In this lesson on inheritance
of qualitative traits,
we focus on segregation 
and independent assortment,
which are Mendel's
first and second laws.
And what I want to do 
instead of just trying to
have you memorize 
these things by brute force,
I'd like to try to connect
them with what you
already understand about 
the mechanics of meiosis.
It all fits together 
as a system.
And I think it's easier 
to remember if we can
see how these connect, 
rather than just trying to
memorize them as completely
separate mechanisms.
So let's start out 
by taking a look at
the inheritance of 
qualitative traits,
where we're just looking at 
one trait at a time.
In the lesson I talk about
Mendel's peas, and
in particular the inheritance
of smooth versus wrinkled peas.
So instead of going through
the same treatment
that I used in the lesson, 
let's take a look at this
as if we're working
through meiosis.
So in the inheritance of
smooth versus wrinkled,
what Mendel did was 
cross a plant that was smooth
with a plant that was wrinkled.
So let's start
with the parents.
One of the parents, 
the smooth parent,
is going to look like this.
It's going to have, remember,
two homologous chromosomes
that are going to carry 
the gene for smooth or wrinkled.
And in the case of
the smooth parent,
both of the homologues are
going to have the capital S,
the dominant gene
for the smooth pea.
So if this is, say, the male
parent, just arbitrarily,
we're going to have 
one of the homologues,
is going to carry the big S
allele at the locus for
the seed smoothness.
And the other homologue
is going to have
that same capital S allele.
It's going to go through
replication and
form sister chromatids, at 
which point our chromosomes
are going to look like this.
So now we have 
the sister chromatids and
we're heading into meiosis.
Meanwhile, on the female side,
the same thing is happening.
So let's say where the
male is big S big S,
the female is our
wrinkled parent,
and it's going to have
genotype little s little s.
So its homologues are
going to look different.
Because it's going to have
the little s allele at the locus
for seed smoothness or
wrinkledness.
And when it goes for
replication prior to meiosis,
now the sister
chromatids look like this.
Getting back to the male,
the homozygous dominant,
when all is said and done 
and meiosis is over,
all of the gametes are 
going to be identical
that come out of this parent.
They're all going
to look like this.
This is our sperm.
We're going to have 
this particular chromosome,
and the allele along there is
going to be called capital S.
All the gametes from 
the male side are going to be
capital S, the only possible.
Look at all these
sister chromatids.
All they are, are capital S.
So that's the only thing
that can be in the gamete.
In contrast, when 
we take a look at the
homozygous recessive,
the only possible allele in
the gamete from this parent is
going to be the small s allele.
Because when we look at
the sister chromatids,
that's all that's there.
So when we cross the
homozygous dominant
big S big S by the homozygous
recessive, little s little s,
then the F1 is always going
to be the combination of
these two particular gametes,
and so the zygote is going to
have the big S allele from, 
in this case, the male
and the little s allele 
from the female.
And so it's going to be
big S little s heterozygote.
Now things get a little
more interesting
when Mendel
self-pollinated this F1.
So let's see
what happens there.
So in this case
looking at the male again,
it's big S little s, and 
the female is also
going to be 
heterozygous big S big S.
Because again, what
we're doing is selfing
the F1 that we just created
that was a heterozygote.
So when you're selfing 
a heterozygote, it means
the male is a heterozygote
and, it's the same plant,
the female is also 
a heterozygote.
So now the question is, 
what sort of
gametes can be produced?
Well again we take a look at
what we have in this cell.
We have a chromosome 
that has a big S and
we have a chromosome 
that has a little s.
And they're going to
go through replication,
so that now we have
the two homologues,
where these two sister
chromatids are both big S,
and these two sister
chromatids are little s.
Now exactly the same thing
happens over here
when the female is
undergoing meiosis.
It's going to 
look identical, because
the genotype is the same.
Big S for one of
the chromosomes,
and because it's
a heterozygote,
little s for the other two
sister chromatids
making up the other homologue.
Now what sort of
gametes do we get from this?
Well, there are two types of
gametes that can result
from meiosis here.
One of the gametes is going
to be a big S allele gamete.
And the other one is going to
be a little s type of gamete.
And they're in equal proportion,
equal frequency.
So these are 
the two types of gametes
that result from 
meiosis in this cell.
And because these two are
identical, then this cell
on the female side is going to
have the same type of gametes.
So it's going to have a gamete
that has the big S allele,
and also gametes that
have the little s allele.
So these are the two types
of gametes on the male side,
and the same two types of
gametes on the female side.
Now the question is,
what sort of progeny,
what offspring, are
going to result?
This is where we use
the Punnett square.
And in the lesson, what I did
was put the types of gametes
from the male side on
the top of this Punnett square.
So one of our gamete types is S,
big S and small s.
And I put the female 
gametes that
I'm just erasing down 
the left hand column.
One of the gametes was big S,
and the other was little s.
And I arrange this 
as a bit of a database.
Then we can do the mating.
We have a male gamete 
that's big S and
a female gamete that's big S.
The zygote that's going to form
is the homozygous dominant.
If the male is small s
and the female big S,
then it's going to
be heterozygous.
Likewise if the female is
small s and the male big S,
it's also going to
be heterozygous.
You notice that when 
I put this notation down,
it doesn't mean that 
one is from the male and
the other is from the female.
You always put the capital first
just by convention.
Then if you have
a recessive s female gamete with
a recessive s male gamete, you
get the homozygous recessive.
So that's how you
use a Punnett square.
You put the gametes
from the male on top,
the gametes from the female
along the left hand margin
of the rows.
They're in equal frequencies.
And then you can figure out
the genotype ratios
and the phenotype ratios.
So the genotype ratios are, 
for big S big S, is 1.
The heterozygote is 2, 
here and here.
And the homozygous 
recessive is 1.
So the genotype
ratio is 1 to 2 to 1.
Genotype.
But, when we ask
what's the phenotype ratio,
and that means what do
the seeds look like to us,
then remember these three
categories all look the same.
Because the capital S is
dominant over the small s.
So these look
the same as this.
So the phenotype
ratio is smooth equals 3,
and wrinkled equals 1, 
or 3 to 1 phenotypic ratio.
So the idea here was to take
a look at Mendel's first law
of segregation, in which we find
that when we have two alleles,
one of those alleles is going
to be found in one gamete
and the other allele 
in another gamete.
And you never find 
those two alleles together
in the same gamete.
They split up into
separate gametes.
They segregate.
And when we follow that
through meiosis and take a look
at what happens in 
the gametes and how they
then come together to form
zygotes, then in this case
we have a genotypic ratio
of 1 to 2 to 1.
The 1's are the
homozygous classes,
homozygous dominant,
the 2 is the heterozygote,
and the 1 on this side is 
the homozygous recessive.
But how they look,
the phenotypic ratio,
00:11:23.550,00:00:00.000
is 3 to 1, with  3 smooth 
and 1 wrinkled.
