- [Voiceover] So today
we're gonna talk about
Co-Dominance and Incomplete Dominance,
but first let's review the
example of a blood type
and how someone with the same two alleles
coding for the same trait
would be called homozygous
and someone with different alleles
would be called heterozygous.
Also remember, the concept of dominant
and recessive alleles
and how the A allele is dominant
over the O allele in this example.
This means that the same phenotype,
blood type A, can result from these
two different genotypes.
Now, the example that I just gave you
was an example of Complete Dominance.
So if a person had a genotype AO,
since our phenotype is just blood type A,
it means that the A allele
is completely dominant over the O allele
and only the A allele from the genotype
is expressed in the phenotype.
But there are actually
three different patterns
of dominance that I want
you to be familiar with
and to explain this I'm going to use
a different example.
Let's say we have this flower
and the red petal phenotype is coded for
by the red R allele
and the blue flower phenotype is coded for
by the blue R allele.
So I'm going to introduce
three different patterns of dominance
and they are complete dominance,
which you've already heard of,
co-dominance, and also
incomplete dominance.
I'm going to explain what these two
new patterns are through
this flower example.
Let's start by looking at
three different genotypes
and the phenotypes that you would see
for each of them under each
different dominance pattern.
We'll start with the genotype, two red Rs,
which we could expect that in all cases
the flower petals will be red
since we only have red Rs in the genotype.
Similarly, if our genotype had two blue Rs
then we could expect that in all cases
the flower petals will be blue
since we only have blue
Rs in the genotype.
Now these three different
dominance patterns
change when we look at
the heterozygous example.
That's what makes these
three patterns different.
Now we're already
familiar with the example
of complete dominance, so if we said
that the red R is dominant over the blue R
then this would make the
heterozygous phenotype
a red flower for complete dominance.
Now what co-dominance is,
is when the heterozygous phenotype
shows a flower with some red petals
and some blue petals.
So it's when the two alleles
are dominant together
they are co-dominant and traits
of both alleles show up in the phenotype.
Now what incomplete dominance is,
is when the heterozygous phenotype
shows a mixture of the two alleles.
So in this case the red
and blue flower petals
may combine to form a purple flower.
Neither allele is completely
dominant over the other
and instead the two, being incompletely
dominant, mix together.
So what did we learn?
Well, if we assume the
heterozygous genotype,
red R, blue R, then
there are three different
dominance patterns that we might see
for a specific trait.
In complete dominance,
only one allele in the genotype,
the dominant allele, is
seen in the phenotype.
And this was the example
with the red flower.
In co-dominance, both
alleles in the genotype
are seen in the phenotype.
This was the example with the flower
with both red and blue petals.
Finally, in incomplete dominance,
a mixture of the alleles in the genotype
is seen in the phenotype
and this was the example
with the purple flower.
