Understanding Blood Group Inheritance

The ABO blood group system in humans is determined by a single gene with three common alleles: I^A, I^B, and i. The I^A allele codes for the A antigen, the I^B allele codes for the B antigen, and the i allele does not code for any antigen (or codes for a non-functional enzyme).

• Alleles I^A and I^B are codominant with each other.
• Both I^A and I^B are dominant over the i allele.

This means:

• A person with genotype I^AI^A or I^Ai has blood group A.
• A person with genotype I^BI^B or I^Bi has blood group B.
• A person with genotype I^AI^B has blood group AB.
• A person with genotype ii has blood group O.

Analyzing Parental Blood Groups and Genotypes

We are given that one parent has blood group A, and the other parent has blood group O.

• A parent with blood group O must have the genotype ii, as the i allele is recessive.
• A parent with blood group A can have two possible genotypes: I^AI^A (homozygous A) or I^Ai (heterozygous A).

To determine the possible blood groups of their child, we need to consider both possibilities for the parent with blood group A.

Case 1: Parent A is Homozygous (I^AI^A) and Parent O is Homozygous (ii)

In this case, the parent with blood group A produces only I^A gametes, and the parent with blood group O produces only i gametes.

All possible offspring will have the genotype I^Ai. The genotype I^Ai results in blood group A because I^A is dominant over i.

Case 2: Parent A is Heterozygous (I^Ai) and Parent O is Homozygous (ii)

In this case, the parent with blood group A produces I^A gametes and i gametes (each with 50% probability), and the parent with blood group O produces only i gametes.

The possible offspring genotypes are I^Ai and ii. The genotype I^Ai results in blood group A, and the genotype ii results in blood group O.

Conclusion on Offspring Blood Groups

Considering both possible genotypes for the parent with blood group A:

• If the parent with blood group A is I^AI^A, all children will have blood group A.
• If the parent with blood group A is I^Ai, children can have blood group A (genotype I^Ai) or blood group O (genotype ii).

Therefore, when one parent has blood group A and the other has blood group O, their child can potentially inherit either blood group A or blood group O. The child's specific blood group depends on whether the parent with blood group A is homozygous or heterozygous.

Revision Table: ABO Blood Groups

Additional Information: Genetics of Blood Type Inheritance

Understanding the principles of Mendelian genetics helps explain blood type inheritance. The ABO blood group gene is an example of multiple alleles (more than two alleles existing in the population for a single gene). The relationship between the alleles (dominance, codominance) determines the resulting phenotype (blood group).

• Alleles: Different forms of a gene (I^A, I^B, i).
• Genotype: The combination of alleles an individual possesses for a gene (e.g., I^AI^A, I^Ai, ii).
• Phenotype: The observable trait resulting from the genotype (e.g., Blood Group A, Blood Group O).
• Dominance: When one allele (like I^A or I^B) masks the expression of another allele (like i) in a heterozygote.
• Codominance: When both alleles are expressed equally in a heterozygote (like I^AI^B resulting in AB blood group).

Pedigree analysis involving family blood types can be used to trace the inheritance patterns and deduce genotypes of individuals.