Dominance:Dominance is the phenomenon in which the expression of only one allele in the presence of other alleles i.e., only one of the parental characters. The alleles I A and I B both are dominant over allele i as I A and I B forms antigen A and antigen B respectively but i does not form any antigen.
Co-dominance:It is the phenomenon when the F1 generation shows both the parental characters are expressed simultaneously. Both the alleles I A and I B are co-dominant since both are able to express themselves in the presence of each other in blood group AB (IA IB) by forming antigen A and B.
Multiple allelism:It is the phenomenon of a gene which expresses more than two allelic forms on the same locus. The ABO blood group in humans is determined by three different allelic forms I A, IB and I for the same character.
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