Jessie (tan, hornless), when matched to Jared (black, normal horns), has produced only 1 in six with horns. Consider the following kids:
Nylan (black, horned)
Anna (black, no horns), Jenna (white, no horns)
Lerris (black, no horns)
Kagan (black, no horns), Maggy (white, no horns)
(Note that Goldie, paired to Jared, has only produced kids with horns.)
According to Dr. Phil Sponenberg of VA Tech, "The hornlessness is pretty well worked out - you should on average get 1/2, but in small groups it can deviate - you got lucky! In my own herd I tend to get just under 1/2, but that is luck!"