Yields on Fannie Mae and Freddie Mac mortgage securities that guide home-loan rates fell to the lowest in almost six months, as the response of European authorities to the sovereign-debt crisis drove investors to the relative safety of US government-related debt. Fannie Mae’s current-coupon 30-year fixed-rate mortgage bonds tumbled 0.10 percentage point to 4.05%, down from 4.67% on April 5 and the lowest since Nov. 30.