Which of the following disorder has been shown to be genetically transmitted by single autosomal dominant genes
First, I need to recall common autosomal dominant disorders. Examples include Huntington's disease, Marfan syndrome, Neurofibromatosis type 1, and Achondroplasia. The question is testing knowledge of genetic inheritance patterns.
Next, the core concept should state the principle of autosomal dominant transmission. The explanation needs to mention that only one copy of the mutated gene is sufficient for the disorder to manifest, and that each child of an affected parent has a 50% risk.
For the correct answer section, I need to pick one of the disorders and explain its mechanism. Let's say the correct answer is Huntington's disease. I'll explain the mutation in the HTT gene leading to huntingtin protein accumulation and neurodegeneration.
Then, each wrong option must be addressed. If options include conditions like cystic fibrosis (autosomal recessive), hemophilia (X-linked recessive), Down syndrome (trisomy), and sickle cell anemia (autosomal recessive), I need to explain why each is incorrect.
The clinical pearl should highlight common autosomal dominant disorders and maybe a mnemonic. For example, "Huntington's, Marfan, Neurofibromatosis—H is for Huntington, M for Marfan, N for Neurofibromatosis."
Finally, ensure the correct answer is clearly stated at the end. Need to keep everything concise, within the character limit, and use proper formatting without markdown.
**Core Concept**
Autosomal dominant disorders require only one copy of a mutated gene for clinical expression. These conditions follow a vertical pattern of inheritance, with each child of an affected parent having a 50% chance of inheriting the mutation. Examples include Huntington’s disease and neurofibromatosis type 1.
**Why the Correct Answer is Right**
Huntington’s disease is caused by a CAG trinucleotide repeat expansion in the *HTT* gene on chromosome 4. This mutation leads to abnormal huntingtin protein accumulation, triggering progressive neurodegeneration. A single defective allele is sufficient to cause the disease, confirming its autosomal dominant pattern.
**Why Each Wrong Option is Incorrect**
**Option A:** Cystic fibrosis is autosomal recessive (requires two mutated *CFTR* alleles).
**Option B:** Hemophilia A is X-linked recessive (affects males more severely; females are typically carriers).
**Option C:** Down syndrome results from trisomy 21 (nondisjunction during meiosis), not a single gene mutation.
**Option D:** Sickle cell anemia is autosomal recessive (requires two mutated *HBB* alleles).
**Clinical Pearl / High-Yield Fact**
Remember "Huntington’s, Marfan’s, and Neurofibromatosis" as classic autosomal dominant disorders. Avoid confusing them with X-linked (e.g., hemophilia) or recessive conditions (e.g., cystic fibrosis). Huntington’s disease is unique for its adult-onset neurodegenerative presentation.
**Correct Answer: C. Huntington’s disease**