Multiple Allels-Human Blood Group

ABO blood groups in human beings
The ABO blood group system in human beings was established by K. Landsteiner. It is based on the presence or absence of certain antigens. There can be two anitigens A or B in the blood, resulting in four blood groups, namely A,B, AB and O. These are called ABO blood groups or Landsteiner blood
groups.
The inheritance of ABO system illustrates a new principle in geneticcontrol of phenotypes.The blood of a person having A group will have the antigen A and aperson having B group will have the antigen B. With these antigens A and Bthere are certain naturally occurrring antibodies in the serum of the blood. The
antibodies in a particular individual will be found only for those antigens which are absent in blood of this individual. The presence of antigens and antibodies occur as follows

Antibodies in the blood of ‘group A’ will agglutinize red blood corpuscles of the blood group B. Similarly the antibody in blood ‘group B’ will agglutinize red blood corpuscles of the blood group A. Since no antibody is found in ‘group AB’ blood, it will not agglutinize any other group . ‘Group O’ will have antibodies
for ‘group A’ and B. Hence ‘group O’ will agglitinize group A and B.Compatibility of donor blood to that of the recipient will occur as follows.

From the table provided it is obvious that ‘group AB’ is universal recipient. ‘Group O’ is universal donor.
The gene for ABO system is conventionally represented by the sym bol ‘I’. Alleles IA and IB code for the enzymes involved in the formation of the antigens A and B respectively and the allele ‘Io’ for no known protein. Thus the genes can result in 6 possible genotypes but four possible phenotypes as found in the table

The alleles IA and IB jointly express themselves in the individual, they are codminant. Both IA and IB are dominant to the recessive allele Io.
Disputed parentage and blood groups
The identification of blood group may help to decide in cases concerned with parentage issues. By knowing the blood groups of parents, it is possible to determine the possible blood groups in the children. The impossibilityof a particular blood group in the progeny can also be pointed out.

Rh blood group
Rh factor in blood was discovered by Landsteiner and Wiener in 1940. It was initially discovered in rabbits, immunized with the blood of Rhesus monkey. The human beings whose blood will get agglutinated with rabbit serum were designated as Rh+, and whose blood if not agglutinated were designated as Rh-. Wrong transfusion can cause agglutination of blood in the recipient Hence before transfusion of blood, along with identification of ABO blood group, it is necessary to test compatibility of Rh factor.The presence of Rh+ child in the uterus of the Rh- mother can cause agglutination in the blood of the fetus. Even though such an unfortunate incident may not happen in the first pregnancy, it could occur in successive pregnancies.
The death of the foetus in such cases is due to haemolytic anemia. This disease is called erythroblastosis fetalis.