Strong Genetic

Phenylketonuria (PKU)

Name: Phenylketonuria (PKU)
Creator: Realised
Published: 2026-03-01

GenePAHrsIDrs5030858, rs5030849, rs75377686InheritanceAutosomal recessiveCarrier frequency~1 in 50 Europeans, ~1 in 25 IrishSystemCarrier Screening

Summary

PKU is a metabolic condition where the body cannot break down phenylalanine, an amino acid found in most protein-containing foods. Without treatment, phenylalanine accumulates in the brain and causes progressive intellectual disability, seizures, and behavioural problems. With treatment, outcomes are entirely different — PKU is one of medicine's greatest screening success stories. Nearly every developed country screens newborns via the Guthrie test (heel prick), and children identified at birth who follow a low-phenylalanine diet from the start achieve normal intellectual development.

Practical takeaway

This matters when planning children. If your partner's ancestry is European, they have roughly a 1 in 50 chance of also being a carrier (higher if Irish — approximately 1 in 25). A carrier test through a GP or genetic counselling service can determine your partner's status. If you are both carriers, a genetic counsellor can discuss what PKU management looks like in practice — including newborn screening, dietary management, and newer treatments like sapropterin (Kuvan), which helps some individuals with milder variants metabolise phenylalanine more effectively. Gene therapy is also in clinical trials. Realised does not calculate reproductive risk or advise on reproductive decisions.

Evidence detail

What Carrier Status Means For You

You are not affected. Carrying one working copy of PAH produces enough phenylalanine hydroxylase enzyme for normal metabolism — you process protein normally and require no dietary changes. Your carrier status becomes relevant if you and your partner both carry a PAH variant. In that case, each child has a 25% chance of having PKU, a 50% chance of being an unaffected carrier, and a 25% chance of inheriting neither variant.

Population Context

PKU carrier rates are highest in people of European descent, particularly Irish and Northern European populations, where the condition was first described and newborn screening was pioneered. The R408W variant is the most common pathogenic variant in Europeans and likely spread through founder effects in Northern European populations. Lower carrier rates are found in African and East Asian populations, though PKU occurs in all ethnic groups. Newborn screening has made PKU one of the most successfully managed genetic conditions worldwide — the PKU diet is restrictive (requiring careful protein management), but the outcomes with early treatment are consistently excellent.

Limitations

23andMe tests 23 of over 1,100 known pathogenic PAH variants. The tested panel covers the most common European variants but may miss rarer variants or those more prevalent in non-European populations. The severity of PKU depends on which specific variants are inherited — some combinations produce classical PKU (requiring strict dietary management), while others produce milder forms that may respond to sapropterin. A "not detected" result reduces but does not eliminate carrier risk. Comprehensive carrier screening through a clinical laboratory covers substantially more variants and provides genotype-phenotype correlation that consumer testing cannot.

Sources (6)

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