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  4. When both alleles express their effect on being present together, the phenomenon is called

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Q. When both alleles express their effect on being present together, the phenomenon is called

UP CPMTUP CPMT 2009Principles of Inheritance and Variation

Solution:

When both alleles express their effect on being present together, the phenomenon is called codominance. As a result the heterozygous condition has a phenotype different from either of homozygous genotypes, e.g. AB blood group in humans. Alleles for blood group $A (I_A)$ and blood group $B (I_B)$ are codominant so that when they come together in an individual, they produce blood group AB. It is characterised by the presence of both antigen A (from $I_A$) and antigen B ($I_B$) over the surface of erythrocytes.