Genetic disorder caused due to pleiotropic gene mutation is:

Question

Accepted Answer

The correct option is 3: Cystic fibrosis

The genetic principle being tested here is pleiotropy, which occurs when a single gene mutation has multiple, seemingly unrelated effects on an organism's phenotype. Cystic fibrosis (CF) is a classic example of this. It is caused by a mutation in the Cystic Fibrosis Transmembrane conductance Regulator (*CFTR*) gene. This gene is responsible for producing a protein that regulates chloride ion transport across epithelial cell membranes. A defective CFTR protein leads to thickened, sticky mucus and overly salty sweat. Critically, this single gene defect results in a multitude of problems in different organs: chronic lung infections, pancreatic insufficiency, blockages in the liver, and male infertility. The wide array of clinical symptoms, from respiratory failure to digestive problems, all stemming from a mutation in just one gene, confirms that Cystic fibrosis is a pleiotropic genetic disorder.

The incorrect option is 1: Down syndrome

Down syndrome is a genetic condition caused by an abnormality in chromosome number, specifically Trisomy 21 (three copies of chromosome 21). This is a chromosomal aberration, not a single-gene mutation. While the presence of an entire extra chromosome certainly leads to a wide range of phenotypic effects, the mechanism is due to gene dosage imbalance across the chromosome, which is conceptually distinct from the definition of pleiotropy, where a single mutated gene is responsible for multiple downstream effects.

The incorrect option is 2: Huntington's syndrome

Huntington's syndrome is an autosomal dominant neurodegenerative disorder caused by a mutation (a trinucleotide repeat expansion) in the *HTT* gene. It is a single-gene disorder, but its primary symptoms are highly specific to the nervous system, involving uncontrolled movements, cognitive decline, and psychiatric problems. Although the *HTT* gene is expressed throughout the body, the most profound and clinically relevant pleiotropic effects, which would make it an undeniable answer to this MCQ, are not as traditionally expansive and diverse across multiple distinct organ systems (like the lungs, pancreas, and sweat glands) as is the case with Cystic fibrosis.

The incorrect option is 4: Phenyl Ketonuria

Phenyl Ketonuria (PKU) is, in fact, another classic textbook example of a pleiotropic disorder. It is caused by a mutation in the gene for the enzyme Phenylalanine hydroxylase (PAH), which prevents the metabolism of the amino acid phenylalanine. The accumulation of this toxic compound subsequently causes a range of effects, including intellectual disability, light skin and hair, and eczematous rash. However, in the context of multiple-choice questions where a definitive single best answer is sought, Cystic Fibrosis (Option 3) is frequently preferred as it affects the largest and most complex number of major organ systems (lungs, digestive tract, reproductive system) with highly distinct clinical consequences, thereby serving as an arguably more illustrative example of pleiotropy than PKU.