What is co-dominance? Explain with reference to the ABO blood group system.

Co-dominance is a genetic phenomenon where both alleles of a gene are fully expressed in the organism’s phenotype. Unlike in dominance-recessive inheritance, where one allele masks the expression of another, in co-dominance, both alleles contribute equally and are visible in the organism’s traits. This means that neither allele is dominant or recessive, and the heterozygote exhibits traits of both alleles simultaneously.

Co-Dominance:

  • Definition: In co-dominance, both alleles of a gene are expressed equally in the phenotype when they are present together in a heterozygous individual.
  • Outcome: The heterozygous individual shows characteristics of both alleles.

Example: The ABO Blood Group System

The ABO blood group system is a classic example of co-dominance, where the alleles for blood type A and B are co-dominant. Here’s how it works:

  1. The ABO Blood Group Gene:
    The ABO blood group is controlled by a single gene, which has three alleles:

    • Iᴬ (allele A): Codes for the A antigen on the surface of red blood cells.
    • Iᴮ (allele B): Codes for the B antigen on the surface of red blood cells.
    • I⁰ (allele O): Codes for no antigen (i.e., it is recessive to both Iᴬ and Iᴮ).

  2. Genotypes and Phenotypes:

    • IᴬIᴬ or IᴬI⁰: The individual will have A type blood (A antigen on red blood cells).
    • IᴮIᴮ or IᴮI⁰: The individual will have B type blood (B antigen on red blood cells).
    • IᴬIᴮ: The individual will have AB type blood (both A and B antigens are expressed on the red blood cells). This is the example of co-dominance, where both A and B antigens are equally expressed, and the individual has the characteristics of both A and B blood types.
    • I⁰I⁰: The individual will have O type blood (no A or B antigens on the red blood cells).

  3. Co-dominance in Action:
    In individuals with the IᴬIᴮ genotype (heterozygous for both A and B alleles), both the A antigen and B antigen are expressed on the surface of the red blood cells. This results in AB blood type, where both antigens are visible and contribute to the phenotype equally. This is a clear example of co-dominance, where neither allele (Iᴬ or Iᴮ) is dominant over the other, and both contribute to the final blood type.

Related Questions:

  1. How do genes control inheritance of characters?
  2. Explain the composition of chromatin material.
  3. Give an account of genes and alleles.
  4. Describe the role of genes in protein synthesis.
  5. State and explain Mendel’s law of segregation.
  6. State and explain Mendel’s law of independent assortment.
  7. Write the differences between:
  8. Name sources of variation.
  9. Give examples of artificial selection.
  10. What kind of gametes would be produced by organisms having the following genotypes?
  11. Two white sheep produce a black offspring. What must the parents’ genotype for colour be? What is the probability that the next offspring will be black?
  12. The diagram shows two varieties of moths in England.
  13. Among the following genotypes, which ones are heterozygous and which are homozygous? Which of the genotypes have the same phenotype?
  14. Name the structure through which the exchange of gases can take place in plants.
  15. Name the ways through which a plant can obtain oxygen.
  16. Why should one not sleep under a tree at night?
  17. Why would very wet soil be dangerous to plants
  18. Why do plants not need a specialized respiratory system?
  19. How does the exchange of gases take place in alveoli?
  20. What is the effect of exercise on the rate of breathing?