In another situation, we could
alleles involved. Recall, Mendel choose things
that had duality just two different alleles.
Now in blood typing, A, B and O blood typing,
we have three potential alleles and as you
can see from the table here, there are a number
of different outcomes. Each individual may
only have two, but in the population, there
are three and so some people are blood type
A and some people are blood type AB and some
people are B and some people is O, all dependent
on which two alleles they get. This is an
example of multiple alleles. Blood typing
is interesting because it also exhibits codominance
that brings us into our next variation from
the phenotypic ratios that we might expect
in Mendelian inheritance. With blood type
A an individual's red blood cells contain
the A antigen or the A name tag and that A
name tag makes it specific to A individuals
because the immune system is not going to
attack it. Now a type B blood itself has type
B antigens and a type AB blood type displays
both antigens and O means simply no
antigens on the surface. The AB individual
in ABO blood typing scheme has codominance.
This means that both phenotypes are being
displayed. This is not an intermediate. For
example, it is not pink. It is distinctly
different in individual with type A blood
cannot take type AB blood nor can B
take AB because it is a distinct phenotype.
It is not blending inheritance. We will look
now at something that looks slightly like
blending, but is also not blending inheritance.
We call blending was the inheritance's pattern
that people were subscribing to mostly before
Mendel's time. Let us look at incomplete dominance
we do get an intermediate phenotype. We will
cross a red flower and the color allele in
this type of flower, these are not p plants
is R for red or W for white in which case
no pigment is produced. The offspring of this
cross will end up producing a pink flower.
Why does it end up producing a pink color?
Well that is because we have one R and so
one enzyme is making red color while the other
enzyme on the other chromosome is not making
red color and so we get an intermediate. In
this case, we have incomplete dominance because
the R allele is not completely dominant as
in covering up a white allele and when we
cross that F1 generation to produce an F2
generation an incomplete dominance, we see
a 1:2:1 genotypic as well as phenotypic
ratio. So I will have to I ask you a question
why is this not blending? It sure looks like
blending doesn't it. Why is it not blending?
Imagine that in this case, we have the red
and the white flower and as I explained, we
have one allele that makes red and one allele
that makes white. It looks like blending because
they become pink. But when we do the F2 cross,
we see the reemergence of white, which is
why all of these questions started anyway.
Why is it that the offspring of the F2 generation
might look something like one of the grandparents?
Incomplete dominance is not blending and nor
is codominance that we saw in the blood cells
and so definitely, we were seeing an exhibition
of Mendelian inheritance patterns however,
the phenotypes display themselves slightly
differently. We end up with different ratios
than we might expect from the simple Mendelian