Prader willi syndrome, ch romosomal defect-
**Core Concept**
Prader-Willi syndrome is a rare genetic disorder characterized by a deletion or malfunction of genes on chromosome 15, specifically affecting the paternal copy. This condition is associated with distinct physical features, developmental delays, and a range of behavioral and cognitive impairments.
**Why the Correct Answer is Right**
The correct answer involves a chromosomal defect, specifically a deletion or mutation on chromosome 15. This genetic anomaly leads to the silencing of paternal genes, resulting in the characteristic features of Prader-Willi syndrome. The paternal gene deletion on chromosome 15 is caused by a microdeletion, which is a small deletion of genetic material. This deletion affects the expression of genes involved in appetite regulation, growth, and development.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the genetic basis of Prader-Willi syndrome.
**Option B:** Fragile X syndrome is a different genetic disorder caused by an expansion of the CGG repeat on the X chromosome, not a deletion on chromosome 15.
**Option C:** Turner syndrome is a chromosomal disorder caused by a missing or partially deleted X chromosome, not a deletion on chromosome 15.
**Option D:** Down syndrome is a chromosomal disorder caused by an extra copy of chromosome 21, not a deletion on chromosome 15.
**Clinical Pearl / High-Yield Fact**
Prader-Willi syndrome is characterized by a range of physical features, including short stature, narrow forehead, and a prominent jaw. Patients often exhibit developmental delays, intellectual disability, and behavioral problems, including hyperphagia and obesity.
**Correct Answer: D. Down syndrome is a chromosomal disorder caused by an extra copy of chromosome 21, not a deletion on chromosome 15.