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

Co-dominance is a type of genetic inheritance in which both alleles of a gene are fully expressed in the phenotype of a heterozygous individual. Unlike in dominant-recessive inheritance, where one allele masks the expression of the other, in co-dominance, both alleles contribute equally and are both visible in the organism’s traits.

Co-dominance:

  • Definition: In co-dominance, neither allele is dominant or recessive, and both are fully expressed when present together in a heterozygote.
  • Outcome: The heterozygous individual shows both traits equally.

Co-dominance in the ABO Blood Group System

The ABO blood group system is an example of co-dominance, where both the A allele and the B allele are co-dominant to each other. The system involves three alleles:

  • Iᴬ (A allele): Codes for the A antigen on the surface of red blood cells.
  • Iᴮ (B allele): Codes for the B antigen on the surface of red blood cells.
  • I⁰ (O allele): Codes for no antigen (recessive allele).

How Co-dominance Works in the ABO Blood System:

  1. Blood Type Phenotypes:

    • A blood type: An individual with IᴬIᴬ or IᴬI⁰ will have A antigens on the surface of their red blood cells.
    • B blood type: An individual with IᴮIᴮ or IᴮI⁰ will have B antigens on the surface of their red blood cells.
    • AB blood type: An individual with IᴬIᴮ will have both A and B antigens on their red blood cells. This is the result of co-dominance, where both alleles (Iᴬ and Iᴮ) are fully expressed, and the individual has both A and B antigens.
    • O blood type: An individual with I⁰I⁰ will have no A or B antigens on their red blood cells.

  2. Co-dominance in Action:

    • Individuals with the IᴬIᴮ genotype (heterozygous for both A and B alleles) will have AB blood type, meaning they will express both the A antigen and the B antigen on their red blood cells. Neither antigen is dominant over the other, and both are visible in the blood type.

Key Points about Co-dominance in the ABO System:

  • Iᴬ and Iᴮ alleles are co-dominant, meaning both the A and B antigens are equally and fully expressed in individuals with the IᴬIᴮ genotype.
  • I⁰ is recessive to both Iᴬ and Iᴮ, meaning it does not express any antigen and is only visible in the O blood type (I⁰I⁰).
  • AB blood type individuals show both A and B antigens, demonstrating co-dominance.

Related Questions:

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