## Phenylketonuria Is A Genetic Disease Caused by a Recessive Allele [block_0] 1. When a newborn baby undergoes a routine heel-prick test, 2. They are undergoing a simple test screening for **phenylketonuria (PKU)**, a genetic disorder that, if untreated, can lead to severe intellectual disabilities. 3. PKU is a classic example of a disease caused by a **recessive allele**. ![](https://cdn.sanity.io/images/w7j7fz60/production/faa68cab9c0d494bf3b843efbe087f6d4823f4ad-1280x720.webp) ## PKU Is Caused by a Defective PAH Gene [block_4] 1. PKU arises from a mutation in the **PAH gene**, which encodes the enzyme **phenylalanine hydroxylase** (PAH). 2. This enzyme converts phenylalanine (an amino acid found in many foods) into another amino acid, **tyrosine**. [{"_key":"ae76b1a1367d","_type":"block","asset":null,"children":[{"_key":"48a9b08eabe1","_type":"span","marks":[],"text":"Without functional PAH, phenylalanine accumulates to"},{"_key":"4ea2d56aa742","_type":"span","marks":["strong"],"text":" toxic"},{"_key":"05b23eb4ee78","_type":"span","marks":[],"text":" levels, impairing brain development."}],"markDefs":[],"style":"normal"}] ## Recessive Inheritance Explains PKU’s Rarity [e914efba2372] 1. PKU follows a **recessive inheritance pattern**: 1. **Carriers (Pp):** Individuals with one normal and one defective allele produce enough functional enzyme to remain **unaffected**. 2. **Affected Individuals (pp):** Symptoms only appear in individuals inheriting **two** defective alleles. | | P | p | | --- | --- | --- | | P | PP | Pp | | p | Pp | pp | [{"_key":"block_10","_type":"block","asset":null,"children":[{"_key":"c187498de69c","_type":"span","markDefs":[],"marks":[],"text":"Many students mistakenly think that carriers (Pp). show mild symptoms of PKU. This is incorrect. "}],"level":1,"listItem":"bullet","markDefs":[],"style":"normal"},{"_key":"8f3ae8d030e0","_type":"block","asset":null,"children":[{"_key":"8258ade3c921","_type":"span","markDefs":[],"marks":[],"text":"Carriers produce enough functional enzyme to remain completely unaffected."}],"level":1,"listItem":"bullet","markDefs":[],"style":"normal"}] [{"_key":"ef0377f07c04","_type":"block","asset":null,"children":[{"_key":"9b12b6c94d850","_type":"span","marks":["strong"],"text":"Chance of Inheritance:"},{"_key":"9b12b6c94d851","_type":"span","marks":[],"text":" Two carriers have a 25% chance of having an affected child (pp) and a 50% chance of producing carriers (Pp)."}],"markDefs":[],"style":"normal"}] ## PKU Can Be Managed with Early Intervention [43d0c0e4b3ca] 1. Newborn screening ensures early diagnosis, allowing **immediate **dietary adjustments to prevent symptoms. 2. **Low-Phenylalanine Diet** 1. To manage PKU, affected individuals must** avoid **high-phenylalanine foods, such as: Meat, Dairy, Nuts [{"_key":"78680a54ab5a","_type":"block","asset":null,"children":[{"_key":"d3fd59ace0cd0","_type":"span","marks":[],"text":"A child diagnosed with PKU avoids restricted foods and consumes specialized formulas to maintain normal development."}],"markDefs":[],"style":"normal"}] [{"_key":"block_20","_type":"block","asset":null,"children":[{"_type":"span","markDefs":[],"marks":[],"text":"How does the availability of genetic screening and dietary management for PKU raise ethical questions about genetic testing and healthcare access?"}],"markDefs":[],"style":"normal"}] ## PKU Highlights Key Genetic Principles [9f5324527cf0] 1. **Recessive Inheritance:** PKU only manifests in individuals with two defective alleles. 2. **Gene-Environment Interaction:** Symptoms can be prevented by controlling phenylalanine intake. 3. **Autosomal Inheritance:** PKU affects males and females equally since the PAH gene is on a non-sex chromosome. [{"_key":"block_28","_type":"block","asset":null,"children":[{"_type":"span","markDefs":[],"marks":[],"text":"In what ways does the study of PKU connect to broader questions in biology, such as the role of nature versus nurture in determining traits?"}],"markDefs":[],"style":"normal"}]