Inheritance of ABO blood groups
Blood group is set by one gene with three alleles: A, B and O. A and B are codominant, both dominate over O. The parents’ genotypes tell you which blood groups a child can have.
Rules
- Group A: AA or AO · B: BB or BO · AB: AB · O: OO
- A and B codominant, O recessive
- Each parent passes on one allele: four equally likely combinations
- Two group A parents can have a group O child if both are AO
Your turn
Three problems on this topic. Type probabilities as a percentage number, e.g. 18.75.
Task
In the ABO system, A and B are codominant and both are dominant over O. The mother has genotype AB (blood group AB), the father AO (blood group A). What is the probability that a child has blood group O, A, B or AB?
Blood group O in %
Blood group A in %
Blood group B in %
Blood group AB in %
Probabilities as a percentage number (25 or 18.75), rounded to 2 decimal places; allele frequencies as a decimal (0.02).
Worked example
In the ABO system, A and B are codominant and both are dominant over O. The mother has genotype AA (blood group A), the father AB (blood group AB). What is the probability that a child has blood group O, A, B or AB?
Worked solution
- 1.Mother’s gametes: A, A; father’s: A, B
- 2.Four equally likely combinations: AA → A; AB → AB; AA → A; AB → AB
- 3.25% per combination: O 0%, A 50%, B 0%, AB 50%
Typical mistake: Reading the genotype off the phenotype: blood group A can be AA or AO. Only an AO parent can pass on the O allele.
Common questions
Can parents with groups A and B have a group O child?
Yes, if the parents are AO and BO. Then each child has a 25% chance of O, A, B and AB.