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Class 12th Biology 2012 Set1 Outside Delhi Board Paper Solution

Question 19

(a) Explain the phenomena of multiple allelism and co-dominance taking ABO blood group as an example.
(b) What is the phenotype of the following:
(i) IAi
(ii) i i

(a) When a gene exists in more than two allelic forms, the phenomenon is called multiple allelism.
When two alleles of a gene are not dominant over each other and expresses themselves independent of the presence or absence of others, the phenomenon is called co dominance. Both these phenomenon can be easily explained on the basis of ABO blood grouping in humans. The blood groups in humans are controlled by the gene I which exist in three allelic forms ‘i’, ‘IA’, and ‘IB’. Thus it shows multiple allelism . IA, and IB codes for glycoprotein that protrudes from the surface of the RBCs while ‘i’ does not code for anything. Therefore IA, and IB are codominant over each other while both are dominant over ‘i’. When both, IA, and IB comes together in an individual, both the types of glycoprotein are produced and hence the blood group becomes AB.
(b) Phenotype of-
(i) IA, i- Blood group A
(ii) - Blood group O