In this video we're going to talk about Gregor
Mendel's Law of Segregation and what observations
let to this conclusion.
Gregor Mendel was one of the first people
to accurately analyze patterns of inheritance.
He deduced the fundamental principles of the
study of inheritance, which we call Genetics.
Gregor Mendel was doing his research and published
his works in 1866.
This was well before the discovery of DNA,
before the discovery of chromosomes.
He didn't know what the physical particles
of inheritance were, but he knew some information
about the properties that they must have.
Gregor Mendel performed most of his research
on a specific type of organism.
These are garden peas and he used these organisms
for several different reasons.
He could easily control the crossing and this
would allow for both self-fertilization and
cross-fertilization.
They made many offspring each time reproduction
happened.
They had a very fast generation time, so he
could get several generations grown in a single
growing season.
And also, they were cheap and easy to grow.
With these organisms, Gregor Mendel carried
out both self and cross-fertilizations.
Now, pea plants are hermaphroditic.
They produce both male and female gametes
in a single flower and they're capable of
self-fertilizing.
So the pollen from a flower can fertilize
itself, resulting in a fruit or in the case
with the pea plants, resulting in a pea pod
filled with seeds or offspring.
What Gregor Mendel would do with these cross-fertilizations
is that before a flower was able to fertilize
itself he would remove the stamens, remove
those sections that would make the pollen
or the male gametes.
He would then take pollen from another flower
and pollinate or fertilized that flower with
the removed stamens.
In that way, he would know all of the seeds
that were produced, all of the offspring,
they had genetic material from two different
parents.
Now before he would even start this experiment,
he would make sure that his starting lines
were true-breeding.
By “true-breeding”, we mean that these
plants would produce offspring 
exactly like themselves, exactly like the
parents, every time they self-fertilized.
So if you had a true-breeding pea plant that
had purple flowers, if you allowed it to self-fertilize,
all of the offspring would have purple flowers.
If you'd allow them to self-fertilize, all
their offspring would have purple flowers.
Yet in that same way, you could have a strain
of pea plants that were true-breeding for
white flowers, in that all their offspring
would have white flowers and the next generation
and the next generation after that.
He always made sure he was starting with two
true-breeding lines, but then he asked “What
happens if I cross these two?’
He would cross those two true-breeding lines
that differed in a single trait.
In this case, that trait that he was looking
at was flower color.
So if he had a purple-flowered pea plant and
a white-flowered pea plant, he knew they were
from true-breeding lines.
But then he used the pollen from one to fertilize
the eggs of the other.
He could then plant those offspring and see
what flower color those offspring would have.
And when he performed this experiment he found
that always the offspring of those two true-breeding
lines, they would have purple flowers.
This type of a genetic cross is known as a
monohybrid cross.
It’s a cross between parent plants that
differ in only one characteristic.
So Gregor Mendel performed this cross many
times, and consistently he would see the results
that the F1 generation, they would all have
purple flowers.
But then if he allowed those F1 generation
to self-fertilize, if they would pollinate
their own eggs to produce seeds or to produce
that next generation.
When he planted that next generation called
the F2 generation, most of them would have
purple flowers, just like the F1, but consistently
white-flowered pea plants would show up again.
And in fact, those white-flowered pea plants
would consistently show up in about 1/4 of
the F2 offspring.
So this was a pattern that Gregor Mendel saw
repeated many times.
I'd like to introduce you a bit to the terminology
that he used so we can describe this situation
that's occurring here.
Gregor Mendel would always take these monohybrid
crosses to 3 generations.
Now, the first generation he would call the
P generation or parental generation and these
were the true-breeding lines.
Whether they were true-breeding for purple
flowers or true-breeding for white flowers.
This is always where he would start.
When he would cross two of those true-breeding
lines together and plant the resulting seeds,
the offspring that were formed were known
as the F1 generation or the first filial generation.
These were the offspring of the parental generation.
And what Gregor Mendel saw is that this F1
generation always showed one particular trait.
Again, if we're talking about flower color,
the F1 generation were always purple, even
though there was a true-breeding purple and
a true-breeding white pea plant in the P generation.
The F1 only showed one trait and that was
purple.
Now when Gregor Mendel allowed this F1 generation
to self-fertilize and he planted their seeds…
Well, those offspring became known as the
second filial generation or the F2 generation.
These were the offspring of the F1 generation.
Now what Gregor Mendel saw was that consistently
there was one trait that showed up in the
F1 generation of the two possible options
from the parents.
That one trait that showed up he called the
dominant trait.
So in this case, purple flower color is considered
dominant to white.
If you cross a true-breeding purple and a
true-breeding white-flowered pea plant together,
all of their offspring had purple flowers.
Purple is the dominant trait.
Having white flowers, which is considered
to be the recessive trait, did not happen
in the F1 generation.
Instead, that recessive trait skipped the
F1 generation.
Probably what was most surprising is that
when Gregor Mendel allowed the F1 generation
to self-fertilize, you might expect all of
their offspring to have purple flowers because
everyone in the F1 generation had purple flowers.
If you're having them self-fertilize, wouldn't
their offspring also have purple flowers?
Well this was the surprising thing.
Consistently, that recessive trait for white
flowers, it showed up in the F2 generation
and it showed up in predictable numbers.
So Gregor Mendel would consistently see in
this F2 generation white-flowered pea plants
showing up about one-quarter of the time.
So again, looking at this cross, purple versus
white, both of these are true-breeding in
the P generation, their offspring, all of
them, have purple flowers.
That's the F1 generation.
But then when we allow the F1 generation to
self-fertilize and we look at the F2 generation,
we get this consistent ratio of about 3/4
of them showing the dominant trait, 1/4 of
them showing the recessive trait.
Now first, Gregor Mendel wasn't sure if this
was just a quirk of flower color or if other
traits would also show this behavior, so he
looked at several different traits and he
found for flower color, for flower position,
for seed color, seed shape, pod shape, pod
color, and stem lengths, all of these seven
traits, one of the two characteristics was
dominant to the other, that the F1 generation
would always show the dominant trait and the
recessive trait would show up predictably
in about 1/4 of the F2 generation.
So from these repeated experiments, Gregor
Mendel formulated several hypotheses.
So these hypotheses that Gregor Mendel came
up with, first, he said that there were going
to be alternative forms of the heritable characteristic,
what we call genes, and he called these different
forms alleles.
So an example of this, if we're talking about
the flower color gene, there are two options,
the purple flower color allele, or the white
flower color allele.
These are two different versions of that gene,
the flower color gene.
Now for each characteristic, an organism inherits
two alleles, One from each parent.
And it turns out, it's not just the traits
which are dominant or recessive, it's actually
the alleles which will be dominant or recessive.
And so, what we mean by this is that, if an
organism has one of each allele, both the
dominant allele and the recessive allele,
they will only show the dominant trait.
So even if they have the purple flower color
allele and the white flower color allele and
purple is dominant to white, that individual
will only have purple flowers.
The last hypothesis that Gregor Mendel came
up with is that gametes each carry only one
allele for each inherited characteristic.
So there's a few terms that I'd like to introduce
and then we can see how these hypotheses brought
Gregor Mendel to his conclusion, what we now
know as the Law of Segregation.
And so, these terms, the phenotype of an organism
is the description of that organism’s physical
traits, their characteristics.
If a pea plant has purple flowers, that is
its phenotype.
If it has white flowers, that is its phenotype.
Whether it's tall or short, yellow seeds or
green seeds, those are all examples of phenotype.
In contrast, the genotype of an organism is
specifically stating which alleles that organism
has.
If I say an individual has two copies of the
dominant or purple flower color allele, that
would be an example of genotype.
If I say an individual has one copy of the
dominant allele, one copy of the recessive
allele, that is another example of genotype.
In fact, there are some terms we use when
describing genotype.
The first of these, “homozygous”, is 
when an organism has identical alleles for
a gene.
So if an individual has two copies of the
dominant flower color allele, they would be
homozygous for that dominant allele.
If they have two copies of the recessive allele,
they would be homozygous for that recessive
allele.
In contrast, if an individual has two different
alleles, they will be heterozygous 
for those alleles.
So having one copy of the purple flower color
allele and one copy of the white flower color
allele, that individual would be heterozygous
for that flower color gene.
These two terms, homozygous and heterozygous,
they are terms we use to describe the genotype
of an organism.
But remember, phenotype is simply described
by the traits or characteristics that an organism
has, whether they look dominant or look recessive,
whether they have purple flowers or they have
white flowers.
So let's see how these terms are used when
we look at the traits and characteristics
of pea plants and their flower color.
So when we're talking about genotype, again
homozygous can either mean homozygous dominant,
having two copies of that dominant allele,
in this case the purple flower color allele,
or homozygous recessive, meaning having two
copies of that recessive allele or a white
flower color allele.
Whenever we use the term heterozygous, we
don't preface it as heterozygous dominant
or heterozygous recessive because the name
itself tells us that we have one of each.
Heterozygous is when the two alleles are not
matching.
So there are three different possible genotypes,
yet there are only two different phenotypes.
The flower color will either be purple or
white.
So when we go back to this cross that Gregor
Mendel made, the P generation was always true-breeding.
Now, another thing that's very important to
realize is that in genetics problems and genetics
terms, true-breeding is actually another way
of saying “homozygous”.
If individuals are true-breeding, they are
homozygous for the trait that they are true-breeding
for.
So if the P allele is for purple flowers and
the p allele is for white flowers, well then,
the true-breeding purple flowered parent is
going to be PP, homozygous dominant, for the
purple flower color allele.
Whereas true-breeding plant with white flowers
will be pp, it will be homozygous recessive,
having two copies of that recessive allele
for the white flowers.
Now in the F1 generation, these plants will
receive one allele from each parent and then
in the F2 generation we see that there will
be most of the offspring showing that dominant
trait, having the dominant phenotype.
Whereas a quarter of them having the recessive
phenotype.
Now what Gregor Mendel was able to do was
to take those observations and to take those
hypotheses that he came up with and to come
up with what's now known as the Law of Segregation.
And that is that each individual, each parent,
will have two alleles for each gene.
When that individual makes gametes, when they
make sex cells, those gametes will have only
one allele for each gene.
Using some of the terms we have learned when
discussing meiosis, parents are diploid, whereas
gametes, the sex cells, the sperm and egg,
they are haploid.
They will have only one allele for each gene.
Now each child, each offspring, will be the
result of two gametes fusing together.
So the offspring will be diploid, just like
the parents were, but part of the genius of
Gregor Mendel's Law of Segregation is that
if you have an individual who is heterozygous,
having two different alleles, when that individual
makes gametes, those alleles will separate
from each other evenly, and so half of the
gametes will have one allele, half of the
gametes will have the other.
Even though one allele is considered dominant
and the other is recessive.
The Law of Segregation says those two alleles
will separate evenly.
That F1 individual can make two different
types of gametes.
Half of the gametes will have the P allele,
half the gametes will have the p allele.
The two members of an allele pair segregate
or separate from each other during the production
of gametes and it's actually this even separation
of the two alleles along with the random fertilization
that happens during pollenization that ends
up giving us this F2 generation, where when
we look at the four possible combinations
of sperm and egg, with again half the sperm
being P, half the sperm being p, half the
eggs being P, half the eggs being p, is that
we end up with offspring of which three-quarters
of them will look purple and one-quarter will
look white, or if we look at the genotypic
ratios, one-quarter of them will be homozygous
dominant, one half will be heterozygous, one
quarter will be homozygous recessive.
We're actually going to see in the next video
how we can take this Law of Segregation and
use it to answer questions about predicted
probabilities of genetic crosses.
Thanks for your attention and I'll see you
in the next video!
