Landsteiner worked in detail on this and came to the conclusion that the mixing or clumping depends on the antigen and antibody reaction present in the blood.

He discovered that there are two antigens in the blood A and B and two serum antibodies that clump or agglutinate them. Correspondingly the blood of all human beings could be classified into four types based on the antigen present in the RBC.

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These are:

1. Antigen A – Blood group A

2. Antigen B – Blood group B

3. Both Antigen A and B – Blood group AB

4. No Antigen – Blood group O

Before we proceed on further discussion we will try to understand what is meant by antigen – antibody reaction.

Blood is a connective tissue in fluid form. It has two main parts – cells and plasma. The cells are RBC, WBC, and platelets. The plasma consists of protein, fibrinogen and the serum.

When the serum is injected into the blood stream of another organism, a substance is produced in the blood which reacts with the serum protein and neutralizes it.

The substance produced by the blood cell is known as antibody and the protein in the serum which causes the production of antibody is called antigen.

Antibody is also-called Agglutinin (because it agglutinates) and antigen is also called Agglutinogen (because it causes agglutination). The reaction between antibody and antigen is very specific and is often compared to the lock and key mechanism.

Landsteiner discovered A, B and O blood groups. His students Von Decastello and Sturii discovered the AB groups. Besides these, scientific investigations has also proved the existence of two other blood groups -Viz MN blood groups and by a single factor.

A, B, AB and O Groups:

In these blood groups, some of them contain natural antibodies against certain others. It has been pointed out already, there are two antigens (agglutinogen) A and B and there are two antibodies (agglitinm)

A and B. Persons of blood group A contain within the serum of their blood an antibody which agglutinates RBC of blood group B. Group B serum has antibody against A. The AB group serum however has no antibodies while O group has both antibodies for A and B.

Hence O group cannot receive blood from any group except itself. But .it has no antigens and can be given to any other group it does not cause the production of antibodies in blood types.

During blood transfusion if the donor’s blood has antigens for the recipient, it will harm the individual it the recipient’s blood has antibodies for the donor’s blood.

For e.g. a person with O group has antibodies for both A and B and cannot receive blood from either because the antigens in A and B cause the production of more antibodies and the serum of O agglutinates RBC of both A and B.

Hence O group people can receive blood only from O group. On the other hand O blood can be given to all others because it has no antigen for A and B.

Hence when it is injected to other groups, it does not cause the production of antibodies in the serum of the recipient and hence no agglutination takes place.

This is the reason why O group is known as the Universal donor. In the same way AB group is known as Universal recipient.

Genetic basis of A, B, 0 Blood Groups:

It is believed by many that O is the original gene, and two dominant mutations from this have given rise to A and B blood groups. The blood groups are determined by a series of three allelic genes viz.

Alleles IA and IB are dominant over are allele. But when they are together, they behave as an intermediate or compound producing the AB blood group. This is an example of co dominance where both the genes have equal expression.

It will be interesting to observe the possible genotypes of offspring and their phenotype blood group when marriage occurs between people belonging to different blood groups.

Sub divisions of A, AB and B:

The A group RJBCs are divided into two sub groups namely A l and A2. The latter is not very common and is found in about 20% of the group there is no agglutination reaction between A RBC and A2 serum and vice versa but both A and A2 are agglutinated by B and O serum.

The B group has two types of antibodies. The A antibody agglutinates both A and A2 corpuscles and the other A2 strongly agglutinates only A but agglutination against A2 is very weak. Similarly the AB group is also known to have two to four sub groups, A B, A2 B, A3B, A4B etc.

Blood group inheritance and parentage: In cases of disputed parentage when blood groups of parents and children are known it can be easily decided (see table) whether a given child is born to particular parents or not.

When the parent’s blood groups are known, their genotype can be worked out and the child’s genotype and blood group can be blood group of mother, child and the unlikely blood group of father.

If the blood group of mother and child is of a particular type, the disputed father cannot have the genotype given in the table in that he is not the father.

It should be noted however that in instances when both the babies and families are of the same group it will not be possible to decide (if the disputed father also has the matching blood group) parentage on the basis of blood group.

Rh factor:

Landsteiner and Wiener (1940) discovered one more type of antigen (agglutinogen) in the RBC’s.Of human beings. It is named after the Rhesus Monkey (Macaca rhesus) and called the Rh factor or Rh antigen.

It is present in about 95% of Indians (about 85% of whites also have it). However the serum does not have the corresponding antibody in the human beings. When blood of rhesus monkey is injected into the rabbit, the rabbit produces antibodies against it.

When this antiserum (serum with antibody) is tested against human blood, agglutination takes place in about cases. In about 15% no agglutination takes place.

Persons whose blood agglutinates with Rh serum of monkey are called Rh positive and where no agglutination takes place they are called Rh negative.

Six groups of Rh factors have been identified in human beings C, c, D, d, E and e. Out of these D and d are commonest.

D is a Mendelian gene hence is dominant over d Almost all human beings (85%) who are Rh positive belong to D group while Rh negative people belong to d group. The importance of Rh factors can be ascertained from the following examples.

A. Assume that an Rh negative individual gets a blood transfusion from a Rh positive individual. At the first instance nothing will happen as there are no natural antibodies in the serum of Rh negative individual.

But the presence of Rh blood will induce antibody production. Nothing will happen however as there are no sufficient antibodies.

Should the same Rh negative individual require another transfusion from Rh positive blood the donor blood will show agglutination due to the antibodies already present? This may be harmful to the recipient.

B. A Rh negative woman marries an Rh positive man. The child of this couple obviously belongs to Rh positive as the gene D is dominant over d. In the first pregnancy However the child is not harmed even though the maternal blood and the child’s blood mix.

Once again the mother’s Rh negative blood will not produce enough antibodies to haemolyse the child’s blood. But with each succeeding conception the antibodies in the mother’s blood accumulate, sometimes the third or even the second child itself may have its blood.

(Rh positive) haemolysed leading to erythroblasts from the bone marrow enter the blood (to compensate for the haemolysed RBC) this condition is called Erythroblastosisfoetalis.

Such a baby is generally still born or even when born highly anaemic. It can be saved with total transfusion of Rh negative blood.

Pseudo alleles:

The alleles are so called because they occupy the same locus virtually permitting no cross over. But in some instances different genes may be so closely linked that there is no cross over even though they are not alleles, such genes which are basically different but because of occupying almost the same locus behave as alleles are called pseudo alleles.

(Lewis 1951) There is some, controversy regarding the exact identity of the pseudo allele. While Lewis (1955) and Green (1963) regard pseudo alleles as two different genes with their own function, Pontecorvo (1958) believes them to be different functional sites within a same gene. The functional part is called cistron, while the recombinant part is called the recon. Pseudo allelism has been observed in the sex linked genes for eye color in Drosophila.

The eye color varies from white to red with a range of eye colors between. All these allies are located, at about 1.5 on the X-chromosome map. Lewis (1951) in one.ot his experiments reported in a cross between apricot and white eye.

In the F2 instead of the white and apricot aggregating he found the wild type also, showing the two genes are actually non allelic. But if they are non allelic they should be two different genes.

But apricot and white are known to be allelic as seen by other experiments, but they behave as non allelic. Lewis called such genes as pseudo alleles.

Significance of allelism:

The study of allelism is significant in that it proves that a gene can mutate several times, that a gene can suppress other genes to different degrees and a gene can be a compound gene with its sub units mutating in diverse directions.