What happens if genetically identical individuals have different exercise habits?
One Twin Exercises, the Other Doesn’t
Short version: The body and brain of the one that exercises changes in significant and material ways. The one that exercises gets more muscle, a more developed brain in the areas of coordination, and in metabolic health:
"It turned out that these genetically identical twins looked surprisingly different beneath the skin and skull. The sedentary twins had lower endurance capacities, higher body fat percentages, and signs of insulin resistance, signaling the onset of metabolic problems. (Interestingly, the twins tended to have very similar diets, whatever their workout routines, so food choices were unlikely to have contributed to health differences.)
The twins’ brains also were unalike. The active twins had significantly more grey matter than the sedentary twins, especially in areas of the brain involved in motor control and coordination."
This is very significant - it shows that even with the same upbringing and same genetics, exercise matters. Movement improves health in a variety of ways, from ability to exercise and move to brain function to immune and diet function.
Genetics matters, but it's what you do with those genetics that matters, too.