A couple has two children affected with tuberous sclerosis. On detailed clinical and laboratory evaluation (including molecular studies) both parents are normal. Which one of the following explains the two affected children in this family ?
First, I recall that tuberous sclerosis complex (TSC) is a genetic disorder, typically autosomal dominant. However, since both parents are normal, this suggests that the children might have a de novo mutation. But wait, if both parents are normal, how can two children have the condition? De novo mutations would mean each child has a 50% chance, but since both parents are unaffected, maybe it's not a de novo mutation in each case. Alternatively, maybe there's a germline mutation in one parent that wasn't detected in the molecular studies. But the question states that molecular studies showed both parents are normal. Hmm.
Another possibility is that the parents are carriers of a mutation in the TSC1 or TSC2 genes, but the molecular studies missed it. However, the question says detailed molecular studies were done, so that's less likely. Wait, maybe the disease follows a different inheritance pattern here. Tuberous sclerosis is usually autosomal dominant with high penetrance. If both parents are normal, then the children can't inherit the mutation from them. So each child would have a new mutation. But having two children with the same de novo mutation is extremely rare. So maybe the parents have a germline mosaicism that wasn't detected in the molecular studies. Germline mosaicism means that the mutation is present in some of the parent's germ cells but not in their somatic cells. So the molecular studies looking at somatic cells (like blood) might miss the germline mutation. That would explain two affected children without the parents showing the condition.
So the correct answer would be germline mosaicism in one of the parents. The options probably include something like that. Let me check the possible options. The distractors might include other explanations like autosomal recessive, X-linked, or other inheritance patterns. But since TSC is autosomal dominant, those are incorrect. Another distractor could be non-paternity or consanguinity, but the question doesn't mention that. The other option might be mosaicism in the parents, which fits here.
So the answer is germline mosaicism. The parents have a mutation in their germ cells that wasn't detected in their somatic cells, leading to two affected children.
**Core Concept**
Tuberous sclerosis complex (TSC) is an autosomal dominant disorder caused by mutations in *TSC1* or *TSC2* genes. Germline mosaicism occurs when a mutation is present in gametes but not in somatic cells, explaining recurrence in families with apparently unaffected parents.
**Why the Correct Answer is Right**
Germline mosaicism in one parent allows transmission of the pathogenic mutation to offspring despite normal somatic cell testing (e.g., blood DNA). This explains two affected children when standard parental molecular studies (assessing somatic tissue) are negative. The mutation arises in the germline (sperm/egg) and is not detectable in the parent’s body tissues.
**Why Each Wrong Option is Incorrect**
**Option A:** *Autosomal recessive inheritance