We don't understand a lot about genetics. Even the most accomplished of scientists would tell you they are just beginning to scratch the surface. Gregor Mendel knew a lot about genetics 125 years ago. Breeders of animals and plants know a lot about genetics. But we have only scratched the surface.
Genetics explain - even control - hair and eye color, body height and shape and other factors of human appearance. Handsome, blond haired, blue eyed people tend to have handsome, blond haired, blue eyed offspring.
Physical strength may be determined in part by genetics. But genetics can have a dark side. A number of diseases have a genetic factor. I know this from my own experience and I pray every day that none of my sons and grandsons will inherit that particular gene or genetic mutation.
Scientists may one day know enough about genetics that we will all be strong, handsome, and disease free. Until then, we must each play the hand we are dealt.