K325 / 2027

Lesson 7 of 11 / Inheritance

ABO: more than two alleles

How can two alleles both be expressed in one person?

I can explain codominance and multiple alleles using ABO genotypes and phenotypes.

  • I can use dominance, codominance, genotype and phenotype terminology.Syllabus

    Syllabus K325, Topic 14(b). Use dominance, codominance, genotype and phenotype terminology.

  • I can explain ABO codominance and multiple alleles.Syllabus

    Syllabus K325, Topic 14(f). Explain ABO codominance and multiple alleles.

About 6 min

Make a guess. You are not marked.

There are three ABO alleles: IA, IB and IO. How many of them does one person carry?

  1. Three, one of each.
  2. One, the one that is dominant.
  3. Two, one from each parent.
Show the answer

Two, one from each parent.

Each body cell is diploid, with two copies of the gene. The three alleles exist across the population.

The key idea

The 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.

Which two ABO alleles could an offspring inherit?
IA IOIB IOIAIBIA IB
Choose a box: one allele from each parent
GametesIBIO
IA
IO

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.

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. 1

    Write full allele symbols

    Do not replace a genotype such as IA IO with the phenotype label A.

  2. 2

    List parental gametes

    Each gamete receives one allele of the ABO gene.

  3. 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.

Watch out for this

A student says: "A person with blood group AB has three alleles because ABO has three alleles." What is wrong with this?

Show the answer

An AB individual has IA and IB, two alleles. Multiple alleles refers to the alternatives present across the population.

Check your understanding

Why is IA IB codominant?

  1. The individual carries IA, IB and IO at once.
  2. Both A and B antigens are expressed in the AB phenotype.
  3. IA and IB blend together to give a new antigen, which is neither A nor B.
Show the answer

Both A and B antigens are expressed in the AB phenotype.

Both alleles are expressed together rather than one masking the other.

Exam-style questions

Check your understanding

A mother is blood group AB and the father is blood group O. Which blood groups can their children have?

  1. AB
  2. A or B
  3. AB or O
  4. A, B, AB or O
Show the answer

A or B

The mother gives IA or IB and the father gives IO, so children are IA IO (A) or IB IO (B).

Quick recall

Card 1 of 3. Answer in your head, then check.

How can two alleles both be expressed in one person?

You can return to this lesson any time.