|MadSci Network: Genetics|
Assuming that all of your blood typing results are correct, you are certain of the child's paternity, & there's no possibility that your baby was inadvertently switched with another in the hospital, the only other likely explanation is that either you or your husband has the Bombay Phenotype, as described in the Wikipedia.
The "expression" of the A & B blood antigens also depends upon another gene known as the H antigen. If either you or your husband do not produce this enzyme (h), then you could actually have a genetic blood type of A, or even AB, but you would appear to have an O blood type when tested. So if your baby inherited the A antigen from one of you but the normal H antigen from the other, it would have a normal A blood type but would be a "carrier" of one non-functioning h antigen "allele", which it could pass on to its own offspring. The blood type of your father & brothers doesn't really have any direct bearing on that of your baby, but since they are all type A, it does suggest that you might be the parent carrying the Bombay Phenotype, with an actual genetic A typing like them. In this case, both of your parents would have to be "carriers" of the Bombay Phenotype, too, in order for you to have inherited 2 non-functioning h antigen "alleles".
The Bombay Phenotype is very rare except in parts of India where a frequency of 1 in 7,600 has been observed, so if you or your husband has any Indian ancestry, it might explain your inheritance of this unusual trait. You may also want to look into genetic testing for you &/or your husband to confirm this possibility, since "individuals with Bombay Phenotype blood groups can only be transfused with blood from other Bombay Phenotype individuals. Given that this condition is very rare to begin with, a person with this blood group who needs an urgent blood transfusion, may be simply out of luck, as it would be quite unlikely that any blood bank would have any in stock", as explained in the Wikipedia entry.
Thanks for your unique question,
Jeff Buzby, Ph.D.
CHOC Research Institute
MadSci Genetics Network
Try the links in the MadSci Library for more information on Genetics.