Fisher showed that total trait variation can be decomposed: VP = VA + VD + VE + VEnvironment. Adjust each component and watch heritability change.
Same seeds, different soil. Heritability is high in both fields — but the difference between fields is entirely environmental.
Identical twins share 100% of DNA. Fraternal twins share 50%. The difference in similarity reveals genetic influence.
Height, BMI, blood pressure — the same concept, very different numbers. Why?