Based on the figure attached at the bottom that I accidentally found, I recalled the ABO antigen theory and typed in the table all the blood types and the genetics, just to relax.

---------------------------------------俺是那土到掉渣的分割线（copy来的，呵呵）------------------------------------

Background:

On human chromosome 9, the gene coding for A Enzyme is the wild type allele, while the gene for B Enzyme a slightly mutated one. Both are dominant (codominant), responsible for convertion of H-antigen into A-antigen and B-antigen, respectively.

There is a non-functional allele encoding O enzyme (no function) (or encoding nothing? premature? forget. will check later). It is unable to carry out the convertion and, unlike A and B, it is recessive.

-----------------------------------------------俺还是那土到掉渣的分割线-----------------------------------------------

Here in the table, blue represents parents, while purple offspring. Capital letters stand for the blood types; lowercase ones for genes (alleles). You can try and check it out for fun.

Say, if you are O type, and you have a sibling with B type, check the corresponding rows and columns and you will find out that: your parents are either A×B, or B×B,  or O×B. And if you know their types already, their genetypes can be deduced and so can yours, probably.  Like, if A×B, then they are ao×bo, you oo, your sibling bo.

Kinda confusing, right? No hurt to have fun, anyway.