The ABO System

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It is important to get the correct blood when you are receiving a blood transfusion. Fortunately doctors understand this and always make sure that they use the right stuff. You don't need to know what blood group you are, they will test you. It only takes a small drop of your blood and a couple of minutes. It is usually sufficient to test for the ABO and Rhesus Factor. In the good old days we could do this test at school, but now that everyone is terrified of catching hepatitis or AIDs we don't do it anymore which is a great pity since it was great fun. Perhaps you have a nice doctor who will show you the test for blood groups on your next visit.

Surgeons must be even more careful when doing organ transplants; this is to prevent the organ being rejected. So when hospitals do transplants they are very careful to compare tissue types between the potential donor and the recipient. Providing that the organ donor and recipient have the same tissue type, the transplant should work.

Since blood groups are inherited it is sometimes possible to prove that someone cannot possibly be the parent of a child. Now that scientists have developed the techniques for comparing DNA it is much easier to prove that someone IS the parent of a particular child.

 
There are four possible blood groups in the ABO system:
AB
A
B
O

Everybody belongs to one of these four groups. You may have other antigens on your red blood cells, e.g. the Rhesus factor. Don't worry if you do not have the same blood group as your parents: when you understand how blood groups are inherited you will realise that it is perfectly possible to have a different blood group. You might be unfortunate and discover that you must have been adopted; your adoptive parents should have told you about this already!

 

These are only two types of blood in the Rhesus system:
Rhesus positive
Rhesus negative

You must belong to one of these two groups.

 
Now we can multiply the ABO system by the Rhesus Factor to get 8 possible blood groups.
AB +ve
AB -ve
A +ve
A -ve
B +ve
B -ve
O +ve
O -ve

So you must belong to one of these eight blood groups. I am A +ve.

 

The inheritance of the Rhesus Factor is straight forward, but the inheritance of the ABO system is a little more complicated and interesting. For the Rhesus Factor there is a dominant gene and a recessive gene, so the pattern of inheritance is just like any other factor. For the ABO system there are three alleles. Two of them are co-dominant and the other is recessive.

 

A and B are antigens found on the surfaces of red blood cells. If you have blood group AB it is because both antigens are present. If you have blood group O it is because neither antigen is present. The ability to form these antigens on the surfaces of red blood cells is inherited. People with Blood Group A only have antigen A on their red blood cells and people with Blood Group B only have antigen B.

 

There are three alleles:
Ia    this one produces the antigen A on the surfaces of red blood cells,
Ib    this one produces the antigen B on the surfaces of red blood cells,
Io    this is a recessive gene; neither antigen is produced.

 
Ia and Ib are co-dominant. That means that if both alleles are present, both antigens (A and B) will be formed on the surfaces of red blood cells and you will have Blood Group AB.

 

Io is recessive; neither antigen is produced; so you will have Blood Group O if you have two of these alleles.

 

Your phenotype is the blood group to which you belong, i.e. AB, A, B or O. Your genotype is your genetic make-up, i.e. which alleles you have inherited. There are 6 possibilities:
Ia Ib
Ia Ia
Ia Io
Ib Ib
Ib Io
Io Io

The first possibility is that you have inherited Ia from one parent and Ib from the other; you will have Blood Group AB.

Or you could have inherited the recessive Io allele from both parents. In this case you will have Blood Group O.

In between, if you have inherited either one or two Ia alleles you will have Blood Group A and if you have inherited either one or two Ib alleles you will have Blood Group B.

 

Now you must start to work out how it all happens. So for example:
If either parent has Group AB it will not be possible for any of the children to inherit Group O.
If both parents have group O, all their children must have Group O.
If both parents have Group A, it is still possible for some of their children to have Group O.

 

You can try out the other combinations. E-mail me with your answer if you are not sure that you are right. However, if you use your common sense, you will be right.

 

[ Mendel's Laws ] [ The ABO System ]

 

Last revised: 19 November 2006.


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