Blood Groups Genetics
Blood groups are inherited. Most familiar are the blood types of the A-B-O group and Rh types, but several others have been found that display the antigen-antibody reaction.
In the seventeenth century, the idea of saving lives by transfusion was first proposed. Many of these early attempts resulted in death until German-born immunologist Karl Landsteiner (1864-1943) found that mixing blood from different individuals would, in some cases, cause the mixed cells to form a clump. This clumping of cells was responsible for the deaths of many of the early transfusion patients. Landsteiner concluded that substances on the surface of the red cells were responsible for the clumping reaction, these proteins were called these antigen A and B. Blood cells without these antigens were placed into the O group (pronounced as the letter "O"). Landsteiner also found a type AB type cell that has both antigens.
The blood types became the first recognized human polymorphisms (Greek for many forms). Polymorphisms are two or more distinct kinds of a trait. Different variants of a particular gene resulting in variant traits are called alleles.
Discovery of the ABO system challenged the Mendelian law of dominant and recessive traits. The blood group is an example of co-dominance and also shows in a population there may be more than two alleles, as in the case of blood groups-- A, B, and O.
The ABO gene is located on chromosome nine at 9q34.1, and one of the three alleles is inherited from each parent. The phenotype is the characteristic the person has. The genotype is the genetic makeup. Blood groups inheritance follow these patterns: A and B are dominant over O; four phenotypes (the way a person appears) are A, B, AB, O; six genotypes (the persons genetic makeup) are AA, AO, BB, BO, AB, OO. O produces no antigens and is recessive. A person with the rare type AB has an A from one parent and a B from the other. The alleles are co-dominant.
The Rhesus (RH) system is an important blood system. Landsteiner and colleagues discovered the factor in 1940, when they produced an antibody to the red cells of the Rhesus monkey in rabbits and guinea pigs. Landsteiner's research showed this factor agglutinated 85% of the human population, and these people were described as Rh positive. If the cells did not clump, they were said to be Rh negative. The problem for humans arises when an Rh negative mother carries an Rh positive fetus. The mother becomes sensitized and builds up antibodies to the Rh factor. If she carries a second child, the mother's antibodies attack the child's red blood cells causing hemolytic disease of the newborn (HDN).
Inheritance of the Rh system is complex. Three genes make the Rhesus antigens, C, D, and E, and are located on chromosome one. Two possible alleles are at each locus c or C, d or D, e or E. An individual with at least one C, D, E is Rh positive. Those with cde/cde(rr) are Rh negative.
The MN blood types are also antigens located on the human blood cell. The antigens are polypeptides coded directly by genes on chromosome four. The genes are close together on the chromosome. Other blood group systems are the P system, the Lutheran (LU) system (rare), the Kell system, the Lewis system, the Duffy system, and the Kidd system.
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