Principles Of Inheritance And Variation

Step 1: Understanding dominance patterns.
- In complete dominance, only one allele is expressed.
- In incomplete dominance, the heterozygote shows an intermediate phenotype.
- In co-dominance, both alleles in a heterozygote are expressed equally and independently.

Step 2: Applying to AB blood group.
- The ABO blood group system is controlled by the I gene with three alleles: I\textsuperscript{A}, I\textsuperscript{B}, and i.
- Individuals with genotype I\textsuperscript{A}I\textsuperscript{B} express both A and B antigens on their red blood cells.
- Thus, both alleles (I\textsuperscript{A} and I\textsuperscript{B}) are equally expressed without blending.

Step 3: Conclusion.
The AB blood group is therefore a classic example of co-dominance.

Final Answer: