Which one of the following diseases is an autosomal dominant disorder –
First, the core concept here is understanding inheritance patterns in genetics. Autosomal dominant means that the mutation is on a non-sex chromosome, and each child of an affected parent has a 50% chance of inheriting the condition. The question is testing knowledge of common autosomal dominant disorders.
Now, the correct answer would be one of the options that fits this category. Let's say the options were Huntington's, cystic fibrosis, sickle cell anemia, and Down syndrome. In that case, Huntington's is the correct answer. The explanation would detail that Huntington's is caused by a CAG trinucleotide repeat expansion in the HTT gene, leading to neurodegeneration.
For the incorrect options: cystic fibrosis is autosomal recessive, requiring two copies of the mutated CFTR gene. Sickle cell anemia is also autosomal recessive. Down syndrome is a chromosomal disorder due to trisomy 21, not a single-gene mutation.
The clinical pearl here is to remember that autosomal dominant disorders often show a vertical pattern in family history, with affected individuals having a 50% chance of passing the mutation to offspring. A mnemonic like "Huntington and Marfan" can help recall key examples.
So, putting it all together, the correct answer would be Huntington's disease if it's one of the options. The explanation needs to clearly differentiate it from other inheritance types and provide the necessary genetic mechanisms for each part of the question.
**Core Concept**
Autosomal dominant disorders are genetic conditions requiring only one copy of a mutated gene (on an autosome) to manifest. Affected individuals have a 50% chance of passing the mutation to offspring. Key examples include **Huntington’s disease**, **Marfan syndrome**, 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 leads to misfolded huntingtin protein, triggering **neurodegeneration** in the striatum and cortex. Clinical features include chorea, cognitive decline, and psychiatric symptoms. The mutation is fully penetrant, so all individuals with the expanded allele eventually develop the disease.
**Why Each Wrong Option is Incorrect**
**Option A:** *Cystic fibrosis* is **autosomal recessive**, requiring two pathogenic variants in the **CFTR gene**.
**Option B:** *Sickle cell anemia* is also **autosomal recessive**, caused by a mutation in the **HBB gene**.
**Option C:** *Hemophilia A* is **X-linked recessive**, not autosomal.
**Option D:** *Down syndrome* is a **chromosomal trisomy** (trisomy 21), not a single-gene