Understanding Phenylketonuria and Pleiotropy

The question asks us to evaluate two statements about Phenylketonuria (PKU). Let's analyze each statement carefully.

Statement I: Phenylketonuria is an example of Pleiotropy

Pleiotropy is a genetic phenomenon where a single gene mutation affects multiple distinct phenotypic traits. In simpler terms, one faulty gene can cause several different problems or symptoms in an organism.

Phenylketonuria (PKU) is a genetic disorder caused by a mutation in the gene that codes for the enzyme phenylalanine hydroxylase. While the primary defect is in this enzyme, the consequences of the resulting biochemical imbalance (high levels of phenylalanine) affect multiple body systems. Symptoms of untreated PKU can include intellectual disability, seizures, behavioral problems, psychiatric disorders, and lighter skin and hair pigmentation compared to unaffected family members. These multiple, seemingly unrelated effects stemming from a single gene mutation are characteristic of pleiotropy.

Therefore, Statement I, which claims Phenylketonuria is an example of Pleiotropy, is correct.

Statement II: Affected individuals lack an enzyme which converts phenylalanine into tyrosine

Phenylketonuria is an autosomal recessive metabolic disorder. It is caused by a deficiency in the enzyme phenylalanine hydroxylase (PAH). This enzyme is crucial because it catalyzes the conversion of the amino acid phenylalanine into another amino acid, tyrosine.

The biochemical reaction is:

\text{Phenylalanine} \xrightarrow{\text{Phenylalanine Hydroxylase}} \text{Tyrosine}

When the PAH enzyme is deficient or non-functional due to a genetic mutation, phenylalanine cannot be effectively converted to tyrosine. This leads to a buildup of phenylalanine in the blood and other tissues, which is toxic to the central nervous system and causes the health problems associated with PKU.

Therefore, Statement II, stating that affected individuals lack an enzyme which converts phenylalanine into tyrosine, is correct. The specific enzyme is phenylalanine hydroxylase.

Based on the analysis of both statements:

• Statement I is correct because PKU is a classic example of pleiotropy, where one gene defect leads to multiple symptoms.
• Statement II is correct because PKU is caused by the lack or deficiency of the enzyme phenylalanine hydroxylase, which is needed to convert phenylalanine to tyrosine.

Thus, both statements are correct.

Revision Table: Phenylketonuria Key Facts

Additional Information: Pleiotropy in Genetics

Pleiotropy is a common genetic phenomenon. Many genetic disorders in humans are pleiotropic, meaning a mutation in a single gene affects multiple traits or organ systems. Understanding pleiotropy is important for studying the full range of effects of a genetic mutation and for developing treatments that address all the affected systems.

Examples of other disorders exhibiting pleiotropy include:

• Marfan Syndrome: A defect in the gene for fibrillin-1 affects connective tissue throughout the body, leading to problems in the skeleton, eyes, heart, and blood vessels.
• Cystic Fibrosis: A mutation in the CFTR gene affects chloride ion transport, impacting the lungs, pancreas, digestive system, and sweat glands.
• Sickle Cell Anemia: A single point mutation in the hemoglobin-beta gene causes abnormal hemoglobin structure, leading to various problems like anemia, pain crises, organ damage, and increased susceptibility to infections.

In all these cases, a single genetic change has widespread effects on the organism's phenotype, demonstrating the principle of pleiotropy, just as seen in Phenylketonuria.