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Explanation of Inheritance of Blood Groups in Humans
1. Overview
The ABO blood group system in humans is controlled by a gene that exists in three different allelic forms: $I^A$, $I^B$, and $I^O$. These alleles exhibit different types of inheritance patterns, namely dominance, co-dominance, and multiple allelism. Below is a structured explanation detailing how each concept applies to this system.
2. Dominance: $I^A I^O$ or $I^B I^O$
When one parent passes the $I^A$ allele and the other parent passes the $I^O$ allele, the individual’s blood group is A. This is because $I^A$ is dominant over $I^O$. Similarly, if the combination is $I^B I^O$, then the individual’s blood group is B, since $I^B$ is dominant over $I^O$. The presence of the $I^O$ (often called the “O” allele) does not express a specific antigen on the red blood cells when paired with $I^A$ or $I^B$; hence it is the recessive allele.
3. Co-dominance: $I^A I^B$
When an individual inherits both $I^A$ and $I^B$ alleles, the blood group becomes AB. Here, both alleles express themselves equally and simultaneously, a phenomenon called co-dominance. Thus, both A and B antigens appear on the surface of the red blood cells.
4. Multiple Allelism: $I^A, I^B, I^O$
Unlike a typical situation where a gene has only two allelic forms, the ABO blood group gene possesses three different alleles ($I^A, I^B, I^O$). This condition is referred to as “multiple allelism.” In such cases, more than two alleles of a single gene can exist within a population, even though a single individual can possess only two alleles at a time.
5. Conclusion
Therefore, the correct set of characteristics to describe the inheritance of the ABO blood groups system in humans is:
Dominance ($I^A$ over $I^O$ and $I^B$ over $I^O$)
Co-dominance ($I^A I^B$)
Multiple allelism ($I^A, I^B, I^O$)
Hence, the correct answer is (a) Dominance, (b) Co-dominance, and (c) Multiple allelism.
