I would highly recommend reading 'A brief history of everyone who ever lived' by Adam Rutherford. IIRC it contains a whole chapter on GWASs (pronounced jee-wazzes).

There are some phenotypes (characteristics) which clearly can be associated with a small number of genes (e.g. autosomal recessive disorders) , however for many phenotypes such as intelligence, it takes a lot more effort to correlate them with underlying genotypes. The fact that intelligence is approximately normally distributed is an indicator that there are many individual effects at play.