As anyone will learn in a simple genetics class, blood types are
determined by the type of antigens produced by an individual (with out
regard to the positive/negitive).  If a person has two parents with A
blood, they can only have A or O blood.  The same for a person with B
(except it's B or O).  However, if one parent has AB blood and the
other parent has A, B or O, the child can only have A, B or AB blood,
because they will either inherit the A or the B from their first
parent and either an A, B, or a sequence for not producing any antigen
at all, which will default to either the A or B inherited from the
other parent.

This is obviously a very basic understanding of this phenonenon.  I
have brought the following problem up in many classes, to the
confusion of many of my teachers:

I have O Positive blood.  My mother has O positive, and my father has
AB negitive.  In theory, this should dictate that I have either A or B
blood, but I don't.  I know that my father was my real father, as I
have a genetically inherited condition (Polysystic Kidney Disease)
that he has.  My blood type is correct, as I recently gave blood and
the Red Cross returned my donor card that said my blood type was O
Positive.  My father's blood type is correct, as he went through
barrages of tests before his kidney transplant about ten years ago.  I
believe my sister also has O positive (but I could be wrong).

My question is, how is this possible?

This is the "Bombay phenotype".  It results from being homozygous
recessive for the 'h' gene needed to produce either the A or B
antigens.

Hello rmn,

I would tend to agree with xarqi about you having the Bombay
Phenotype.  This is not a problem and does not in itself affect your
health in any way.  To sum it up briefly, Bombay phenotypes occur when
you have 2 recessive "h antigen" genes.  These antigens, when active,
are responsible for the production of the A or B antigens from the
inherited genes.  When H antigens are expressed (as in most people)
the genetic ABO profile of a person and their phenotypic ABO profile
match and make sense.  Since standard ABO and crossmatching tests done
at hospitals, blood banks, etc do not screen for the H antigen, your
case may have gone unnoticed.  You may specifically ask to be tested
for H antigen at most hematology labs should you need further
confirmation.

If this is confirmed, you may even pass on your father's A or B
genotype to one of your children (even if your wife is O).

Here is a good website to explain what Bombay Phenotype
http://www.bh.rmit.edu.au/mls/subjects/abo/resources/frequency4.htm

Good luck

telnady

Here is another set of references that may help you understand this.
The second page can also be referenced via a tab on the first page,
and both are a little less "technical" than the source mentioned in
the previous comment, but the conclusion is the same.

http://anthro.palomar.edu/blood/ABO_system.htm
http://anthro.palomar.edu/blood/Bombay_pheno.htm