in Heredity and Variation by

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  1. The blood type is an example of Codominance.
  2. Codominance is the phenomenon in which the two alleles of a gene express themselves independently when present together.
  3. The different types of blood groups in human beings is controlled by Gene I.
  4. Gene I exists in three allelic forms – IA, IB and i.
  5. The alleles IA and IB are dominant over the allele i.
  6. The production of different types of glycoproteins by these alleles on the surface of RBCs forms the basis of difference between the blood groups.
  7. The allele IA produces the glycoprotein A, while IB produces the glycoprotein B.
  8. The allele i does not produce any glycoprotein.
  9. When the alleles IA and IB come together in a person, AB blood group is produced. The person with AB blood group has both glycoproteins A and B on the surface of RBCs.
  10. The products of both the alleles IA and IB are seen in the phenotype.
  11. Hence, AB blood group is an example of Codominance.

No.

Type of blood group

Glycoprotein (Antigen)

Genotype

1.

A (Dominance)

Antigen A

IA IA OR IA i

2.

B (Dominance)

Antigen B

IB IB OR IB i

3.

AB (Codominance)

Antigen A and B

IA IB

4.

O

Antigen absent

ii

 

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