When I have an adoption situation where the child is not of either the parent's blood line; I create a new tree. It is only connected to the main tree by the adoption. This is a custom adoption tag, that gives the child's name and refers to her tree.
Doing it this way, I can trace the children of the adopted child, and if I ever earn about the parent of the adopted child, I can trace them also.
If I can not find the parents of the adopted child, her tree starts with her, as her blood lines are unknown. Done this way, the adopt child's line can be traced by genetic testing as far back as you can find the child's blood ancestors.
Again genealogy is about blood ancestor, while it gives a social picture of the family its purpose is to provide the blood line on the person.
We have an example of why genealogy must contain only blood line ancestor. That example is the identification of the body of Richard III. This identification was further hindered by the age of Richard III's remains which meant the MDNA, had to be used. The MDNA link can be broken if if only male connections could be found. With Richard III, fortunately, a complete all female line was found between Richard III and a living member of his family.
If there was a gender change in the above line, this test will fail since the blood DNA links will be lost. Even with gender changes, the DNA is not changed. If one of the apparent females in Richard III line had their gender changed, the MDNA link would be broken.