4.3.6

Codominance

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Codominance

Human blood group can be split into four categories: A, B, AB and O.

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Multiple alleles

  • There are multiple alleles for the gene that controls blood group: IA, IB, and IO.
  • IA and IB are expressed equally (neither allele is dominant or recessive). This is known as codominance.
  • IO is recessive.
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Cell markers

  • Red blood cells have ‘markers’ on their cell membrane.
  • People with the A blood group have ‘A-markers’ on their cells; those in blood group B have ‘B-markers’; those with AB blood group have both ‘A- and B-markers’; those in blood group O have no markers.
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Genotypes

  • The genotypes IAIA and IAIO result in blood group A.
  • The genotypes IBIB and IBIO result in blood group B.
  • The genotype IAIB causes blood group AB.
  • The O blood group is caused by genotype IOIO.

Jump to other topics

1The Nature & Variety of Living Organisms

2Structure & Functions in Living Organisms

3Organism Functions

3.1Respiration

3.2Gas Exchange

3.3Transport

3.4Excretion

3.5Coordination & Response

4Reproduction & Inheritance

5Ecology

6Biological Resources

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