Blood groups are not random labels, they are inherited chemistry. Understanding one page of genetics explains why your group is what it is, why a child’s group can differ from both parents, and why some groups are rare while others are everywhere.
The ABO system
Your red cells carry sugar-based markers called antigens. The ABO system has two of them, A and B:
- Group A, A antigens on the cells; anti-B antibodies in the plasma.
- Group B, B antigens; anti-A antibodies in the plasma.
- Group AB, both antigens; neither antibody (the universal red-cell recipient).
- Group O (neither antigen; both antibodies (red cells fit everyone) the universal donor).
Your immune system attacks markers it does not carry. That single fact is why compatibility charts look the way they do, why O− red cells can go to anyone, and why the centre’s crossmatch (not a chart) gives the final word.
How a child inherits a group
You get one ABO gene from each parent. A and B are dominant over O; A and B are co-dominant with each other. So:
- A + A parents → child is A (usually), but can be O, if both parents secretly carry an O gene.
- A + B parents → child can be A, B, AB or O, any of the four.
- One parent AB, one O → child is A or B, never AB, never O.
- O + O parents → child is always O.
This is why a child’s group occasionally “doesn’t match” the parents, completely normal, and no basis for doubts about paternity. Blood-group testing alone cannot establish parentage.
The Rh factor (the + and −)
The D antigen is separate from ABO. Have it → Rh positive (about 80–95% of Indians, depending on community). Lack it → Rh negative. Like ABO, it is inherited: two Rh+ parents can have an Rh− child if both carry the recessive trait.
Rh matters most in two places: pregnancy (an Rh− mother carrying an Rh+ baby can develop antibodies, hospitals prevent this with anti-D injections) and transfusions (Rh− patients must receive Rh− red cells).
Why negative groups are rare, and why banks run short
Negative genes are simply less common in India’s population. Rough figures: AB− is around 1 in 250 people, B− about 1 in 90. The rule that hurts supply: an Rh− patient can only receive Rh− red cells, so a small donor pool faces every negative-group emergency. The rare-group register →
The Bombay blood group
India has one more twist: a very rare trait (called hh or Bombay blood group) where a person produces no H antigen at all. Standard tests label them “O”, yet their blood is incompatible with ordinary O blood, they can only receive from other Bombay-type donors. It is found more often reported in India than in most of the world, which is why rare-type registries matter here more than anywhere.
What this means for you
- Know your group, camps and centres test it free. Don’t know it? Register as “not sure” and the record updates after your first donation.
- Your group + your district decide who you can rescue in minutes, not hours.
- Negative donors are structurally precious, one registration covers patients nobody else can. Register →
One caution: compatibility charts are simplifications for red cells. Plasma works partly inverted (AB plasma is universal), and every transfusion is confirmed by laboratory crossmatch. Charts inform; the blood bank decides.