Most people think of any super-powered humans as mutants. Nothing could be further from the truth! Many meta-humans gain their powers in other ways, such as direct bestowal from a god (or similar entity, such as a demon), chemicals, devises or whatever. In reading this, it's important to keep this in mind
What defines a mutant is the meta-gene complex, also called the (MGC). This is a collection of 18 changes scattered over a number of chromosomes. Most people who have the meta-gene complex (at the last survey this was about 1/6th of the population) aren't "mutants". That requires activating the complex. Nobody knows how that happens. The whole concept of a "mutant" is, therefore, rather fuzzy. It's solid enough for the mass media and politicians, but once you get beyond the rabble rousers it becomes less and less precise.
It should be noted that while there is no "mutant test", it is possible to genetically test to see if the meta-gene complex is active. While there are people with an active complex who don't have super powers, all mutants with measurable abilities have an active complex.
Information on the mutation process, and a bit on how it applies to Sidhe mutations
- The important parts changed as part of the power spike that exists during the manifesting period. That's when 'most' mutants change at a foundation level... the stuff that's 'deeper' than just biology. It's like the discussions about TG Fiction and how most transformations won't work they way they're depicted. Taking hormones as an adult might have result in breasts, some subtle changes to looks as a result of skin/subcutaneous fat distribution and such... but it won't make your hips wider (bone placement doesn't change), won't change your voice, etc... UNLESS it happens early in life that you undergo those basic growth cycles into adulthood under their influence. But even then, it won't happen quickly, and it won't change your sexual organs (although they will probably be very small) or genetics...
- Even worse, if you were to be changed via a retro-virus that replaces all your male cells with female ones... you won't even benefit as much as you would from hormone treatments. Because you'd still have glands and organs producing male hormones and a fully formed body. For changes to occur, the body would have to be able to reject the old organs and tissue... and somehow survive the rejections long enough to either grow replacements or alter the existing ones in ways that the human body doesn't have a system to do. And, like with hormones, the adult body may replace cells regularly... but there's not a lot of difference between genetically male and female cells... its the larger picture organs, that only really form at one stage in your development, that would have to change for anything else to happen.
- So, the critical thing medically and mutant mechanics wise to understand is that manifestation is not just a matter of establishing a new normal and then slowly (or even rapidly through regeneration) healing your way towards it. Manifestation is an enormous energy spike defining not only what the end result of your mutation is going to be, but generating a road map for your body to get their. Certain traits have to evolve and come into play before others, so that when those others occur, you can survive them. If things get out of order... or energy isn't available to complete something that protects you before the next stage goes into effect... that's when you get burnout problems. Like an energizer tapping into the earth's magnetic fields to channel lightning before their body develops a nervous system capable of bearing high voltage electrical loads. Many times, mutants who are in manifestation have working regeneration... but by the time they are fully manifested, they do not. This is part and partial of the core foundation having to evolve into place extremely rapidly to avoid stages where critical organs have to replace baseline ones... and if it doesn't happen quickly enough, you die. Because of the excessive demand on the regeneration that's available, it burns out the trait and the remainder of the mutation happens more slowly over time.
- With adequate support and nutritional resources on hand at the moment of manifestation, it could be possible that 'every' mutant would end up with at least a moderate level of regeneration instead of it burning out in the process.
- So... summary - no, Fey did become a Sidhe almost immediately. Her physical appearance from a Sidhe that looked like Nick to one that looks like Nikki... took some time.