En route to Thorung La I learned something new from a doctor in Manang at the daily HRA Altitude Talk.
High-altitude adaptation has a genetic component.
Some people who live at high altitudes suffer breathlessness, palpitations and dizziness, while others have no health problems, and now a new study reveals which genes may explain the difference.
The genetic changes, described today (Aug. 15, 2013) in the American Journal of Human Genetics, allow people to take in enough oxygen from the thin mountain air without developing the heart attacks and strokes of chronic mountain sickness.
“We have ascertained there is a major genetic component that allows populations at high altitude to live better,” said study co-author Dr. Gabriel Haddad, a pediatric pulmonologist at the University of California at San Diego. …
Altitude Sickness: Genetics May Explain Why Only Some Fall Ill

I’ve been over 5000m dozens of times, never having any altitude sickness.
Perhaps I’ve got good genes. 🙂
Read more on wikipedia – High-altitude adaptation in humans – Genetic basis:
… The underlying molecular evolution of high-altitude adaptation has been explored and understood fairly recently. Depending on the geographical and environmental pressures, high-altitude adaptation involves different genetic patterns. …