blood types are usually made by A, B and O. A and B are dominant allele while O is recessive.
I for example, have a blood type of A. but in real, it is actually AO, with A being the dominant and O the recessive.
people with AO, or BO are normally referred as blood type A or B. therefore O is the universal donor as anybody can accept O type blood. and AB type are codominance as both A and B are dominant allele.
my dad is O (which mean he is OO) and my mom is A (she could be AO or AA, both are referred as type A blood).
OO + AA = 100% AO
OO + AO = 50% OO, 50% AO
simply take the 1st allele of parent 1, plus 1st allele of parent 2 to form the 1st group.
then 2nd allele of parent 1, plus the 1st allele of parent 2 to form the 2nd group.
3rd group will be 1st P1 + 2nd P2
4th group will be 2nd P1 + 2nd P2
eg, ab + cd => 25% ac, 25% ad, 25% bc, 25% bd
if the parents are:
AA + AA = 100% AA
AO + AA = 75% AA, 25% AO
AB + AA = 50% AA, 50% AB
AO + AO = 25% AA, 50% AO, 25% OO
BB + AA = 100% AB
and so on...
so your blood type does not always follow your parents.
you could be O even if your parents are both type A. (AO+AO)
but one thing for sure; if both your parents are O, you *MUST BE* O! unless....

ok, enough highschool biology...