In response to the original poster's question about how the mother could be type AB and the child type O, I offer two POSSIBLE explanations:1.) A non disjunction event occurred during Anaphase I of oogenesis, and the egg that was later fertilized ended up with no copies of Chromosome 9. In other words, when your mother was producing eggs many years ago, one of those eggs had a problem during replication and ended up with no copies of the chromosome that carries the bloodtype allele. Therefore, your mother's bloodtype genes had NO IMPACT on your bloodtype genotype. In this case, you would have inherited only one chromosome 9 (from your father), and thus would have whatever genotype that allele corresponded to (so if your father is bloodtype O, you would definitely be bloodtype O). If this explanation is true, you only have 45 chromosomes (called monosomy) and are missing a copy of chromosome 9. A genetic counselor could confirm this by performing a karyotype.2.) The different between the Bloodtype A allele (iA) and blood type O allele (i) is very small: deleting a single G base at position 261 in the iA allele will cause it to become a i allele. The different between these two alleles is one base. It is possible that a spontaneous mutation occured, causing a iA allele in your mother's eggs to become an i allele, and hence you became a type O child.
