Question

How does the gene I control ABO blood groups in humans? Write the effect of the gene on the structure of red blood cells.

Answer 1

Hint: ABO blood grouping is a case of codominance and multiple allelism. Multiple allelism is a case in which a single gene is controlled by multiple alleles. Multiple alleles are more than two alternative forms of the same gene. Phenotypically, humans show three types of blood groups whereas genotypically, there are six types of blood groups.

Complete answer:
 Gene i controls ABO blood grouping in humans as there are three different alleles- ${ I }^{ A }$, ${ I }^{ B }$ and i. ${ I }^{ A }$ and ${ I }^{ B }$are dominant over i allele which is recessive. So whenever ${ I }^{ A }$ and i alleles are present together it gives a phenotype of blood group A and when IB and i are present together it gives a phenotype of blood group B because ${ I }^{ A }$ and ${ I }^{ B }$ are dominant over i allele. Whereas when ${ I }^{ A }$ and ${ I }^{ }$ are present together they are expressed equally as ${ I }^{ A }$, ${ I }^{ B }$ and show a blood group AB. Gene i controls the type of sugar polymer and these sugar polymers protrude out from the surface of RBCs.

Additional Information:
- ABO blood grouping is an example of dominance (shown when gene i is present with gene ${ I }^{ A }$ or gene ${ I }^{ B }$ as ${ I }^{ A }$ and ${ I }^{ B }$ is dominant over i), multiple allelism (blood grouping is determined by three alleles ${ I }^{ A }$, ${ I }^{ B }$ and i), and codominance (${ I }^{ A }$ and ${ I }^{ }$ when present together are co-dominant i.e., they express themselves equally when presenting together).
- Multiple alleles are located on the same locus of homologous chromosomes. They are formed by mutation.
- The sugar polymers on the surfaces of RBCs contain the gene responsible for blood grouping in an individual.

Note: The alleles ${ I }^{ A }$ and ${ I }^{ B }$produce sugars of types A and B respectively and allele does not produce any kind of sugar. O blood group is considered as a universal donor and blood group AB is considered to be a universal recipient. Moreover, the Rh factor or Rhesus factor is also checked before blood transfusion. If an Rh -ve person is given Rh +ve blood then in the first encounter there would not be any problem but the recipient person's body starts creating antibodies and in the second case of encounter these antibodies cause agglutination which is fatal and may even lead to death.