Phenylketonuria Is A Genetic Disease Caused by a Recessive Allele
- When a newborn baby undergoes a routine heel-prick test,
- They are undergoing a simple test screening for phenylketonuria (PKU), a genetic disorder that, if untreated, can lead to severe intellectual disabilities.
- PKU is a classic example of a disease caused by a recessive allele.
PKU Is Caused by a Defective PAH Gene
- PKU arises from a mutation in the PAH gene, which encodes the enzyme phenylalanine hydroxylase (PAH).
- This enzyme converts phenylalanine (an amino acid found in many foods) into another amino acid, tyrosine.
Without functional PAH, phenylalanine accumulates to toxic levels, impairing brain development.
Recessive Inheritance Explains PKU’s Rarity
- PKU follows a recessive inheritance pattern:
- Carriers (Pp): Individuals with one normal and one defective allele produce enough functional enzyme to remain unaffected.
- Affected Individuals (pp): Symptoms only appear in individuals inheriting two defective alleles.
| P | p | |
|---|---|---|
| P | PP | Pp |
| p | Pp | pp |
