Question

To find a gene, you have to associate a phenotype with a genotype (e.g. all individuals with white flowers have a certain allele). It is MUCH harder to find which genes are involved in polygenic traits than it is to find the genes responsible for discrete traits. Why?

Answer 1

T's harder to find one of the many genes involved in a polygenic trait vs. the only gene involved in a discrete trait." 

It is probably not very difficult to find ONE OF THE GENES involved in a polygenic trait (as this answer suggests), but it IS difficult to find ALL OF THE GENES involved in the trait. 

This sounds like a high school Biology or first year university question - chances are they're testing your reading comprehension as much as your scientific knowledge.