Apart from the Y sex chromosome, all of Jesus' genetic material would have come from his mother Mary. The gene for blood type is not on the Y chromosome, and therefore came from Mary, who also would have blood type AB.
Okay. I may have this wrong, and I'm sure someone more knowledgable will correct me if I'm wrong.
First, mother and father both pass on on an allele for blood type. The dominate type will determine the type of the child (father's do have an x chromosome)
So if father passes B and mother passes A then the child will be AB---this works the other way around too.
There must be two alleles to have AB. A and B are codominant, so that's how that happens.
Now---if Jesus had type AB, then one allele came from father, and one came from mother. Arguing that he had type AB kind of makes the existence of that other allele necessary. AB are not attached to one allele, but are the merging of two separate alleles.
So if that blood type is correct, then EITHER Mary contributed TWO alleles---which goes against all nature. OR she was either type A, type B or type AB, and passed either A or B on, to be met by a father that was the same and passed the opposing type. So are we saying that God passed on one of those alleles?
Don't know if that made any sense, but it's the best I can do.