A young woman is referred for genetic counseling. She has a 3-year-old boy with developmental delay and small joint hyperextensibility. The pediatrician has diagnosed fragile X–associated mental retardation. She is currently pregnant with her second child at 14 weeks of gestation. The family history is unremarkable.
Questions:
1. What is the genetic mutation responsible for fragile X–associated mental retardation? How does it cause the clinical syndrome of developmental delay, joint hyperextensibility, large testes, and facial abnormalities?
2. Which parent is the probable carrier of the genetic mutation? Explain why this parent and the grandparents are phenotypically unaffected.