The more traditionally used method is the single–gene analysis approach which gives information on a few genetic variants at a time. However, complex traits are not determined by a few genetic variants but rather by hundreds to thousands of genetic variants. Therefore, PRS is a more advanced method approach because it takes all of the hundreds to thousands of SNPs that are associated with the trait of interest into account and calculates a single net effect.

To put this simply, the single gene approach is like trying to figure out the picture from a few random pieces of a puzzle, whereas PRS gives you all the pieces of the puzzle put together so you can see the image very clearly.