Sounds like Yosemite converted the drive to a core storage volume and that is preventing you from manipulating the partitions.
command-r boot back to recovery then from the Utilities menu launch Terminal and enter the command below exactly like I have it included the quote marks. That will delete the main volume and make the disk appears normally in Disk Util.
Now start Disk Utility again and go to the erase tab. Then select the drive brand name itself from the top of the left column. Now select Mac OS Extended (Journaled) in the dropdown and apply that format.
Then quit Disk Util and click reinstall OS X. That will prompt for your AppleID then download and resonate Yosemite for you. Once the install completes and the setup wizard starts, just power down the Mac and you are done.
Code:
diskutil cs delete "Macintosh HD"