This follows anime canon.
Envy was the supposed result of Hohenheim's failed attempt to resurrect the son of himself and Dante. This was 400 years ago, according to their timeline. From what I gathered, this occured after the two of them had begun switching bodies, since they hadn't been able to do so until they first created the Philosopher's Stone.
But what really gets me is this--when Envy reveals his true form, he resembles Hohenheim. We know that he and Dante had jumped bodies several times, and that his last body was not the one that he had possessed 400 years ago. So why is it that Envy's true form looked like the biological father of Edward and Alphonse Elric?
We know for a fact that Envy was close to 400 years old, being born shortly after Dante and Hohenheim began switching bodies. So why would he have looked like Hohenheim's last form, since that wasn't the body he had when Envy had been conceived in his first incarnation as a human? Their bodies began to rot sooner after each jump, so there's no way that Hohenheim could have had the same body for 400 years. The only explanation would have been that he had acquired a body somewhere through the years that was genetically similar to the one that Envy had been spawned from, perhaps a descendant of that body. Being a more emotional person than Dante, I think that it would stand to reason that Hohenheim would take such an action, perhaps to feel more normal.
Well, those are my thoughts, I hope they made for an interesting addition and would highly appreciate any feedback. Thank you for the opportunity to express this to you.