B1 Intermediate US 29 Folder Collection
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Well, before we even knew what DNA was, much less how it was
structured or it was replicated or even before we
could look in and see meiosis happening in cells, we had the
general sense that offspring were the products of some
traits that their parents had.
That if I had a guy with blue eyes-- let me say this is the
blue-eyed guy right here --and then if he were to marry a
brown-eyed girl-- Let's say this is the brown-eyed girl.
Maybe make it a little bit more like a girl.
If he were to marry the brown-eyed girl there, that
most of the time, or maybe in all cases where we're dealing
with the brown-eyed girl, maybe their kids are
Let me do this so they have a little brown-eyed baby here.
And this is just something-- I mean, there's obviously
thousands of generations of human beings, and we've
observed this.
We've observed that kids look like their parents, that they
inherit some traits, and that some traits seem to dominate
other traits.
One example of that tends to be a darker pigmentation in
maybe the hair or the eyes.
Even if the other parent has light pigmentation, the darker
one seems to dominate, or sometimes, it actually ends up
being a mix, and we've seen that all around us.
Now, this study of what gets passed on and how it gets
passed on, it's much older than the study of DNA, which
was really kind of discovered or became a big deal in the
middle of the 20th century.
This was studied a long time.
And kind of the father of classical genetics and
heredity is Gregor Mendel.
He was actually a monk, and he would mess around with plants
and cross them and see which traits got passed and which
traits didn't get passed and tried to get an understanding
of how traits are passed from one generation to another.
So when we do this, when we study this classical genetics,
I'm going to make a bunch of simplifying assumptions
because we know that most of these don't hold for most of
our genes, but it'll give us a little bit of sense of how to
predict what might happen in future generations.
So the first simplifying assumption I'll make is that
some traits have kind of this all or nothing property.
And we know that a lot of traits don't.
Let's say that there are in the world-- and this is a
gross oversimplification --let's say for eye color,
let's say that there are two alleles.
Now remember what an allele was.
An allele is a specific version of a gene.
So let's say that you could have blue eye color or you
could have brown eye color.
That we live in a universe where someone could only have
one of these two versions of the eye color gene.
We know that eye color is far more complex than that, so
this is just a simplification.
And let me just make up another one.
Let me say that, I don't know, maybe for tooth size, that's a
trait you won't see in any traditional biology textbook,
and let's say that there's one trait for big teeth and
there's another allele for small teeth.
And I want to make very clear this distinction between a
gene and an allele.
I talked about Gregor Mendel, and he was doing this in the
1850s well before we knew what DNA was or what even
chromosomes were and how DNA was passed on, et cetera, but
let's go into the microbiology of it to understand the
So I have a chromosome.
Let's say on some chromosome-- let me pick
some chromosome here.
Let's say this is some chromosome.
Let's say I got that from my dad.
And on this chromosome, there's some location here--
we could call that the locus on this chromosome where the
eye color gene is --that's the location of
the eye color gene.
Now, I have two chromosomes, one from my father and one
from my mother, so let's say that this is the chromosome
from my mother.
We know that when they're normally in the cell, they
aren't nice and neatly organized like this in the
chromosome, but this is just to kind of show you the idea.
Let's say these are homologous chromosomes so they code for
the same genes.
So on this gene from my mother on that same location or
locus, there's also the eye color gene.
Now, I might have the same version of the gene and I'm
saying that there's only two versions of
this gene in the world.
Now, if I have the same version of the gene-- I'm
going to make a little shorthand notation.
I'm going to write big B-- Actually, let me do
it the other way.
I'm going to write little b for blue and I'm going to
write big B for brown.
There's a situation where this could be a little b and this
could be a big B.
And then I could write that my genotype-- I have the allele,
I have one big B from my mom and I have one
small b from my dad.
Each of these instances, or ways that this gene is
expressed, is an allele.
So these are two different alleles-- let me write that
--or versions of the same gene.
And when I have two different versions like this, one
version from my mom, one version from my dad, I'm
called a heterozygote, or sometimes it's called a
heterozygous genotype.
And the genotype is the exact version of the alleles I have.
Let's say I had the lowercase b.
I had the blue-eyed gene from both parents.
So let's say that I was lowercase b, lowercase b, then
I would have two identical alleles.
Both of my parents gave me the same version of the gene.
And this case, this genotype is homozygous, or this is a
homozygous genotype, or I'm a homozygote for this trait.
Now, you might say, Sal, this is fine.
These are the traits that you have. I have a brown from
maybe my mom and a blue from my dad.
In this case, I have a blue from both my mom and dad.
How do we know whether my eyes are going to be brown or blue?
And the reality is it's very complex.
It's a whole mixture of things.
But Mendel, he studied things that showed
what we'll call dominance.
And this is the idea that one of these traits
dominates the other.
So a lot of people originally thought that eye color,
especially blue eyes, was always dominated
by the other traits.
We'll assume that here, but that's a gross
So let's say that brown eyes are dominant
and blue are recessive.
I wanted to do that in blue.
Blue eyes are recessive.
If this is the case, and this is a-- As I've said
repeatedly, this is a gross oversimplification.
But if that is the case, then if I were to inherit this
genotype, because brown eyes are dominant-- remember, I
said the big B here represents brown eye and the lowercase b
is recessive --all you're going to see for the person
with this genotype is brown eyes.
So let me do this here.
Let me write this here.
So genotype, and then I'll write phenotype.
Genotype is the actual versions of the gene you have
and then the phenotypes are what's expressed
or what do you see.
So if I get a brown-eyed gene from my dad-- And I want to do
it in a big-- I want to do it in brown.
Let me do it in brown so you don't get confused.
So if I've have a brown-eyed gene from my dad and a
blue-eyed gene from my mom, because the brown eye is
recessive, the brown-eyed allele is recessive-- And I
just said a brown-eyed gene, but what I should say is the
brown-eyed version of the gene, which is the brown
allele, or the blue-eyed version of the gene from my
mom, which is the blue allele.
Since the brown allele is dominant-- I wrote that up
here --what's going to be expressed are brown eyes.
Now, let's say I had it the other way.
Let's say I got a blue-eyed allele from my dad and I get a
brown-eyed allele for my mom.
Same thing.
The phenotype is going to be brown eyes.
Now, what if I get a brown-eyed allele from both my
mom and my dad?
Let me see, I keep changing the shade of brown, but
they're all supposed to be the same.
So let's say I get two dominant brown-eyed alleles
from my mom and my dad.
Then what are you going to see?
Well, you could guess that.
I'm still going to see brown eyes.
So there's only one last combination because these are
the only two types of alleles we might see in our
population, although for most genes, there's
more than two types.
For example, there's blood types.
There's four types of blood.
But let's say that I get two blue, one blue allele from
each of my parents, one from my dad, one from my mom.
Then all of a sudden, this is a recessive trait, but there's
nothing to dominate it.
So, all of a sudden, the phenotype will be blue eyes.
And I want to repeat again, this isn't necessarily how the
alleles for eye color work, but it's a nice simplification
to maybe understand how heredity works.
There are some traits that can be studied in this simple way.
But what I wanted to do here is to show you that many
different genotypes-- so these are all different genotypes
--they all coded for the same phenotype.
So just by looking at someone's eye color, you
didn't know exactly whether they were homozygous
dominant-- this would be homozygous dominant --or
whether they were heterozygotes.
This is heterozygous right here.
These two right here are heterozygotes.
These are also sometimes called hybrids, but the word
hybrid is kind of overloaded.
It's used a lot, but in this context, it means that you got
different versions of the allele for that gene.
So let's think a little bit about what's actually
happening when my mom and my dad reproduced.
Well, let's think of a couple of different scenarios.
Let's say that they're both hybrids.
My dad has the brown-eyed dominant allele and he also
has the blue-eyed recessive allele.
Let's say my mom has the same thing, so brown-eyed dominant,
and she also has the blue-eyed recessive allele.
Now let's think about if these two people, before you see
what my eye color is, if you said, look, I'm giving you
what these two people's genotypes are.
Let me label them.
Let me make this the mom.
I think this is the standard convention.
And let's make this right here, this is the dad.
What are the different genotypes that their children
could have?
So let's say they reproduce.
I'm going to draw a little grid here.
So let me draw a grid.
So we know from our study of meiosis that, look, my mom has
this gene on-- Let me draw the genes again.
So there's a homologous pair, right?
This is one chromosome right here.
That's another chromosome right there.
On this chromosome in the homologous pair, there might
be-- at the eye color locus --there's the brown-eyed gene.
And at this one, at the eye color locus, there's a
blue-eyed gene.
And similarly from my dad, when you look at that same
chromosome in his cells-- Let me do them like this.
So this is one chromosome there and this is the other
chromosome here.
When you look at that locus on this chromosome or that
location, it has the brown-eyed allele for that
gene, and on this one, it has the blue-eyed
allele on this gene.
And we learn from meiosis when the chromosomes-- Well, they
replicate first, and so you have these two chromatids on a
But they line up in meiosis I during the metaphase.
And we don't know which way they line up.
For example, my dad might give me this chromosome or might
give me that chromosome.
Or my mom might give me that chromosome or might give me
that chromosome.
So I could have any of these combinations.
So, for example, if I get this chromosome from my mom and
this chromosome from my dad, what is the genotype going to
be for eye color?
Well, it's going to be capital B and capital B.
If I get this chromosome from my mom and this chromosome
from my dad, what's it going to be?
Well, I'm going to get the big B from my dad and then I'm
going to get the lowercase b from my mom.
So this is another possibility.
Now, this is another possibility here where I get
the brown-eyed allele from my mom and I get the blue eye
allele from my dad.
And then there's a possibility that I get this chromosome
from my dad and this chromosome from my mom, so
it's this situation.
Now, what are the phenotypes going to be?
Well, we've already seen that this one right here is going
to be brown, that one's going to be brown, this one's going
to be brown, but this one is going to be blue.
I already showed you this.
But if I were to tell you ahead of time that, look, I
have two people.
They're both hybrids, or they're both heterozygotes for
eye color, and eye color has this
recessive dominant situation.
And they're both heterozygotes where they each have one brown
allele and one blue allele, and they're going to have a
child, what's the probability that the child has brown eyes?
What's the probability?
Well, each of these scenarios are equally likely, right?
There's four equal scenarios.
So let's put that in the denominator.
Four equal scenarios.
And how many of those scenarios end
up with brown eyes?
Well, it's one, two, three.
So the probability is 3/4, or it's a 75% probability.
Same logic, what's the probability that these parents
produce an offspring with blue eyes?
Well, that's only one of the four equally likely
possibilities, so blue eyes is only 25%.
Now, what is the probability that they produce a
So what is the probability that they produce a
heterozygous offspring?
So now we're not looking at the phenotype anymore.
We're looking at the genotype.
So of these combinations, which are heterozygous?
Well, this one is, because it has a mix.
It's a hybrid.
It has a mix of the two alleles.
And so is this one.
So what's the probability?
Well, there's four different combinations.
All of those are equally likely, and two of them result
in a heterozygote.
So it's 2/4 or 1/2 or 50%.
So using this Punnett square, and, of course, we had to make
a lot of assumptions about the genes and whether one's
dominant or one's a recessive, we can start to make
predictions about the probabilities
of different outcomes.
And as we'll see in future videos, you can actually even
go backwards.
You can say, hey, given that this couple had five kids with
brown eyes, what's the probability that they're both
heterozygotes, or something like that.
So it's a really interesting area, even though it is a bit
of oversimplification.
But many traits, especially some of the things that Gregor
Mendel studied, can be studied in this way.
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Introduction to Heredity

29 Folder Collection
張瑩 published on December 29, 2019
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