Many kinds of blood types have been found ever since Landsteiner discovered the ABO blood types in 1901. However, the ABO blood types remain the best known. The differences in blood types are classified mainly by differences in sugar chain structures on the surface of red blood cells. In Japan, personalities are often classified by blood type, but the ABO blood types are classified according to structural differences in the sugar chains on the surface of red blood cells, and no relationship has been discovered between the sugar chain and personality.
▲ The blood type having type A sugar chain is referred to as type A, the blood type having type B sugar chain is referred to as type B, and the blood type having only the sugar chain structure common to A and B is referred to as type O. Further, the blood type having both sugar chains of types A and B is referred to as type AB.
As a result of detailed investigations of each of the genes of ABO blood types in the 1990s, it has been discovered that A and B genes are blueprints of enzymes that respectively add a different sugar on the sugar chain portion of a complex glucide called "type H substance." The substance synthesized by the work of the respective enzyme is referred to as "type A substance" in the case of the A gene and "type B substance" in the case of the B gene, and a type A person has type A substance, a type B person has type B substance, and a type AB person who has both A and B genes has both type A and B substances on the surface of his/her red blood cells.
In contrast, the O gene is a blueprint that lacks both enzymes that transfer sugar chains that characterize types A and B. Accordingly, a type O person has type H substance, which is parental to type A and B substances without modification on the surface of red blood cells.
The reason a person with the AO combination becomes type A or a person with the BO combination becomes type B is that if any A or B gene exists, all of the type H substances become type A or type B substance. One may recall learning in biology class that type A and B genes have dominance over type O according to Mendel's law of heredity.
The base sequence of ABO blood type genes of representative primates has been clarified recently, and based on this it has been hypothesized how blood types came to be.
However, even with the most advanced research, the biological meaning of blood type genes has not yet been discovered. Its "meaning" shall be further investigated in the future.