It can be difficult to predict the ABO blood type of children based on the phenotypes of their parents, due to the fact that a third antigen (H) on the surface of red cells can prevent the expected ABO blood type options from occurring. 

Usually, when an A blood type mother has an O type child, the father is expected to be type O or at least to carry the O allele recessively (OO, AO, or BO genotype).

The child has inherited an O allele from both parents.  However, an O blood type child can also be born to parents who do not have the O allele if a recessive form of the allele for the H antigen also is inherited from both parents.

The H antigen is a precursor to the A and B antigens.  For instance, the B allele must be present to produce the B enzyme that modifies the H antigen to become the B antigen. As well, the A allele must be present to produce the A enzyme that modifies the H antigen to become the A antigen.  

However, when only recessive alleles for the H antigen are inherited (hh), as in the case above, the H antigen will not be produced and thus, the A and B antigens will not be produced either .  The result is an O phenotype, by default since a lack of A and B antigens is the O type.  This seemingly impossible phenotype result has been referred to as a Bombay phenotype.

The ABO blood system is further complicated by the fact that there are two subtypes of type A and two subtypes of type AB.  These are A₁,  A₂,  A₁B,  and A₂B.

Additional Resources:

~ First identified in Mumbai, from which the group derives its name, so far there have been   just 179 such cases reported in Indian....read article.

~ A description of Bombay Blood (Reads as Type O+ hh) from Wikipedia...read here.

The Indian blood group system (In) is named because 4% of the population in India possess it. It is a classification of blood based on the presence or absence of inherited antigens that reside within the CD44 molecule that is expressed on the surface of blood cells.

Could your Autoimmune Condition, Chronic Pain or Illness be somehow

related to your Rh-Negative Blood or Genetic Family Line?

Microchimerism is defined as the presence of a small number of cells that originated from another individual and are therefore genetically distinct from the cells of the host individual. This phenomenon may be related to certain types of autoimmune conditions and diseases.  The cause or mechanisms responsible for this relationship are unclear and the effects are still being researched.

What is know is that during pregnancy, a two-way traffic of immune cells may occur through the placenta. Exchanged cells can multiply and establish long-lasting cell lines that are immunologically active even decades after giving birth.

In humans the most common form of microchimerism is fetomaternal microchimerism which is also known as fetal cell microchimerism or fetal chimerism. This occurs when cells from a fetus pass through the placenta and establish cell lineages within the mother. Reports and studies show that fetal cells have been documented to stay within the mother and multiply for several decades. The exact phenotype of these cells is still unknown, although several different cell types have been identified.  Some of these include various immune lineages, mesenchymal stem cells, and placental-derived cells.

The potential health consequences and or benefits of these cells are currently unknown, but are being researched. One popular hypothesis is that these fetal cells might trigger a graft-versus-host reaction in the mother and could lead to autoimmune disease.

The Rhesus (Rh) blood group is the most polymorphic human blood group system, and it is clinically significant in transfusion medicine. About 15% of Caucasoid people are RhD-negative, whereas in the Asian population, the RhD-negative blood type only occurs in 0.1% to 0.5%. However, approximately 30% of apparently RhD-negative Taiwanese people actually were RhD_el.

Read Study, click here >