ABO blood group system is an example of

Options

A. (a) dominant-recessive relationship
B. (b) polygenes
C. (c) co-dominance
D. (d) co-dominant and dominant-recessive relationship

Explanation

### Step-by-Step Solution:

1. **Understanding the ABO Blood Group System**: The ABO blood group system is determined by three alleles: IA, IB, and i (Io). These alleles combine in different ways to produce the four blood types: A, B, AB, and O.

2. **Identifying Allele Dominance**: In this system:
- IA (A allele) is dominant over i (O allele).
- IB (B allele) is also dominant over i (O allele).
- Both IA and IB alleles are co-dominant to each other, meaning that when they are present together (as in the AB blood type), both traits are expressed.

3. **Exploring Genotypes and Phenotypes**: The possible genotypes and their corresponding phenotypes are:
- IAIA or IAi → Blood type A
- IBIB or IBi → Blood type B
- IAIB → Blood type AB (showing co-dominance)
- ii → Blood type O

4. **Applying the Law of Dominance**: According to the law of dominance, in a heterozygous condition, the dominant allele will mask the effect of the recessive allele. In this case, IA and IB are dominant over i.

5. **Identifying Co-dominance**: Co-dominance occurs when both alleles in a heterozygous condition are fully expressed. In the case of blood type AB (IAIB), both A and B antigens are expressed on the surface of the red blood cells.

6. **Conclusion**: The ABO blood group system exemplifies both the law of dominance (where IA and IB dominate over i) and co-dominance (where both IA and IB are expressed in AB blood type). Therefore, the ABO blood group system is an example of both co-dominance and dominant-recessive relationships.

### Final Answer:
The ABO blood group system is an example of both co-dominance and dominant-recessive relationships.