One of TV's most durable and wholesome sitcoms is about widower Steve Douglas and his boys, who shared a house with their maternal grandfather, and later, their crotchety uncle. The series was originally scheduled to be a vehicle for the Lennon Sisters of `Lawrence Welk Show' fame. Characters came and went, but this was an endearing, highly rated series throughout its 1960-72 run; when it moved from ABC to CBS in 1965, Mike, the eldest, was written out, and replaced by adopted son Ernie.