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Codominance

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

Multiple alleles

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.
Cell markers

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.
Genotypes

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.
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Classification of Living Organisms

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Organisation of the Organism

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