So far each gene had two alleles. Many genes have more in the population. The gene for the ABO blood types has three: IA, IB and i. It mixes the ideas of codominance and ordinary dominance.
Key idea
IA and IB are codominant; both are dominant to i. A person has two of the three alleles, which makes six genotypes and four blood types.
| Blood type | Genotypes | Markers on red blood cells |
|---|---|---|
| A | IAIA or IAi | A |
| B | IBIB or IBi | B |
| AB | IAIB | A and B |
| O | ii | neither |
Why it works
IA and IB each code for an enzyme that adds a different sugar marker to the surface of red blood cells. The i allele codes for no working enzyme. One IA is enough to add A markers, so IAi is type A. With IA and IB, both markers are added: type AB.
Worked example: All four types from one couple
A parent with type A blood (IAi) and a parent with type B (IBi) have children. What blood types are possible?
- Gametes: IA or i; IB or i.
- Boxes: IAIB (AB), IAi (A), IBi (B), ii (O).
Answer: AB, A, B and O, each with chance ¼.
Worked example: Working backward
Two parents with type A blood have a child with type O. What are the parents' genotypes?
- Type O is ii, so the child got i from each parent.
- Each parent is type A and carries i.
Answer: Both are IAi.
Worked example: What cannot happen
Can a parent with type AB blood have a child with type O?
- The AB parent is IAIB and gives either IA or IB.
- A type O child (ii) needs an i from each parent.
Answer: No.
Common mistake
Because three alleles exist, some people think a person can carry all three. Each person has only two copies of the gene, one on each chromosome of the pair. The third allele exists elsewhere in the population, in other people.