Questions: The I^A and I^B alleles in the A B O blood group antigens on red blood cells are... incompletely dominant variably expressed codominant epistatic

The answer is: codominant.

Explanation for each option:

1. Incompletely dominant: This term refers to a situation where the phenotype of the heterozygote is intermediate between the phenotypes of the two homozygotes. This is not the case for the $I^{A}$ and $I^{B}$ alleles in the ABO blood group system.

2. Variably expressed: This term refers to the degree to which a trait is expressed in individuals with the same genotype. While there can be some variability in the expression of blood group antigens, this term does not accurately describe the relationship between the $I^{A}$ and $I^{B}$ alleles.

3. Codominant: This term means that both alleles in a heterozygote are fully expressed, resulting in a phenotype that shows both traits simultaneously. This is the correct description for the $I^{A}$ and $I^{B}$ alleles in the ABO blood group system, as individuals with both alleles (genotype $I^{A}I^{B}$) express both A and B antigens on their red blood cells.

4. Epistatic: This term refers to a situation where one gene can mask or modify the expression of another gene. This is not the case for the $I^{A}$ and $I^{B}$ alleles in the ABO blood group system.

In summary, the $I^{A}$ and $I^{B}$ alleles in the ABO blood group antigens on red blood cells are codominant.