to inherit the recessive
allele from her mother,
and then she got the normal
allele from her father.
But then when she ended
up having two sons,
both of them inherited the X chromosome
with that recessive allele,
and therefore they're color blind.
Whereas here, if you have
a father who's color blind,
it's guaranteed that the daughters
are at least going to be carriers.
If the mother is not a carrier
and she's not color blind,
they're at least going to inherit that,
because if they're a
girl, the then father had
to have passed on the X chromosome.
All right now, let's
look at scenarios beyond
just the dominant recessive
that we've been talking about.
Because in reality, a lot
of things don't fall under
that category where the alleles
are either dominant or recessive.
A lot do, a lot of the alleles
that create your traits
are either dominant or recessive.
But there are other
scenarios where they're not.
So this is what we call
beyond Mendel's Laws,
because they look for
patterns of inheritance
other than the dominant
recessive relationship
that Mendel was looking at.
So here's an example.
I'll give you an example of flowers,
and I'll also give you an example of
the human species of this scenario.
And either one of those are fair game
as far as a Punnett Square
question for your quiz.
So it's either gonna be on the flowers,
or it's gonna be on the
human scenario I'll give you.
So sometimes alleles aren't
one dominant, one recessive,
in some scenarios they're both dominant.
But when you have an
individual who is heterozygous,
meaning they have one of
each of the dominant alleles,
you end up getting a
mixture of the phenotype.
And this is what we call
incomplete dominance.
So here the alleles
either encode a pigment
in which makes the flowers red,
or there's a different
allele that encodes a pigment
that makes the flowers white.
Now when you get a cross
between these individuals,
and you get one red allele
and one white allele,
it's like mixing two colors on a pallette,
you get an intermediate phenotype.
Make sure you understand that
word, intermediate phenotype.
Which is essentially a
combination of the two phenotypes,
and you get something in the middle.
So here's a scenario where
if you take two heterozygous individuals
and cross them together you
can see that you ultimately get
these statistics of
homozygous for the red allele,
heterozygous, one red, one white allele,
and homozygous for the white allele.
Notice again, you don't
have dominant recessive,
in this case they're both dominant,
but we call them incomplete dominance.
Neither one is dominant over the other,
and so you get a mixing
of the two phenotypes.
What's another example of this?
Well in humans your hairs, as
they grow out of your head,
either are round and
therefore make straight hair,
that's one allele.
The other is where the
hairs are actually flat
and they end up curling up on themselves,
and that's what makes curly hair.
Now for the sake of making
sure you understand these
are the same gene, we're
gonna do S1, and then S2.
So these are dominant alleles.
This one makes straight hair.
(marker thudding)
This is curly hair.
What do you think the
intermediate phenotype is?
Wavy, so naturally wavy hair is an example
of incomplete dominance,
where you end up having one
of each allele, and you get
this intermediate phenotype,
this is incomplete dominance.
So that's really another
example in human genetics.
- [Student] Does this exist
for recessive genes, too?
(Instructor sighs heavily)
- [Instructor] I can't
think of any examples.
I can't think of any.
I'm sure there are, but
I can't think of any
off the top of my head.
Let's see...
So let me give you a scenario.
Let's say that you had two parents
with naturally wavy hair,
where they have this
intermediate phenotype.
What are the chances that they'll have
a child with curly hair?
So straight hair,
you have to have two of
the straight hair alleles,
curly hair, you have to have
two of the curly hair alleles,
so I'll so this.
(marker thudding)
And then you have two
parents who have wavy hair,
what are their genotypes?
- [Student] S1 and S2.
- [Instructor] S1 and S2, and?
(students murmuring answers)
S1, S2, okay.
We have to do the Punnett
Square, S1, S1, S1, S2,
S1, S2 and S2, S2.
What are the chances that they'll
have a child with curly hair?
It's the same as this right here.
This is in fact the same scenario,
if you were to cross two
pink flowers with each other,
you get 25% chance that
they're red, 50% chance
that they're pink, and 25%
chance that they're white.
In the case of these
two parents, 25% chance
their children have
straight hair, 50% chance
that they have wavy hair,
and 25% chance that
they'll have curly hair.
So it's got to be one
or the other of those.
we call these four o'clock flowers,
there's a number of
different types of flowers.
There's also snapdragons, which
I think these are, as well.
But there's several types of flowers
that show this same type of
relationship with each other.
All right, now, the last
few Punnett Square questions
you're gonna have are
on this last concept.
So you've got a couple on blood type.
But this one of those that
people find pretty fascinating,
because they're always interested
in understanding how blood type works.
Now before we can get into
the Punnett Squares of this,
we first have to describe
a little bit about
how blood types work and what they are.
Now if you remember
back from cell biology,
we talked about what was
called a glycoprotein.
Which is essentially a protein
with a carbohydrate attached to it,
that's embedded in the cell membrane.
Now we use these for things
like cell recognition.
Remember these are the oligosaccharides
that ultimately determine what is self
and what it's for.
So we learned this back in
when we discussed cell biology,
and we discussed how proteins were made
and how the Golgi apparatus attaches
its carbohydrates to the proteins.
Well that's what we're
looking at right here.
Because that's what your blood type is.
It's very particular glycoprotein
that is attached to the surface
of your red blood cells.
So if you have blood type A you have
what we call the A antigen.
So antigen and glycoprotein are the same.
(marker thudding)
In this case, antigen is that
surface recognition molecule
that your body uses to recognize
what is self and what it's for.
So some people have the
A antigen on the surface
of their red blood cells.
Other people hae the B
antigen on the surface
of their red blood cells.
Some people have neither one,
and that's what gives us blood type O.
But in some cases people
have both A and B antigens.
In fact, this is one of the
least common blood types
in the US, statistically speaking.
But there are individuals
which are blood type AB.
Now here's why it's
beyond Mendel's genetics.
Up until this point we
have talked about genes
having pretty much two alleles,
a dominant and recessive
allele, two dominant alleles.
Now we're looking at three alleles.
Blood type is a single gene,
we call it the ABO gene,
so you can guess why,
that has three different alleles to it.
Now two of the alleles are dominant,
and one allele is recessive.
Which is why we use the I as a capital
to indicate the dominant allele,
and a lower case I to
represent the recessive allele.
So if you have the recessive allele,
this doesn't make either antigen.
Essentially it's like putting
nothing on the surface.
So that's why this is recessive.
Because if you have even
one allele that makes
the A glycoprotein, and
the other is the recessive,
you'll still make the A glycoprotein,
you'll still be blood type A.
Same thing for B, if
you make the B antigen,
but your other allele is recessive,
you'll still make the B antigen
and your red blood cells
will be blood type B.
If you have both the A and the B antigens,
then both of those are expressed
on the surface of your red blood cells,
and you're blood type AB.
And only if you have the
two recessive alleles,
do you have blood type O.
Because in the presence of any of these
other two dominant alleles, you'll either
have blood type A or blood type B.
So blood type O is recessive.
Now remember we talked about recessive
does not mean that it is the least common.
In fact, 45% of the US has blood type O.
So it's one of the most common blood types
in the US, AB being the least.
The next most common is blood type A,
third most common is blood type B.
Now, you'll notice I'm
ignoring the RH factor,
which is the plus or the minus.
I'll talk a little bit
more about that later,
but for now we're just gonna focus on
the A or the B antigen.
Now there are more than
just these four genotypes.
Because in fact, if
you have two A antigens
you're also blood type A.
And if you have two B antigens,
you're also blood type B.
So in fact, if you're blood type A
there are two possible
genotypes for that phenotype.
And if you're blood type B,
there are two possible
genotypes for that phenotype.
It's only blood type AB and blood type O,
that you end up having
one genotype associated
with that phenotype.
There's only one way to get blood type AB,
there's only one way to get blood type O.
So here's where things
get a little interesting.
Here is the concept regarding these.
We already know the
dominant/recessive relationship,
that's old news, you already know that.
But what happens when you
get two dominant alleles?
Well this is not like the
example we just went over.
This is not incomplete dominance.
It's what we call co-dominance.
Now there's a big difference
between the two, let me explain.
Co-dominance means that neither allele
is dominant over each other,
they are both expressed independently.
Neither is dominant, so you don't get
this intermediate phenotype.
Let me give it to you
in a scenario like this.
If, it's a big if,
if these flowers' alleles
represented co-dominance,
what would you expect
this flower to look like,
or this plant?
- Some reds and whites.
- Reds and white.
- [Instructor] Good, red and white,
you would not get anything in between.
You'd have some petals which are red,
and some petals which are white.
If you we saw the same
scenario with the human,
the straight and the curly hair.
If you plucked out a hair it would be
either straight or curly,
but not in between, not
wavy, if it were co-dominant.
So really that's the big difference.
Incomplete dominance you get
a mixing of the phenotypes
and you get that intermediate phenotype.
Co-dominance on the other
hand you get both of them,
you get both expressed at the same time.
You get blood type A
and B antigens expressed
on the surface of your red blood cells,
and therefore, you're blood type AB.
Let's look at some of the
Punnett Square scenarios.
Now if I tell you that
someone's blood type AB,
or blood type O, I do not have
to tell you anything more,
you know what their genotype is.
As I mentioned, there's
only one genotype associated
with blood type AB,
there's only one genotype
associated with blood type O.
It's blood type A and B
that become a little tricky,
because there are two possible genotypes.
You could be homozygous for the A antigen,
or you can be heterozygous.
You could be homozygous for the B antigen,
or you can be heterozygous.
Now when we say blood
type A, heterozygous,
you already know that the other one has
to be the little i,
because it can't be this,
because then you can't be blood type A.
So if I say, "Oh, they're blood type A
and they're homozygous,"
the alleles are the same.
If they're blood type A heterozygous
then they have one A antigen
and then the recessive one.
'Cause this is the only
other heterozygous scenario
in that situation, you can't be AB.
So the same thing for B.
If you're homozygous blood type B,
or heterozygous blood type B,
those are the differences
between those genotypes.
Okay, now with that knowledge,
let's look at some scenarios.
If you have two parents with blood type O,
what are the chances you'll
have a child with blood type O?
(marker thudding)
100%, this is why they don't genotype,
or blood type kids in the schools anymore.
Because they have been kids that are like,
I'm blood type A, and my
parents are blood type O.
Hm, yeah, so ultimately,
(chuckles) we don't do that anymore.
All right, let's look at a
parent who's blood type AB,
and a parent who's blood type O.
What are the chances they'll
have a child with blood type A?
(marker thudding)
50%, okay?
Let's do one parent is blood
type A and is heterozygous,
the other parent is blood
type B and is heterozygous.
What are the alleles then?
If they're blood type A and
heterozygous what are they?
- [Student] IA.
- [Instructor] IA.
- [Student] Little i.
- [Instructor] Little i.
And blood type B heterozygous?
All right B.
- Little i.
- Little i.
And guess what you get?
(marker thudding)
Everything.
25% chance for all four blood types.
That would be an interesting family.
So 25% chance blood type A, blood type B,
blood type A, and blood
type O, okay, yeah?
- [Student] So what is the deciding factor
if someone, for the person
to get a particular one?
So if they had 25% of each one,
what is the actual deciding factor
of what blood type you get?
Or is it just random?
- [Instructor] It all depends upon
what genetics they inherit.
So if the father passes on his B antigen
and the mother passes on her O antigen,
then the child would be blood type B.
So it all depends upon
when you undergo myosis
and you split your homologous chromosome
which allele you pass on.
And then the combination will
ultimately determine that.
But here is where you're gonna
have four different possible
phenotypes from that scenario.
In fact, that's shown right here,
in this scenario right here.
You're gonna have a question like this,
but here's the more
difficult of the question,
so let's do this.
Let's say you have a
child who is blood type O,
and you have a parent who's blood type A,
and another parent is blood type O.
What is the genotype of the parents?
- [Student] IA, little
i, little i, little i.
- [Instructor] How do you know that?
- [Student] 'Cause that was for the O,
you have to pass on the recessive.
- [Instructor] Good, Law of Segregation.
In order for the child to be blood type O,
we know this parent's little i, little i.
But that means that this
parent has to be heterozygous.
Otherwise they can't inherit
that second O allele.
Let me give you another scenario.
Same thing, parents are
A and O, child is now A.
Which of those genotypes
can you figure out?
(students answering off microphone)
Do you know the child's?
- 100% has to be IA.
- IA, little i.
- [Instructor] Why does
it have to be that?
- [Student] That's the only way to make-
- [Instructor] That's the only way.
I mean this person is blood type O,
so they only have two little i alleles.
And because this person's A they got to
at least have the A antigen.
Now do you know this parent?
- [Student] No.
- [Instructor] No, you cannot,
from just this information,
you cannot figure out, because
the parent could be this,
or they could be this, either one works.
So in that scenario you
would not be able to tell us
the genotype of that parent.
But you would be able to of
the child and of that parent.
Before I go onto the last little bit,
sometimes people are interested to know
why certain blood types
can't be mixed with others.
This isn't any test questions,
but just for your sake,
for your knowledge.
If you're blood type A,
then you have the A antigen,
we've learned that already.
But your body will
produce antibodies against
the other antigen that is created,
which is why we create
these anti-B antibodies.
Now the same thing is true
for someone with blood type B.
They produce B antigens on the surface
of their red blood cells,
but they'll produce the
A antibodies as a result.
Because the body will not,
it needs to recognize
anything else for it.
Now there are many other antibodies
that are produced besides these,
but these are amongst those that
are produced by those individuals.
If you're blood type A and B,
then the body essentially doesn't produce
either of these antibodies,
because this is what happens when you mix
the antigen with antibodies.
Let's say someone with blood type B,
or sorry, someone with blood type A
gets a transfusion of blood,
but it was the wrong
blood, so we screwed up.
They put blood type B and mislabeled it.
This is what happens when the blood type
gets into the person who has blood type A.
The person with blood type A
produces these B antibodies,
they attach to it, they cause
this agglutination response,
and then the bursting of the cells.
It would be the equivalent
of pumping mass amounts
of bacteria into somebody's
body, it will kill them.
This immune response if you pump enough
of this foreign blood into someone
will kill them essentially.
- [Students] What's the
purpose of blood cells
being able to detect foreign blood?
- [Instructor] Well ultimately,
we don't just produce
antibodies against the other one,
we produce antibodies
against everything else.
So if you're blood type
A, you have millions
of different antibodies.
Your body will only eliminate ones
that were recognized cells.
So your body has the potential to create
the antigens against A,
but it destroys those cells
that would actually make that.
So this is kind of misleading in saying
that we only produce that.
We produce millions against
any foreign antigen,
this is one amongst those millions.
And then if somebody has this blood type,
they essentially destroy these cells
that would make anti-A
and anti-B antibodies,
of the millions that we produce.
So really it's just about
recognizing anything foreign.
But this is amongst those
antibodies that we make.
Now here's the thing here.
If you're blood type O, you
do not have any antigens,
and therefore, the body doesn't destroy
any of these antibodies against A and B.
So here's the thing,
blood type O are called
the universal donors, why?
Because when you usually
filter out everything
but the red blood cells.
That's why when you do a transfusion
you're only passing on the cells.
There's nothing here to
be recognized as foreign.
So each of these cell types
can receive blood type O
without any problems, now
I'm ignoring RH factor.
We'll talk about that in a second.
If you're blood type A, sorry,
but they don't need your blood. (laughs)
They may, they may,
there are some scenarios
where you might need an AB negative,
because of some individuals, or whatnot.
But most of the time
they're not in high demand.
Blood type O you're in high demand,
because it's the universal donor.
Now if you're blood type
AB, you're better on
the receiving end, because you can pretty
much receive blood from anybody.
'Cause it doesn't matter
the blood has A antigens,
B antigens or no antigens,
you're not gonna reject any of it,
because of your immune system.
Now the same thing applies to what we call
the plus or the minus.
So you say, "Oh I'm A positive."
Well those are two different antigens.
One's the ABO gene, the other
is what we call the RH factor.
And you either have it, or you don't.
So if you have it, that's
where the positive comes in.
If you don't have it, that's
where the negative comes in.
Now this applies to actually pregnancies.
If women give birth to a
child who is RH positive,
and they themselves are RH negative,
the first child won't have any problem.
But in the birthing
process there's usually
some mixing of blood.
And when the mother's
exposed to that RH antigen
then she'll start producing antibodies
in surplus against that.
The next time she has a kid,
then those antibodies can actually attack
the fetus as its developing.
And these mothers usually have to go
on in immunosuppressant drugs to be able
to have the child, because of that issue.
Now it doesn't, for whatever reason,
it's not an issue of the
mother's blood type O,
and the child's blood type
A, that's not the problem.
But that RH factor somehow
crosses that placental barrier
and the antibodies can actually
attack the fetus' blood.
So anyway, that's blood
type in a nutshell.
Now, let's talk about the
last set of relationship
between genes, alleles, and traits.
Everything up to this point,
including the exceptions,
the blood type which has three alleles,
the incomplete dominance like with
the straight and curly hair,
those are all what we
call Mendelian genetics.
And the reason why we call
them Mendelian genetics
is because they're
one-on-one relationships,
one gene, one trait, you know?
You've got the ABO gene which yes,
makes multiple types of blood types,
but it's still one gene, one trait.
Most things in, most organisms
are never that simple.
Most of the time our traits
are what we call polygenic.
Poly means many, genic means genes.
So in reality, most of our phenotypes,
even something that you
might consider as simple
as hair or eye color, are polygenic.
That means that it actually
requires multiple genes
and their respective
alleles combined together
to create your hair color, your eye color.
So it's not just a
one-on-one relationship.
So polygenic, this is how most
things are, even your skin.
Your skin is determined by as
many as six different genes
with their respective alleles.
In fact, this is kind of an example.
This only shows three different
genes labeled A, B and C.
But in reality is more
like six different genes.
So this just kind of illustrates
that you have dominant/recessive
alleles for each gene.
And depending on how many dominant,
and how many recessive
alleles you have for those,
ultimately determine your
natural skin pigmentation.
So polygenic, many genes, one trait.
And this is for hair, eye
color, height, you name it,
most of the time this is what it is.
This is why it's not always
as clear cut and simple
to say, "Oh, both parents have brown hair,
then you're gonna have
a brown-haired child."
It's not that simple, it's a
little more complex than that.
Now the next concept,
there are scenarios where
the opposite is true, where one gene
can actually influence multiple traits.
We call this pleiotropy.
Now we've talked a little
bit about Marfan syndrome,
this is actually a dominant
disorder, a dominant gene.
However, there's not just one
symptom of Marfan syndrome.
Some people have
skeletalgenic abnormalities.
Other people have heart problems,
other people have lung problems,
other people have skin problems, why?
It's a single gene, it's
a connective tissue gene,
has to do with creating the
fibronectin, and whatnot,
and the connective tissues,
but all of these have
to make a tissue in them.
And depending upon what
happens during development
you can have any number
of phenotypes associated
with this one genetic disorder.
In fact, that's what they
found with some family trees,
is they were looking at genetic disorders.
Here's one where it's
a porphyrian mutation.
And they found that some
people had mad behavior,
others had constipation,
dark colored urine.
From generation to generation
there were different symptoms,
and they're like, "Man, you
guys are all screwed up."
Well, it was the inheritance of one gene!
One problematic gene from
one generation to the next.
So that's pleiotropy.
A massively, severe case of pleiotropy
that's interesting in and of
itself, it's not inherited.
So it stops after the mutation occurs,
but it's called androgen insensitivity.
So what is this? It's a female that is XY.
So how does that happen?
Well remember I told
you when you get defects
on the Y chromosome, but
they're usually not inherited?
Well, this is a mutation
in a protein, a receptor
that responds to testosterone.
In women, 'cause they are women
that have androgen insensitivity,
they actually have testes
instead of ovaries.
Now they sit where the
ovaries should have been,
but they develop testes.
But due to the mutation
of this one receptor
on the Y chromosome, the
testes secrete testosterone,
the body can't respond to it,
so it develops as a female.
So it has all of the secondary
characteristics of a female
from breast development, to
long hair and everything.
The only difference is
they don't have a uterus.
So women, if you've had your period,
you don't have androgen insensitivity.
So you're not an XY.
But this is like a one in
a billion chance for this to
happen, it is not very likely.
And it's not inherited,
because obviously it
causes sterility, as well.
That's just another extreme
example of pleiotropy
where one gene can actually change
the entire sex of the individual,
instead of being male, it's female.
All right, now two more concepts.
Sometimes, we see
scenarios where two people
have the same phenotype,
but all of their children are normal.
And so the question becomes,
well how does that happen?
In fact, two different examples.
You've got dwarfism and
you've got deafness.
And these are genetics
that can be inherited.
But I grew up with friends
whose parents were deaf,
but all of their children were hearing.
And then there are times where you
have two parents with dwarfism,
but all their children are normal height.
So how does that happen?
Because statistically you
would at least expect one
or two of them to have the
same genetic disorders.
Well this is what we call
genetic heterogeneity.
Basically, you arrive
at the same phenotype,
but you have different
mutations in different genes.
And the reason for this kind of goes back
to polygenic inheritance.
Multiple genes all combine together
to form a particular phenotype.
Well if you mutate one gene,
then you cause some problem
in that ultimate phenotype.
If you mutate a different
gene you may cause
the same ultimate result.
So that's why two
parents with dwarfism may
have dwarfism in common,
but not because of the
same genetic abnormality.
And when they have children,
because they have different genetics,
the recombination of those genetics
restores normal functionality,
and all their children are normal height.
Or all their children can hear.
Because there's many genes
that go into creating
the hearing of the cochlea and
the inner auditory ossicles,
and basically hearing in general.
Blood clotting as well is
another example of this,
or many different genes all coming down
to the process of blood clotting.
So two people have poor blood clotting,
it may not be for the same reason.
So that's genetic heterogeneity.
Can two parents with blood type O have
a child with blood type A?
Yes, but it is rare.
But it illustrates the last
concept called epistasis.
Where one gene can actually supersede
the expression of another.
Now we've already gone
over one example of this.
Let's go back, I didn't
point it out at first,
'cause I didn't want to add
more on, but let's go back.
Albinism is an example of epistasis.
So what happens in albinism is,
we talked about how there
about six different genes
that cause the skin
pigment colors to occur.
But there's one gene to control them all.
Yes, Lord of the Rings, scenario.
One gene that ultimately
if you mutate that gene
it sets everything at
a default and goes down
to what we call albinism, where
the hair is light colored,
the eyes are light colored,
the skin is completely white.
There's no pigmentation whatsoever.
And that's what you're
seeing here in this family
is that these kids
should have darker skin.
But that one gene supersedes all
of the others in this expression.
And ultimately makes it so that they don't
have any skin pigmentation whatsoever.
Now, what's another scenario?
I talked about blood type.
Let me show you how this works.
We know that the ABO
gene makes the antigen.
And depending upon which
antigen gets attached
to the surface of the red blood cell
will determine what
your phenotype is, okay.
Well guess what?
There's a gene that supersedes this one
that ultimately attaches
the antigen to the surface.
If you have a mutated
version of this gene,
it doesn't matter what your
genotype for the ABO gene is,
your default is gonna be blood type O.
Even though you make the A antigens
it never gets attached to
the surface of the cell,
because of the mutation in this gene.
So in this scenario, the
person's phenotype is O.
But their genotype is this.
Now, take another person
who is actually blood type O
because of the genotype,
cross them together,
what's the child's genotype gonna be?
- [Student] A.
- [Instructor] Or phenotype is gonna be?
It's gonna be A.
Now how does this occur?
How does it, when you recombine these
all of a sudden they're normal
functionality is restored?
Well in this parent, this gene is mutated.
So it doesn't attach the
antigen to the surface.
Over here, they have normal genes.
When you recombine these it
restores the functionality
of that gene, and now this
can be expressed just fine.
This is what we call the Bombay phenotype.
Where two parents with blood type O
can have a child with
a different blood type,
with blood type A, or whatnot.
But it's very rare, so
it's not one the things
you would look for first in that scenario.
There's not a question on this,
but this is another interesting concept
that I want you to keep in mind.
And then we'll do one
last question or scenario
to finish off this lecture.
Most of the time people ask the question,
"Are we nature, or are we nurture?"
we're both, okay?
We're both genetic and environment.
Sometimes environment can have more
of an influence over our
phenotype than the genetics do.
What's an example of
this? Your fingerprint!
Your actual fingerprint,
not your DNA fingerprint,
your actual fingerprint, is about 90%
environmentally influenced,
10% genetically influenced.
That's one huge scenario.
Now on the other flip side,
as we talked about with blood type.
That's pretty much 100%
genetics with no environment.
So it all depends upon what
trait you're looking at
to determine what the overall relationship
between how much genetic influence,
how it's environmental influence.
Now, to give you a
scenario of how environment
can really kind of screw things up.
Let's look at the Siamese
cat, or the Himalayan rabbit.
What do you notice about both of them?
They're black in their extremities,
but their core fur is what.
Well, here's what they found.
They don't have multiple
genes going on here,
they just have one gene that's defective.
And when the temperature in
their core body gets too high
then it doesn't make the black pigment.
That's why the extremities are black,
but the core body where most
of the heat is held is white.
Well, how do they know this?
Like scientists do, they strap an ice pack
to the back of the Himalayan rabbit,
and let the fur grow out,
and then when they pull
it off, boom, it's black.
So they showed by lowering
the surface temperature
of their skin, that that's
exactly this is supposed to do.
But because of the environment,
or the temperature of the rabbit,
then it defaults to white,
because it messes up the protein.
Anyway, an interesting point.
But to illustrate that we're
both nature and nurture,
we're not pure genetic.
Which is why if you clone yourself,
you're not going to have
the same individual,
unless you have the exact
same environmental influence
in both in fetus, an embryo,
as well as throughout your life.
Otherwise, it's no different
than identical twins
that start out genetically the same,
but due to environmental differences
ultimately become different over time.
