Introduction to Heredity


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 brown-eyed. 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 difference. 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 oversimplification. 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 chromosome. 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 heterozygote? 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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