which parent is O

did they stop teaching basic genetics in high school? or are there way too many under-aged readers in this site?

Um...I know you've gotten a TON of comments already, but I just wanted to try and make you feel better. I mean, you may want to talk to your parents anyway, cause it IS uncommon, but I'm a biological anthropology major with a focus in forensic anthropology and genetics, and it actually IS possible for a child of two separate blood types to end up with a different one. The average human has six mutations in their genetic structure, and although that doesn't seem like a lot, some of those mutations can evidence themselves quite clearly. Also, it is possible for some genes to be hidden and pop up later, even though they showed no evidence in the parents. We did a specific study citing a case exactly like this in Biological Anthropology when we focused on blood types, and it DOES happen, so...just something to keep in mind.

if your father is O u r adopted if your mother is O u r either adopted or she cheated on ur father ;)

OMG! It is freaking scary the number of scientific inaccuracies in this thread. #156, you are close, but wrong about the Bombay Phenotype. See http://en.wikipedia.org/wiki/Bombay_phenotype#genetics Yes it is possible, and here is how. But first some blood SCIENCE. Blood type O is recessive, but both A and B are dominant. Thus: -/- is O A/- is A A/A is A and A/A, B/- is AB So if both parents are O, then the offspring MUST be O. If one parent is AB and the other is O, then the offspring MUST be either A, or B but cannot be O or AB. If (p1) is A and (p2) is B, then the offspring can be anything: A, B, AB, or even O, depending on whether or not the parents are homozygous or heterozygous. Back to the OP's case. There are four genes, eight alleles, four each person, being discussed here. There are actually many more, but these are the "important" ones: A/A, B/B, +/+, h/h for a homozygous presence. -/-, -/-, -/-, -/- for the homozygous absence, which would be expressed as phenotype blood type O. let * represent either (for when it doesn't matter) If the parents were (P1) A/-, -/-, -/-, */-, [phenotype blood type A] (P2) */-, B/*, -/-, h/h, [phenotype blood type O] can result in an offspring: (f1) -/-, B/-, -/-, h/-, [phenotype B] Thus, it is possible. Note: The h/h Bombay genotype suppresses the expression of A and B phenotypes because the molecular precursor is absent. The h/h genotype effectively hides the genotype of the A, B, or AB blood type. Take molecular biology, cell biology, or metabolism classes to understand this one. The rest of you: Your high school genetics fails you here. Do not be so quick to think you are an expert and know everything. OP: You should set the genetics aside and care more about how your parents treat you.

My guess is your mom is the O, and she cheated?

And here I thought me and my sister were the only ones with this problem. We're both B and our parents our both O. I always did prefer the mutant theory, lol.

hahaha #3 FAIL

Wow, with the exception of a couple of comments, most posters here are completely clueless. Ask your parents and an actual geneticist before trusting any advice on here.

surprise!