Studying genetic influences
Genes affect our susceptibility to several common illnesses, but so do many other things, like our diet, surroundings and lifestyle. It's a bit like betting on a horse race - the horse, rider, course and weather can all affect the outcome in a way that is hard to predict. Many genetic and non-genetic factors affect our health (and personality and appearance), but scientists don't yet know what they all are or how they interact. Ways of finding out include population studies and studies of twins.