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D3.2.7—Phenylketonuria (PKU)

PKU is an inherited recessive condition affecting phenylalanine metabolism, managed by identifying affected individuals and controlling diet in inheritance problems.

Syllabus
First assessment 2025
Objective
D3.2.7
Level
HL

Exam analysis

Chance of appearing3%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper3
Typical marks1–2

Common command terms

  • Explain
  • Outline

Recent exam appearances

May 2025Paper1A ["HL"] · TZ232[ 1 ]D3.2.7—Phenylketonuria (PKU)
May 2024Paper3 ["HL"] · TZ222(b)[ 2 ]D3.2.7—Phenylketonuria (PKU)
May 2024Paper3 ["HL"] · TZ222(a)[ 2 ]D3.2.7—Phenylketonuria (PKU)
May 2016Paper3 ["HL"] · TZ019(b)(ii)[ 1 ]D3.2.7—Phenylketonuria (PKU)

Coverage 2016–2025 · Updated 16 Jul 2026

PKU Links a PAH Allele to Metabolite Accumulation and Brain Injury

two loss-of-function PAH alleles → little or no phenylalanine hydroxylase → phenylalanine is not converted efficiently to tyrosine → phenylalanine and harmful derivatives accumulate → untreated neural development is damaged

Intervention Why it helps
newborn screening detects the metabolic problem before severe symptoms appear
early low-phenylalanine diet limits substrate accumulation while still supplying controlled essential phenylalanine
continued monitoring keeps blood concentrations within a safer range as diet and growth change

The genotype does not change with treatment, but the environment does. Dietary control can greatly change the phenotype, making PKU a clear genotype–environment interaction.

Two Unaffected PKU Carriers Can Have an Affected Child

Genotype PKU status Reproductive meaning
PP unaffected, not a carrier passes P
Pp unaffected carrier can pass P or p
pp affected passes p
Pp × Pp P gamete p gamete
P gamete PP unaffected Pp carrier
p gamete Pp carrier pp affected

For each pregnancy: 1/4 affected, 1/2 unaffected carrier, 1/4 unaffected non-carrier. The probability resets for every fertilization; previous children do not change the next gamete combination.

Carrier and affected are not synonyms. A heterozygous carrier has a working dominant allele and usually does not show PKU, but can transmit the recessive allele.

Inheritance Predictions Move from Chromosomes to Alleles to Phenotypes

homologous chromosomes carry two alleles in a diploid parent → meiosis separates one allele into each gamete → fertilization restores a two-allele genotype → allele relationship and environment shape phenotype → offspring data test the prediction

For any genetic cross:

  • define allele symbols and their relationship
  • write parental genotypes, not only phenotypes
  • derive gametes before drawing the grid
  • combine gametes, then translate genotype to phenotype
  • state probabilities and the assumptions behind them
If the simple 3:1 expectation fails First question to ask
heterozygote has a third phenotype incomplete dominance?
both products appear codominance?
phenotype changes with conditions environmental or plastic response?
more than two alleles occur in the population multiple-allele locus such as ABO?

Phenylketonuria (PKU)

[ 4 ]

Discuss the causes and treatments of phenylketonuria.

Concept essentials

  • PKU is usually autosomal recessive, so carriers can be unaffected.
  • Diet can influence the PKU phenotype by limiting phenylalanine intake.
  • The condition links an allele to an enzyme defect and a metabolic outcome.