what is standard recommendation of Microsoft to keep FSMO role on Domain controller while we have ADC

Dear all,

we have one domain controller & one Additional Domain controller (windows2012r2) but now all FSMO role holded on domain controller so i am just want to know what would be better for FSMO role hold.either i transfer some role to ADC or keep all role on DC only........please suggest me.what would be happen if DC down with all hold FSMO role.......

Thanks & Regard

Naved Ali