A person has met with an accident and needs blood transfusion. However, when tested for blood group, his blood was found to agglutinate blood of types A, B as well as O. His both parents have blood group A. What is the genotype of this person for this trait? How can we explain these observations?

Question: A person has met with an accident and needs blood transfusion. However, when tested for blood group, his blood was found to agglutinate blood of types A, B as well as O. His both parents have blood group A. What is the genotype of this person for this trait? How can we explain these observations?

Normally, humans have four main blood groups: A, B, AB and O. These are decided by a gene with three alleles: Iᴬ, Iᴮ and i. The alleles Iᴬ and Iᴮ are dominant and code for A and B antigens on red blood cells, while the i allele is recessive and does not produce any antigen. Blood group O people have genotype ii and their red blood cells show no A or B antigens.

But these A or B antigens are not formed directly. They are actually built upon a base molecule called the H antigen. This H antigen works like a platform or foundation where A or B sugars attach to form the A or B antigen. The H antigen itself is made by a completely different gene, called the H gene, which has two forms:
  • H (dominant) – makes the H antigen normally
  • h (recessive) – does not make the H antigen
If a person has hh genotype, then no H antigen is formed, so even if the person has Iᴬ or Iᴮ alleles, the A or B antigen cannot be displayed on red blood cells. In this condition, the person's red blood cells appear to be without any antigen, just like group O, but their immune system reacts not only to A and B antigens but also to H antigen. This condition is called the Bombay phenotype.

Now, in this question, the person's blood agglutinates (clumps) with A, B and O blood, which is unusual because:
  • A and B blood groups contain A or B antigens, and also H antigen
  • O group does not have A or B antigens, but still has H antigen
Since this person's blood reacts even to O group, it clearly shows that his blood lacks the H antigen and has developed anti-H antibodies, which attack even the H antigen found in O blood. This is only possible in Bombay phenotype, where genotype is hh.

Now, the parents are both A blood group, which means they could be IᴬIᴬ or Iᴬi. Also, they might each carry one recessive h allele: Hh. In that case, they can pass Iᴬ or i and h to their child. If the child receives Iᴬ or i from both sides and h from both sides, then their genotype becomes:
  • ABO system: IᴬIᴬ or Iᴬi
  • H gene: hh
Due to hh, the person cannot express A antigen, even though their ABO genotype is A type. That's why the blood behaves like O, but is not truly O, it is Bombay phenotype, which was first discovered by Dr. Y. M. Bhende in 1952 in Bombay, hence the name.

This person cannot receive blood from A, B or O donors, because their blood contains anti-A, anti-B and also anti-H antibodies. So, they can only receive blood from another Bombay phenotype individual, which is very rare.

Final Genotype of this person:

  • ABO gene: IᴬIᴬ or Iᴬi
  • H gene: hh




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