Germline Mosaicism (rare inheritance mechanism)

**What it means:** Germline mosaicism is a rare phenomenon where a parent has a mutation in some of their reproductive cells (sperm or eggs) but NOT in their body cells. This means the parent tested "normal" on standard blood tests but could still pass the mutation to children. **Implication for MHS families:** While most MHS cases are truly de novo (random, one-time event), germline mosaicism means the recurrence risk for parents of an MHS child is slightly higher than zero (estimates range from 1-2%). This is why genetic counseling is important for families.