My first buff dewlap goose had no dewlap for the first couple years of her life, and then she started to slowly develop one. She is 10 years old now, and you cannot tell her dewlap apart from any other dewlap toulouse. I have another buff girl with a less pronounced dewlap and her mate has gorgeous dewlap and their babies develop dewlaps like their father.
Finally, while I have not crossed my dewlap toulouse with any other breeds, I do have one gander out of an african dewlap gander/production toulouse goose accidentally born here a couple years ago. He is a very good looking bird, with a large dewlap.
One final note... I've never had a dewlap toulouse successfully hatch her own babies, but my production toulouse are the exact opposite. In fact, my one buff production toulouse is the best hatcher I've ever had, and would hatch two large clutches per year if I let her.