Question

How many phenotypes are possible in the ABO blood group?

Answer 1

Hint: The presence or absence of the antigens A and B, which are carried on the surface of red blood cells, determines the ABO blood group system, which classifies human blood based on the hereditary characteristics of red blood cells (erythrocytes). Karl Landsteiner and Weiner created this system. ABO blood grouping is genetically determined and is an example of multiple allelism.
Concept used:
Multiple allelism is a phenomenon in which one gene has more than two allelic forms. Like the I gene responsible for blood group type, it has three allelic forms A, B and O.

Complete solution:
To solve this question, one should at first know the difference between phenotype and genotype.
A phenotype is the physically visible characteristic and is controlled by genes whereas a genotype is the genetic makeup which controls the phenotype and expression of a character.
For instance, ${I^A}{I^A}\,or\,{I^A}{I^O}$ are two different genotypes but controls the expression of the same phenotype ‘A’ blood group.
Now, in ABO blood grouping, total possible genotypes can be calculated by the following formula:
$No\,of genotypes\,possible\,in\,multiple\,allelism\, = \,\dfrac{{n(n + 1)}}{2}$ Where n is the total no of alleles.
As in ABO blood group three alleles are traced which are A, B and O total genotypes possible would be:
$No\,of genotypes\,possible\,in\,multiple\,allelism\, = \,\dfrac{{3(3 + 1)}}{2} = \dfrac{{12}}{2} = 6$
Also, A allele and B allele are dominant over O allele but are codominant with each other.
Now based on the above information we can create the following table:

Possible genotypes	Possible phenotypes / Blood group type
${I^A}{I^A}\,or\,{I^A}{I^O}$	A
${I^B}{I^B}\,or\,{I^B}{I^O}$	B
${I^A}{I^B}$	AB
${I^O}{I^O}$	O

Thus, for six genotypes four phenotypes are present in ABO blood grouping.

Note:
Sugars are the antigens of the ABO blood group. They are made by a sequence of processes involving the transfer of sugar units catalyzed by enzymes. The sort of enzymes a person has, and hence the type of sugar antigens that end up on their red blood cells, is determined by their DNA. The Rh blood type's antigens, on the other hand, are proteins. The RHD gene produces the D antigen, a large protein found on the red blood cell membrane.