So lets think a little bit about genetics as its related to the human A B O
blood type
this is a really interesting situation because it exhibits
both codominance and multiple alleles
in the situation of codominance when there
are two different alleles both
are expressed equally this is also a situation in which we have multiple
alleles
for one particular gene locus in the human A B O group there are three
alleles that may be involved with determining ones blood type
course any individual only has
two of each allele but there are three
in the population one of these we refer to as the
I A allele capital I capital A is just a standard convention but you could use
a different
system if you wanted to but the IA allele codes for
antigen A and I've just represented antigen A on this red blood cell
this circle is representing a red blood cell
and the little triangles are just representing
the A antigen that's found on the
surface of the red blood cells the I B
allele codes for antigen B and I've represented
antigen B as a little semicircle on this
circle which represents a red blood cell
then we have the little i allele and the little i
allele doesn't code for an antigen like antigen A or antigen B and so
I've drawn a red blood cell here that doesn't have an antigen
displayed on its surface so three different alleles
are involved with determining the human A B O
blood type so I've
created a little chart here that lists the different
human blood types and it shows the genotypes that are available
so blood type would be the phenotype or the physical characteristic
in this situation and then the genotypes are
which combination of alleles the individual might have
so folks with type A blood could be
homozygous I A I A or
they could be I A little i in both situations
they have the A antigen
on their red blood cells people that have type B
blood are either I B I B
or I B little i
in both situations they have the B
antigen on the surface of their red blood cells
then there are folks that have type AB blood
in order to have type AB blood
one must have both the I A allele
and the I B allele and so type AB blood
would look like this the I A
allele would cause the A
antigen to be produced and displayed on the surface of the red blood cell
membrane
and the I B allele would cause
the B antigen to be produced
and displayed on the surface of the red blood cell so they have
both antigens on their red blood cells
then we have the type O
blood type and the only possible way to get type O blood
is for one to have a homozygous little
i situation so little i little i
results in type O blood and this is what a type O blood cell would look like no
antigens displayed on the surface of those
membranes so human A B O blood type
is an example of codominance and
multiple alleles