Q.Explain the phenomena of dominance, multiple allelism and co-dominance taking human ABO blood group as an example.
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Start your 14-day free trial to unlock the full solution →The ABO blood group system in humans is a single example that beautifully illustrates three different genetic principles: dominance (where one allele masks another), multiple allelism (where more than two allele forms exist for a gene), and co-dominance (where two alleles express themselves equally in the heterozygote).
To understand these three phenomena, you first need a clear picture of how the ABO blood group is inherited. The gene responsible for the ABO blood group is located on chromosome 9 and exists in three common allelic forms: I^A, I^B, and i. The I^A and I^B alleles produce specific antigens (A and B antigens respectively) on the surface of red blood cells, while the i allele produces no antigen. This single gene, with its three alleles, gives rise to four blood group phenotypes: A, B, AB, and O.
Now, let us see how each genetic principle operates within this system.
Dominance is the phenomenon where one allele (the dominant one) expresses itself in the phenotype even when present with a different allele (the recessive one). In the ABO system, both I^A and I^B are dominant over the i allele. This means that if a person inherits one I^A allele and one i allele (genotype I^A i), only the A antigen is produced — the i allele is completely silent. Such a person has blood group A. Similarly, a person with genotype I^B i has blood group B. The i allele is recessive to both I^A and I^B, so blood group O (genotype ii) occurs only when a person inherits two i alleles, one from each parent.
The dominance of I^A and I^B over i is a classic case of complete dominance. The recessive i allele does not produce any functional antigen, so its presence is only revealed in the homozygous condition.
Multiple allelism refers to the existence of more than two alternative forms (alleles) of a single gene within a population. While any individual diploid organism can carry only two alleles (one from each parent), the gene pool of the species may contain many more. The ABO blood group gene is the textbook example of this: three alleles — I^A, I^B, and i — exist in the human population. This is why we see four possible blood groups instead of just two. The NCERT textbook specifically highlights that multiple alleles arise due to mutations at the same gene locus over evolutionary time, and the ABO system is the most familiar illustration.
Remember: multiple allelism is a population-level concept. An individual person can have at most two of the three alleles, but the species as a whole has three. This is why the ABO system is such a clear example — it shows that a single gene can have more than two allelic forms. …
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