According to an interview with Jaleel White, the show would not have returned after the 1989-1990 season if Steve Urkel didn't join the cast. Urkel's character being a hit convinced ABC to keep the series and give it another chance.

I don’t know how true it is, ratings were fine in season 1 even before he came

If the show had not come back, I wonder if Jo Marie would've returned to Perfect Strangers.

God bless you and Jo Marie always!!!

Holly