Hi Robin,
Welcome to WikiTree.
I'm sure that your question has arisen before, however I'll give you a quick overview that might be adequate for your needs. If not, please reply.
It's important that you always enter the blood relatives appropriately, and of course that means showing the children of their respective birth parents (if known).
In effect, in the case of the entry for your father, show both spouses of your grandmother, and show your father born of his blood father. On the profile for his father, enter the birth of your father, but elaborate on the short parental envolvement in his biography.
For the adoptive father, you can show his as the non-biological father, and again elaborate on the adoption in his biography. You can also explain in greater detail in your faher's biography.
I hope that's of some assistance.
Norm.