In an Autosomal Recessive (AR) disorder, one parent is normal and the other is carrier and the child is also affected. What is the reason –
First, the core concept here is Autosomal Recessive inheritance. Normally, for a child to be affected, both parents must be carriers. Each parent has one normal and one mutated allele. The child has a 25% chance of getting both mutated alleles. But in this case, one parent is normal (so they should have two normal alleles) and the other is a carrier (one normal, one mutated). Wait, that doesn't add up. How can the child get two mutated alleles if one parent is normal?
Hmm, maybe there's a new mutation. If the normal parent has a new mutation in their gamete, the child could inherit the mutated allele from the normal parent and the carrier parent's mutated allele. That would result in two mutated alleles. But new mutations are rare but possible. Alternatively, maybe the normal parent is actually a carrier due to a de novo mutation? But the question states the parent is normal. So another possibility is consanguinity, but the question doesn't mention that. Or maybe the parents aren't the biological parents? But the question doesn't suggest that either.
Wait, another angle: if the disorder is actually autosomal dominant, but the question says it's AR. No, the question states it's AR. So the correct answer must be a new mutation in the normal parent's gamete. That's the most plausible explanation. The normal parent is supposed to have two normal alleles, but a new mutation would give one mutated allele. Then the carrier parent gives the other mutated allele, resulting in the child being affected. So the answer is a new mutation in the normal parent's gamete.
Now, the options. Let's imagine the options. The correct answer would be something like "New mutation in the normal parent's gamete." The other options might be things like consanguinity (but parents aren't related), both parents are carriers (but the question says one is normal), or the parent is a carrier due to mosaicism (but the question says the parent is normal). So each wrong option can be explained based on the scenario given.
**Core Concept**
Autosomal Recessive (AR) disorders require two pathogenic variants (one from each parent) for clinical manifestation. Typically, both parents are carriers (heterozygous), and the child has a 25% chance of inheriting both mutated alleles. However, de novo mutations or consanguinity can disrupt this pattern.
**Why the Correct Answer is Right**
The correct explanation is a **de novo mutation in the gamete of the parent labeled as "normal."** While the parent is clinically unaffected, their germline (sperm or egg) may acquire a new pathogenic variant during meiosis. If the carrier parent contributes the second mutated allele, the child inherits two pathogenic variants, resulting in the AR disorder despite one parent appearing phenotypically normal. This scenario is rare but well-documented in conditions like cystic fibrosis or Tay-Sachs disease.
**Why Each Wrong Option is Incorrect**
**Option A:** "Both parents are carriers" is incorrect because the