Phenylketonuria, SNPs, multiple alleles and ABO blood groupsSpec D3.2.7, D3.2.8, D3.2.9
In short
Phenylketonuria (PKU) is a recessive genetic condition caused by mutation in an autosomal gene for the enzyme that converts phenylalanine to tyrosine. Single-nucleotide polymorphisms create many alleles of a gene in a gene pool, but each individual inherits only two. ABO blood groups are an example of multiple alleles: Iᴬ, Iᴮ and i.
Phenylketonuria
Phenylketonuria (PKU) is a recessive genetic condition caused by a mutation in an autosomal gene that codes for the enzyme phenylalanine hydroxylase, needed to convert phenylalanine to tyrosine.
- Only people with two recessive alleles (pp) have PKU. Heterozygotes (Pp) make enough enzyme from their one functional allele, so they are unaffected carriers.
- Without the enzyme, phenylalanine from dietary protein builds up in the blood and damages the developing brain, causing intellectual disability. Less tyrosine is made, so less melanin.
- Babies are screened soon after birth with a blood test. Affected children follow a diet low in phenylalanine, which prevents most harm: an example of environment affecting phenotype.
Single-nucleotide polymorphisms and multiple alleles
A single-nucleotide polymorphism (SNP) is a position in a gene where the base differs between individuals. Each new SNP arising by mutation can create a new allele, so any number of alleles of a gene can exist in the gene pool (all the alleles in an interbreeding population). A diploid individual, though, only inherits two alleles of each autosomal gene.
ABO blood groups
The ABO gene has three common alleles. Iᴬ and Iᴮ code for enzymes that add different sugars to a glycoprotein antigen on red blood cells; i codes for a non-functional enzyme. Iᴬ and Iᴮ are both dominant to i, and codominant with each other.
| Blood group | Genotypes | Antigen on red blood cells |
|---|---|---|
| A | IᴬIᴬ or Iᴬi | A |
| B | IᴮIᴮ or Iᴮi | B |
| AB | IᴬIᴮ | A and B |
| O | ii | Neither |
An ABO cross
A father of blood group A (Iᴬi) and a mother of blood group B (Iᴮi) have a child. Determine the possible blood groups of their children and the probability of each.
- Father's gametes: Iᴬ or i. Mother's gametes: Iᴮ or i.
- Punnett grid: IᴬIᴮ, Iᴬi, Iᴮi, ii.
- Phenotypes: AB, A, B, O, one of each.
Answer: Each child has a 0.25 (25%) chance of each blood group: A, B, AB or O.
Use exactly the notation Iᴬ, Iᴮ and i for ABO alleles. Writing A, B and O as alleles usually loses the mark.
Written and checked against the IB Biology SL specification · Updated October 2026