What's in a Name: Statistical Genetics
Mendel published his results 1866 but they were rediscovered only in 1900. The Hardy-Weinberg model is an application of Mendel's rules to a population that is not under selection forces. So the one builds on the other, and Hardy-Weinberg is a simplification model-wise, and Mendel's rules are not detailed enough either. It's the same relation as with a physical law like gravitation and a mathematical model of its consequences applied on many entities, like models of a solar system's gravitational behaviour.