In human ABO blood group inheritance, an individual carrying both the I^A and I^B alleles … — JAMB UTME

In human ABO blood group inheritance, an individual carrying both the $I^A$ and $I^B$ alleles expresses both A and B antigens on the surface of their red blood cells, resulting in blood type AB. Which genetic phenomenon is directly demonstrated by the simultaneous, full expression of both alleles in the heterozygous state?

A
Incomplete dominance
CodominanceAnswer
C
Complete dominance
D
Epistasis

Answer

Codominance

The correct answer is codominance because both the

I^A

and

I^B

alleles contribute fully and independently to the phenotype. Heterozygous individuals (

I^A I^B

) produce both A and B functional agglutinogens (antigens) on their red blood cell surfaces without blending or masking.

Step-by-Step Solution

Analyze the expression of alleles

I^A

and

I^B

in a heterozygous individual (

I^A I^B

Both antigen A and antigen B are independently produced and present on the erythrocyte membrane.

Neither allele masks the other, nor do they blend to form an intermediate antigen structure.

Relate this joint phenotypic expression to standard non-Mendelian genetic definitions.

The simultaneous full expression of two different alleles at a locus is defined as codominance.

This contrasts with complete dominance (where one allele masks another) and incomplete dominance (where a intermediate phenotype is formed).

Key Concept

Codominance in human blood groups

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