#i don't usually get to flex my science muscles with fire emblem

[iturbide]

(hey you should totally flex ur science muscles on crests/relics because that stuff STILL wilds me out it's so cool even if i only really truly understood part of it)

oh no bany what have you unleashed

okay so for anybody not in the know about this (which is...probably everybody?), I have a deep fascination with science shit.  Biology was absolutely my jam in school, and sometimes I still just go Wiki-diving into weird little science topics for no reason besides “I saw something cool mentioned in a post/article/fic and now I want to know more.”  I have a Science Hat that I mostly only get to wear when Pokemon comes into play, but somehow or another Three Houses managed to hit just the right balance of magic and technology to let me run wild on the science side.  Usual disclaimer applies, this is all just personal speculation and headcanon territory.

Anyway let’s talk about the Crest Virus.

I have this very weird theory.  That dragon blood, when it enters the human system, essentially acts like a virus -- more specifically, like a retrovirus.  The genetic sequence from the dragon ‘donor’ invades the host’s cells and begins to rewire them completely: it splices its sequence into the host’s DNA and begins to produce a shocking variety of new proteins and other molecules both within the cell and attached to the cell’s exterior surface.  It also reproduces its gene package and ships it off within the host organism to continue spreading the genetic material -- which is a key strategy in the early ‘infection’ stage, when the host’s immune system is going on the attack; the repackaged genes originated internally and hijack some of the host’s unique personal antibodies, tricking the immune system into thinking that they’re not invaders at all.  Once all host cells have the ‘donor’ genes (characterized by the altered cellular receptors, which won’t accept the gene package), the virus itself becomes inert and cannot be transferred to another individual. 

Depending on the quantity of dragon blood received (and possibly depending on the method of transfer, as well -- direct injection would be vastly more efficient than ingestion), the ‘infection’ process could be very rapid, overtaking the host within a matter of days for large quantities that rapidly gain a foothold in many locations; or very slow, requiring weeks or even months for small amounts where the virus must spread from one point out while fighting the immune system at every turn (since a human immune system will get wise to infection pretty damn fast and can attack its own infected cells; this autoimmune response may be what’s responsible for the deaths in the Twisted experiments, since their reserves of dragon blood carrying an active virus can’t be replenished and they’re trying to be sparing with their limited stock.  Ultimately, though, anyone who survives the procedure ends up with a Crest due to all the extra genetic material inserted into their genome -- which could additionally explain the extraordinarily long lifespans in first-generation Crestbearers. 

In humans and many other organisms, DNA replication is not a perfect thing; with every copy, a tiny bit of genetic information is snipped off the end.  To defend against genetic deterioration, chromosomes end in a number of repeating non-coding sequences called telomeres; their decay is what causes ageing and associated physical decline in humans.  By adding in so much additional genetic data, it could stave off normal ageing, especially if the new code is added in a place that it defends key human genes from deterioration.  It could also explain the bizarre case of Maurice: somehow or other, the Crest-bearing genes ended up in the wrong place, and vital human genes deteriorated through repeated divisions over time -- and once they decayed too far, the dragon genome inserted in his cells took over, transforming him into a beast who retained his human consciousness.

As an added “bonus,” though, this particular virus doesn’t just affect somatic cells the way most viruses do.  It also affects reproductive ones, as well -- though in that instance, it’s not the sex cells themselves that get the virus, but the germ cells that produce them within the reproductive organs.  Because the Crest genes undergo meiosis along with everything else, the genome is incomplete, and because it’s not a normal part of the human genome there’s no guarantee that the sequence will be completed upon fusion with another gamete the way it would be in dragons.  And this is part and parcel of why Crest inheritance is such a gamble: a thousand years after the death of the Ten Elites, the Crest genomes are scattered all over, and there’s never any guarantee that meiosis is going to preserve the gene or completely strip it away.  People lucky enough to get all the correct genes markers end up with a Major Crest; people with partial gene markers end up with a Minor Crest; and those with incomplete gene markers end up with none at all; they could still be a carrier, technically, but key Crest markers would be required to actually produce a Crest in any offspring (and that’s assuming the meiosis is kind).

Now then from there we can turn into the madness of Relics.

So dragons are, to put it gently, weird.  It might be possible to kill their physical bodies, but their Creststones (effectively their heart) clearly retains some measure of life even after death; Sothis’ Creststone within Byleth imparts a literal part of her within them, after all, but beyond that a Relic is inert until its proper Creststone is embedded in it, at which point it starts twitching.  It’s made of bone, but it twitches and moves simply from having its heart attached.  It’s important to note, though, that while the Relic might move, it’s not technically ‘active’ at that point because it’s not yet a complete system; the heart is present, as is the body, but there is no blood.  Relics can only be activated, and their special powers accessed, by the appropriate Crestholder, because their blood in proximity to the bones and heart fools what remains of the dragon into thinking that the system is complete.

Unfortunately, repeated use of a Relic by someone without the appropriate Crest stresses the unbalanced system, and eventually the remnant of the dragon within the Creststone will try to rectify the matter -- by trying to recreate its body, using both its true bones as part of the Relic and the body of whoever’s unfortunate enough to be holding it both as the basis of the change.  Which is exactly what happened to Miklan.  Miklan wasn’t actually granted a Crest, though: the Creststone just consumed his body, so even if he had survived he wouldn’t have ended up with a Crest.

And I think that mostly sums up my weird thoughts on how Crests and Relics work it’s a lot of mumbo jumbo mostly but it’s a ton of fun to me.