RAM issues

I got a 2012 i5 27" iMac. My first Mac. Had my last pc for 12 years. I know I was way overdue gotta say besides windows issues and reinstalls never had a single hardware problem. Was excited to get a Mac and info love it. Not as hard a switch as I thought but, after 10 months it started having issues. It started shutting down and not restarting followed by 3 beeps. Went to the Genius Bar and turns out to have been the ram. Had the own ram(2x2) and I added some pny(2x4) turns out it was the own ram that croaked. I was shocked. Has anyone else had this problem? Is it just that own ram is low grade or something? What would cause this? I don't wanna have this happen in the same time frame if there is something I could do to avoid a repeat?

 

I wasn't thinking it was apples fault. I own 1 if everything they make. Lol. There parts(ram) are supplied by all the same manufacturers. Will one bad ram module infiltrate another module? I got time machine running in an external I think I'm safe as far as data. Thanks