Question

1. The ABO Blood System is an example of both a Multiple Allele System and codominance. Identify the following aspects of the ABO system and perform the punnett square below to demonstrate the possible genotypes and phenotypes.
   1. All Possible Genotypes:

2. All possible Phenotypes:

Answer 1

The ABO blood group is controlled by three alleles that are I^A, I^B and I^o. And in case of a mating between persons with A (heterozygous for A , I^A I^o ) blood group and  person with B (heterozygous for B, I^B I^o ) blood group, we will get all the possible genotypes and phenotypes i.e., the case of codominance and all the four phenotypes, as A, B, AB and O.

a. The possible genotypes through punnet square is as follows

Gametes	IA	Io
IB	IAIB	IBIo
Io	IAIo	IoIo

We can also take another combination of I^AI^B and I^AI^B to get homozygous condition for A and B blood group but in this we will not get genotype for the O blood group. The punnet square of this combination is as follows,

gametes	IA	IB
IA	IAIA	IAIB
IB	IAIB	IBIB

b. The phenotypes are as follows

For punnet square 1 For punnet square 2

I^AI^B = AB blood group I^AI^A = A blood group

I^BI^o = B blood group    I^AI^B = AB blood group

I^AI^o = A blood group    I^BI^B = B blood group

I^oI^o = O blood group

NB:

Both I^AI^o and I^AI^A genotype is responsible for A blood group phenotype. Similarly I^BI^o and I^BI^B genotype is resposible for B blood group.