Sex change surgery has always been imperfect. Most sex change operations have proven to be successful to some degree but they're still quite crude in the process. Patients require a lifelong supply of hormones since their bodies don't produce them naturally. Their bodies might appear masculine or feminine (depending on their preference), but they're still unable to function as a true biological males and females. MTF patients are unable to bear children on their own while FTM patients are unable to father their own children. This is due to the fact that they don't have the appropriate sex organs for it.
For MTF patients, their original penis was fashioned in order to appear as a vagina. Cosmetically, the vagina appears nearly identical to what one would think a vagina would look like. However, they don't have a way to develop ovaries for the patient. Ovaries provide biological females with their regular supply of hormones. For MTF patients, they have to be supplied with estrogen in order to maintain their feminine appearance. For bearing their own children, they also don't have a way to develop their own uterus. A uterus is important since this is where the fetus develops before they are birthed by the mother. Despite this, MTF patients are more successful since they are cosmetically identical to biological females albeit with subtle differences.
For FTM patients, they tend to be cruder than their counterparts. Fashioning a penis for an FTM patient is much harder than modelling a vagina from a pre-existing penis. That's why a lot of FTM patients omit having bottom surgery. But for those who choose to undergo bottom surgery, they will develop a penis from a skin graft. This is a very crude process as the skin grafted penis doesn't have the ability to have an erection. There is also no way to develop the testes of the patient hence some choose to put artificial implants. Given that they don't have testes, they also don't have a way to supply their bodies with testosterone, hence relying on external supplies of hormones. Their artificial penis cannot produce sperm hence they don't have the ability to father any children.
Doctor Grugger has been researching about improving the state of sex change surgery. He believes that the methods we have right now are far too primitive for what our technology has to offer. He has a theory that transplanting the sex organs between two parties will solve majority of the issues, but there is a glaring issue with his theory. Organ rejection is common for those who undergo transplants. Patients are subjected to various drugs in order for their bodies to accept the transplanted organ. Doctor Grugger doesn't want this for his patients as this will simply cause his patients the same problem of being dependent on a lifelong drugs. Only this time, instead of hormones, the drugs will instead affect their immune system.
A new theory then emerged from his brilliant mind. Transplants have a lower rejection rate if it's from a close relative. There's a much higher chance of success if say a kidney is transplanted from a mother to a child. The same probably goes with genital transplants. It's never been done before as there is an issue with the ethics of the procedure. He wants to test his theory but he needs to find two subjects that are willing (or unwilling) to be his variables for his breakthrough theory.
He has two specific subjects in mind.