How Blood Groups Work

There are 4 different blood groups in the ABO system; A, B, AB and O
There are 2 different blood groups in the Rhesus Factor system; Positive and Negative

This gives a total of 8 different combinations; A+, A-, B+, B-, AB+, AB-, O+, O-

You get one gene in the ABO system from your Mum and one from your Dad.
You get one gene in the Rhesus system from your Mum and one from your Dad.

For the Rhesus Factor, the gene for Rhesus positive is dominant to the gene for Rhesus negative. This gene gives you Rhesus antigens on the surface of the cell. So, if you have one positive gene and one negative gene, you will be Rhesus positive because the positive gene will be making the antigens. One positive gene, One negative gene= Rhesus positive
Two positive genes = Rhesus positive
Two negative genes = Rhesus negative

The ABO system is a little different because you can have both A and B antigens on the cell at one time. The genes for these antigens are considered co-dominant. If you have no antigens on the cell, you have O blood group. O is recessive.

One A gene + One O gene = AO = A blood group
Two A genes = AA = A blood group
One B gene + One O gene = BO = B blood group
Two B genes = BB = B blood group
One A gene + One B gene = AB = AB blood group
Two O genes = OO = O blood group

A parent who is AB blood group has one A gene and one B gene that they can pass on. If they pass on the A, they could have a child who is AA, AO of AB depending on what the other parent contributes.

If a Mum is Rhesus negative, (has two negative genes, and no Rhesus antigens on their cells) and has a partner who is Rhesus positive (two Rhesus positive genes OR one positive and one negative gene), their child may have antigens on their blood cells (if they inherit the positive gene from Dad).

If the baby has antigens on their blood cells, and these go into Mum's blood, Mum's blood can make Antibodies to the Antigen. Antibodies don't harm Mum, but if she has another baby that is Rhesus positive, Mum's antibodies will increase and these can cross the placenta and damage the baby's blood cells. It is to avoid this potential, that Rhesus negative Mums (with rhesus positive partners) are given the Anti-D injection.