Codominance
Biology: Edexcel IGCSE, topic 4.3.6
Codominance
Human blood group can be split into four categories: A, B, AB and O.
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
- 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
- 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.
Test yourself on Codominance
Someone with the genotype IAIB and someone with the genotype IAIO are in which blood group(s)?
How many different types of cell markers for blood group are there?