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Co-dominance

Hey All AIPMT Biology Aspirants, read out the next AIPMT Biology Study material/ Notes of Codominance, useful for AIPMT Biology.

Another exception to law of dominance is Co-dominance. Co-dominance arises when a heterozygote (F1 offspring) shows characteristics found in each of the associated homozygotes (parents). F1 offspring resembles both parents.

  • Perfect example of Co-dominance is seen with human blood grouping ABO. AB blood group is an example of Co-dominance.
  • ABO blood groups are controlled by the gene I. Gene I has three alleles I A, I B and i
  • Allele I A determines A antigen, and allele I B determines B antigen which are present on surface of RBCs. These antigens are two different forms of a single protein. But allele i determines no antigen.
  • Because humans are diploid organisms, each person possesses any two of the three I gene alleles.
  • So these three different alleles can result in possible six different genotypes and four phenotypes.
Allele from one Parent  Allele from other Parent  Genotype of offspring Blood group of offspring

 

I A I A I A I A A
I A I B I A I B AB
I A i I A i A
I B I A I A I B AB
I B I B I B I B B
I B i I B i B
i I A I A i A
i I B I B i B
i i i i O

 Only colored combinations are possible six genotypes, with four phenotypes A, B, AB, O. Other combinations are repeated to show possibility of variation of alleles from both parents, just for simplicity.

  • I A and I B are completely dominant over i, i.e. when only I A and i are present, I A allele is dominant over i and expresses itself and blood group is A (because i does not produce any antigen).
  • Similarly for I B and i , I B allele is dominant over allele i , and blood group is B.
  • But when both I A and I B are present together, they both express their own antigens on surface of RBCs. RBCs have both A and B antigen on their surface and the blood group is AB. This is because of Co-dominance. Both the alleles are equally dominant.
  • So when two alleles appear to contribute independently to the phenotype of the heterozygotes, they are said to be codominant. Neither allele is dominant over the other.
  • Both I A and I B are equally dominant so both are written in uppercase.
  • Also, codominant alleles are represented by superscripts on the symbol for the gene, which in this case is gene I
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