Inheritance of Blood Groups

The transfer of genes from one generation to another is known as inheritance. Similarly, blood groups are transferred from parents to their offspring. Specific genes in the body control specific blood groups. There are three major blood groups: A, B, O, and all are controlled by gene i. Various combinations lead to different blood grouping systems. RBCs contain antigens. These are the same as blood groups and act like cell-antigens. The plasma membrane of RBCs contains sugar polymers on their surface, and this sugar is controlled by gene I. This gene I is inherited by the parents and has multiple forms, which are IA, IB, i. IA and IB are different from i. Blood group A has A-antigen, blood group B has B-antigen, and blood group O has no antigen. The I gene has both dominant and recessive characters I and i. Blood group O is controlled by two recessive i genes.

Codominance- Blood group inheritance is due to the phenomenon of codominance. There are three major blood groups, namely, A, B, and O. Multiple combinations are possible.

Alleles of Gene I:

The three alleles:

• The gene I has three alleles IA, IB, iO
• Alleles IA and IB are completely different from allele iO because they produce sugar polymers wherein i doesn’t have any antigen.
• When IAiO × IAiO are crossed, i.eBoth parents have IA genes and recessive iO, 75% of offspring will have blood group A and 25% will have blood group O.
• When IAIB × ii are fused, 50% have blood group A and 50% are blood group B.
• IAiO × IBiO are crossed to get 50% chances of AB blood group, 25% O and 25% A blood group in the children.

ABO Blood Grouping

Blood Group A:

• The possibility of having blood group A is that both parents have blood groups A(IAIO or IAIA  ). 50% from each parent was introduced.
• Another possibility could be that one parent has dominant A gene and the other has a recessive i gene. So the dominant gets expressed.
• Antibody present on blood serum is Anti-B.

Blood Group B:

• Similarly, for blood group B, both the dominant characters IBIB is expressed from both the parents. 50% from both the parents is expressed.
• The resulting blood group will be B in conditions where either parent has recessive i allele and dominant IB.
• Antibody present on the blood serum will be Anti-B.

Blood group AB:

• For the baby to have AB blood group, there is only one condition, one parent should have IA and the other must have IB allele.
• This blood group has no antibody present on it.
• When IA and IA both are present, they express their own type of sugar because of codominance, hence, RBC has both types of sugars.

Blood group O:

• Like AB, blood group O also has one possibility, both parents must have recessive iO allele to express the blood group O.
• Both antibodies Anti-A and Anti-B are present on the blood serum.

Rh Grouping

Rh antigen:

•  The Rh antigen is not a blood group but an indication of presence of Rh antigen in a particular blood group denoted by D. They are transmembrane proteins expressed on the surface of erythrocytes.
• All blood groups showing positive + sign have Rh antigens and those blood groups showing a Rh negative- sign do not have this extra antigen.
• The Rh antigen was first isolated in rhesus monkeys and is found in the surface of human erythrocytes. Around 90% of Indians are Rh+.
• When a Rh- person is exposed to Rh+, a specific antibody is formed in the blood called the anti-D antibody leading to blood clotting. This reaction is called agglutination. Therefore, it is important to check for Rh antigens before transfusion.

Rh factor:

• Another group of antigens commonly found in RBCs of many people is called the RH factor.
• It was discovered by Landsteiner and Wiener.
• The presence of this antigen in human blood is called  Rh positive whereas absence shows Rh negative.
• This importance of Rh factor can be studied by referring to the condition of erythroblastosis fetalis, where the Rh of the mother becomes incompatible with the foetal blood group.
• This disorder arises due to transplacental transmission of maternal antibodies to the foetus. This usually occurs during parturition or delivery of the first baby, the baby’s blood comes in contact with the mother’s blood through the placenta and starts developing antibodies against Rh antigen in her blood.
• But during the second pregnancy, the already prepared large amounts of antibodies start entering the blood circulation of the foetus through diffusion and begin to destroy the RBCs. This can either cause anaemia to the baby or jaundice. In extreme cases it can lead to death.
• To avoid these emergencies, the mother is injected with an artificial antibody similar to the antibody present inside her against the Rh+ve prior to the second pregnancy.. This is how the foetus is kept out of danger.

Conclusion

The study of blood groups is a must because it is imperative to be aware of blood groups during the transfusion process and pregnancy. Mendel’s law of codominance shows that the dominant one gets expressed if a particular character has two genes or alleles. This idea is helpful to predict a baby’s blood group o by studying the parents’ blood groups. There are three major blood groups–A, B, and O. Multiple combinations are possible. The gene I controls the blood groups alleles. Accordingly, one can check the blood group of a particular person.