Source / episode info
- **Episode:**428
- **Title:**Divine Intervention Episode 428: X-Linked Dominant Disorders (for Step 1-3)
- **Published:**2022-11-22
- Source:Episode page
One-liner
This episode details the inheritance patterns of X-linked dominant traits (e.g., father transmits to all daughters; mother has a 50% chance), and reviews four high-yield disorders: XLH (PHEX/FGF23), Rett Syndrome (MECP2), Fragile X Syndrome (FMR1/CGG repeats), and Ehlers-Danlos Syndrome (COL4E5).
High-yield summary
- X-Linked Dominant Inheritance: If a father has the trait, all daughters are affected because they inherit his single X chromosome. Sons are unaffected as they receive the Y chromosome from him.
- XLH: Caused by mutations in the PHEX gene -> leads to high levels of FGF23. FGF23 causes phosphaturia (hypophosphatemia) and impairs 1-hydroxylation of calcitriol, resulting in low active Vitamin D.
- Rett Syndrome: An X-linked dominant disorder primarily affecting females; characterized by developmental regression, loss of motor milestones, seizures, and speech impediments, due to a defect in the MECP2 gene.
- Fragile X Syndrome: A trinucleotide repeat expansion disorder involving CGG repeats in the FMR1 gene. Classic findings include macroorchidism (large testicles), long face, and large ears; associated with intellectual disability and high rates of ADHD.
- Ehlers-Danlos Syndrome (EDS): Characterized by connective tissue defects, often due to mutations in genes like COL4E5. Clinical triad includes skin hyperextensibility/bruising, vascular issues, and nephritic syndrome (with a characteristic "basket weave" pattern on kidney EM).
Learning objectives
- Describe the unique inheritance patterns (father to daughters vs. mother to offspring) for X-linked dominant disorders.
- Pathophysiologically explain XLH by linking PHEX mutation -> high FGF23 -> hypophosphatemia and low active Vitamin D.
- Recognize the classic clinical triad of Fragile X Syndrome (macroorchidism, long face, large ears) associated with FMR1 gene defects.
- Identify the key features of Rett Syndrome (regression in females) linked to the MECP2 gene.
- Correlate connective tissue findings (e.g., nephritic syndrome, basket weave pattern) with collagen defects like those seen in EDS (COL4E5).
Board exam buzzwords