As you know, humans have four main blood types. The first, second and third are quite common, the fourth is not so widespread. This classification is based on the content of so-called agglutinogens in the blood - antigens responsible for the formation of antibodies. The second blood group contains antigen A, the third contains antigen B, the fourth contains both of these antigens, and the first contains antigens A and B, but there is a "primary" antigen H, which, among other things, serves as a "building material" for the production of antigens contained in the second, third and fourth blood groups.
The blood group is most often determined by heredity, for example, if the parents have the second and third groups, the child can have any of the four, in the case when the father and mother have the first group, their children will also have the first, and if, say, the parents have the fourth and the first, the child will have either the second or the third. However, in some cases, children are born with a blood group that, according to the rules of inheritance, they cannot have - this phenomenon is called the Bombay phenomenon, or Bombay blood.
Bombay blood does not contain antigens A and B, therefore it is often confused with the first group, but it also lacks the H antigen, which can become a problem, for example, when determining paternity - after all, a child does not have a single antigen in the blood that does his from the parents.
The Bombay phenomenon was discovered in 1952 in India, where, according to statistics, 0.01% of the population have "special" blood, in Europe, Bombay blood is found even less often - about 0.0001% of the population.
A rare blood group does not give its owner any problems, except for one - if he suddenly needs a blood transfusion, then only the same Bombay one can be used, and this blood can be transfused to a person with any group without any consequences.