Lesson 6 of 10 / Inheritance
ABO: more than two alleles
How can two alleles both be expressed in one person?
In this lesson: Explain codominance and multiple alleles using ABO genotypes and phenotypes.
About 6 min
The key ideaThe ABO gene has three main alleles in the model, but each person has two. IA and IB are codominant; both are dominant over IO.
Plain-text symbols IA, IB and IO represent the usual I with superscript A, B and O. Keep the allele symbol IO distinct from the blood-group phenotype O.
| Gametes | IB | IO |
|---|---|---|
| IA | ||
| IO |
Selected: IA IB -> Group AB. Each displayed combination has probability 1/4. Each gamete supplies one ABO allele. IA and IB are codominant; both are dominant over IO. The grid gives expected probabilities for these exact parental genotypes.
Expected phenotypes: Group AB: 1/4; Group A: 1/4; Group B: 1/4; Group O: 1/4
Rows: parent 1 gamete alleles. Columns: parent 2 gamete alleles. Boxes are equally likely combinations for the stated crosses, not a required sequence of offspring. Repeated genotypes can occur in different boxes. IA, IB and IO are plain-text allele symbols; A, B, AB and O are phenotypes.
Explanation
Multiple alleles means that more than two alternative alleles of a gene exist in the population. The ABO model uses IA, IB and IO. A diploid individual still has only two of them, one inherited from each parent.
IA and IB are codominant. An IA IB individual expresses both A and B antigens on red blood cells and has blood group AB. Codominance means both are expressed; it does not mean they blend into an intermediate antigen.
IA is dominant to IO, so IA IA and IA IO both give group A. IB IB and IB IO both give group B. IO IO gives group O. These are six possible genotypes producing four phenotypes in this model.
For a cross, list the alleles each parent can transmit and combine them. For IA IO x IB IO, the four equally likely combinations are IA IB, IA IO, IB IO and IO IO, giving AB, A, B and O with probability 1/4 each. This is an inheritance model; blood-transfusion compatibility is a separate application.
Step by step
- 1
Write full allele symbols
Do not replace a genotype such as IA IO with the phenotype label A.
- 2
List parental gametes
Each gamete receives one allele of the ABO gene.
- 3
Apply both relationships
Use codominance between IA and IB, and their dominance over IO.
Worked example
Group O from A and B parents
Explain how a group A parent and a group B parent can have a group O child in the simple ABO model.
One way to explain it
The parents can be IA IO and IB IO. Each can transmit IO, giving an IO IO child with group O. The probability is 1/4 for this specified cross.
Why this answer works
- Phenotype alone does not identify whether A or B is homozygous.
- The O child must receive IO from each parent.
- The probability depends on the stated heterozygous genotypes.
Is this true? "A person with blood group AB has three alleles because ABO has three alleles."
An AB individual has IA and IB, two alleles. Multiple alleles refers to the alternatives present across the population.