Multiple Alleles
The term multiple allele is a condition where more than two genes occupy the same locus, on the same pair of homologous chromosomes, in different organisms. Each of these genes expresses a totally different character. The inheritance of A B O blood groups in man is an example of multiple alleles. The four blood groups A, B, AB and O are due to the presence of three genes occupying the same locus on the same pair of chromosomes in different human beings. The discovery of blood groups dates back to the year 1900. A German doctor by name Carl Landsteiner discovered the four blood groups in man. He was examining the reason for the instant death of many persons immediately after a blood transfusion. He isolated the plasma and RBC from the blood samples of different persons. He found that whenever the plasma was mixed with the RBC of the same person, the mixture was smooth. However, when the plasma and RBC belonged to different persons, the mixture was found to be smooth in some cases and clumped in others. Detailed analysis conducted by Landsteiner showed that the human blood contained two specific substances called antigens, which are responsible for either smooth mixing or clumping. He named these antigens as antigen A and antigen B. Based on the presence or absence of these antigens, he classified the human blood into four groups namely A, B, AB and O.
The following table represents the antigens and the corresponding antibodies found in the four blood groups of man.

Blood Group----- Antigen in RBC--------Antibody in plasma
A----------only antigen A -------------------Antibody-b
B-----------only antigen in B------------Antibody-a
AB------ Both antigens A and B ---------------------------Nil
O------- Neither-----------------------------------Both antibody a and b





Blood Transfusion

The transfer of blood from one person to another is called blood transfusion. In all cases of blood transfusion, it is necessary to match the blood group of the recipient with the blood group of donor. The following table represents the blood group matching.







From the table it is clear that persons with blood group AB can receive blood from any other person. Hence they are commonly described as universal recipients persons with blood group O can donate blood to any other person. Hence, they are commonly described as universal donors.

Blood groups............ Genotypes
A ........ IAIb or IAIO
B ......... Ibb or IbIO
AB ...... IAIb
O ....... IOIO


The fruit-fly, Drosphila melanogaster has 15 alleles for eye colour. In rabbits, there are 4 alleles for colour. In all these cases, at any given time, only two of the alleles can occupy the same locus on a pair of homologous chromosomes.