<P> Blood groups are inherited from both parents . The ABO blood type is controlled by a single gene (the ABO gene) with three types of alleles inferred from classical genetics: i, I, and I . The I designation stands for isoagglutinogen, another term for antigen . The gene encodes a glycosyltransferase--that is, an enzyme that modifies the carbohydrate content of the red blood cell antigens . The gene is located on the long arm of the ninth chromosome (9q34). </P> <P> The I allele gives type A, I gives type B, and i gives type O. As both I and I are dominant over i, only ii people have type O blood . Individuals with I I or I i have type A blood, and individuals with I I or I i have type B. I I people have both phenotypes, because A and B express a special dominance relationship: codominance, which means that type A and B parents can have an AB child . A couple with type A and type B can also have a type O child if they are both heterozygous (I i, I i) The cis - AB phenotype has a single enzyme that creates both A and B antigens . The resulting red blood cells do not usually express A or B antigen at the same level that would be expected on common group A or B red blood cells, which can help solve the problem of an apparently genetically impossible blood group . </P> <Table> <Tr> <Td_colspan="8"> Blood group inheritance </Td> </Tr> <Tr> <Th> Blood type </Th> <Th> </Th> <Th> O </Th> <Th_colspan="2"> </Th> <Th_colspan="2"> </Th> <Th> AB </Th> </Tr> <Tr> <Th> </Th> <Th> Genotype </Th> <Th> ii (OO) </Th> <Th> I i (AO) </Th> <Th> I I (AA) </Th> <Th> I i (BO) </Th> <Th> I I (BB) </Th> <Th> I I (AB) </Th> </Tr> <Tr> <Td> O </Td> <Td> ii (OO) </Td> <Td> O OO OO OO OO </Td> <Td> O or A AO OO AO OO </Td> <Td> AO AO AO AO </Td> <Td> O or B BO OO BO OO </Td> <Td> BO BO BO BO </Td> <Td> A or B AO BO AO BO </Td> </Tr> <Tr> <Td> </Td> <Td> I i (AO) </Td> <Td> O or A AO AO OO OO </Td> <Td> O or A AA AO AO OO </Td> <Td> AA AA AO AO </Td> <Td> O, A, B or AB AB AO BO OO </Td> <Td> B or AB AB AB BO BO </Td> <Td> A, B or AB AA AB AO BO </Td> </Tr> <Tr> <Td> I I (AA) </Td> <Td> AO AO AO AO </Td> <Td> AA AO AA AO </Td> <Td> AA AA AA AA </Td> <Td> A or AB AB AO AB AO </Td> <Td> AB AB AB AB AB </Td> <Td> A or AB AA AB AA AB </Td> </Tr> <Tr> <Td> </Td> <Td> I i (BO) </Td> <Td> O or B BO BO OO OO </Td> <Td> O, A, B or AB AB BO AO OO </Td> <Td> A or AB AB AB AO AO </Td> <Td> O or B BB BO BO OO </Td> <Td> BB BB BO BO </Td> <Td> A, B or AB AB BB AO BO </Td> </Tr> <Tr> <Td> I I (BB) </Td> <Td> BO BO BO BO </Td> <Td> B or AB AB BO AB BO </Td> <Td> AB AB AB AB AB </Td> <Td> BB BO BB BO </Td> <Td> BB BB BB BB </Td> <Td> B or AB AB BB AB BB </Td> </Tr> <Tr> <Td> AB </Td> <Td> I I (AB) </Td> <Td> A or B AO AO BO BO </Td> <Td> A, B or AB AA AO AB BO </Td> <Td> A or AB AA AA AB AB </Td> <Td> A, B or AB AB AO BB BO </Td> <Td> B or AB AB AB BB BB </Td> <Td> A, B, or AB AA AB AB BB </Td> </Tr> </Table> <Tr> <Td_colspan="8"> Blood group inheritance </Td> </Tr>

Inheritance of the a b o blood groups is an example of
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