Is a person's blood type related to his/her parents blood type? What makes a blood type?

Answer:
The first 7 answers are either dead wrong or off the mark to some extent. The blood group is determined by proteins (called antigens) on the surface of the red blood cells in our blood stream.
There are two kinds of antigens- A and B. There could also be no antigen (O)
Humans have two sets of chromosomes and hence genes for everything. We get one set from our mother and one from our father.
Thus we have two genes for the blood group antigen each, one from the mother and one from the father.
If both genes are for blood group A or if one is for blood group A and the other is for no antigen (O) then we have blood group A (Written as AA or AO)
If both genes are for blood group B or one for B and one for no antigen (O) then the blood group is B (written as BB or BO)
If there is one gene for A and the other for B we have group AB
If both genes are for no antigen (O) then we have blood group O.
Obviously the blood groups depends on the parents blood group but we can have a large number of combinations.
1. Parents are A (AA or AO) and B (BB or BO). The kids could be A (AO), B (OB), AB or O (OO).
2. If the parents are A (AA or AO) and O (OO). The kids could be A (AO) or O(OO) but cannot be B or AB.
3. If the parents are AB and A (AA or AO). The kids could be A (AA or AO), B (BO), or AB (BA) but cannot be O.
4. If the parents are AB and O. The kids can only be A (AO) or B (BO).

and so on and so forth.

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