> Genetic predisposition to something != inevitability.

That depends on the thing. Whether you grow five or six fingers on your hands is determined by your genes without much reference to anything else. Whether you reach a "natural" height or develop stunted by malnutrition is also determined by your genes. It is more responsive to environmental conditions than the number of fingers on your hand is - but that responsiveness is determined by your genes.

My wife is -3 myopic, I'm -6 from childhood. And our 3 kids of 19, 17 and 10, have all a perfect sight.

They went outside often but nothing extreme. We have big luminous windows. And they spend a fair amount of time on screens too.

It can be so multifactorial, I have no idea why this luck. As I was convinced they would all wear glasses.

All of my immediate family members need glasses but me, which is pretty interesting.