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Codominance and blood groups
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Sometimes neither allele is dominant. Both show in the phenotype of a heterozygote.
What codominance means
In codominance, both alleles in a heterozygote are expressed in the phenotype.
Neither allele hides the other.
Example: in roan cattle, a heterozygote has a coat with both red hairs and white hairs.
Neither allele hides the other.
Example: in roan cattle, a heterozygote has a coat with both red hairs and white hairs.
ABO blood groups
The gene has three alleles: IA, IB and IO.
IA and IB are codominant. Both are dominant to IO.
IA and IB are codominant. Both are dominant to IO.
A cross
Parents IAIO (group A) × IBIO (group B).
Offspring: IAIB (AB), IAIO (A), IBIO (B), IOIO (O).
Each blood group has a 1 in 4 chance.
Offspring: IAIB (AB), IAIO (A), IBIO (B), IOIO (O).
Each blood group has a 1 in 4 chance.
A man has blood group AB. Which alleles can he pass on?
- His genotype must be IAIB, because both codominant alleles show.
- Each sperm gets one of the pair.
Answer: IA or IB
In codominance both alleles are expressed in the heterozygote. In ABO blood groups, IA and IB are codominant and both are dominant to IO. IAIB is group AB; IOIO is group O.
The interactive lesson includes the diagrams for this topic.
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