For the last in the series, we'll deal with the data that we have on Wesen in general, since there's a fair amount of that as well. A lot of it amounts to either things which are implied to work by magic, but some of it also says interesting things about the ability of Wesen to interchange various things between species, such as diseases, drugs, and genetic material. We'll start with that genetic material part, also known as Wesen hybrids.
It was a discussion of Wesen interbreeding that started this whole essay in the first place, and if I had a smidgen more biology knowledge or the time to research the subjects relevant I could probably go on for the length of a doorstopper. In the interest of not hurting either of our brains, let's just summarize and speculate wildly, which is much more fun. We know that cross-breeding between Wesen and human creates a viable offspring, because we have the example of one Sean Renard, half-hexen sexybiest royal pain in the ass. In the first season we also had the Ziegvolk-Human pregnancies but since we never saw those women carry to term we don't know how viable those offspring would have been; Renard's example suggests they would be physically healthy and capable. We know that crossbreeds between Wesen are potentially physically possible at least through artificial genetic mutation, because we have Pierce the Lowen-Geni Innocuo crossover. Who is violently unstable, but it's impossible to say without an exhaustive study of just what was done to him (which we're not going to get and I can't say I'm sad about that) whether that was a result of the choices made during the course of the experiment or whether all such experiments would be unstable. And let's please not to be repeating that, yes? Yes. On the other hand, we also know that natural cross-breeds creating a child is possible, because we have the pregnant Seelengut at the end of The Good Shepherd. Whether or not that offspring would be healthy and viable or whether there would be similar aforementioned instabilities or other genetic difficulties is another question never answered. Traditionally, species hybrids are infertile, although there are less than a hundred known instances of mules (a hybrid offspring between a male donkey and a female horse) bearing offspring to one of the parent species. In the case of Wesen and human hybrids, barring certain other social phenomena taking place, the resultant hybrid does seem to be capable of siring or bearing offspring, indicating a high degree of compatibility, perhaps higher than that of a Wesen subspecies hybrid.
Although that's all the information we have on cross-breeding from within the text, we may also be able to extrapolate from other aspects of Wesen life that we learn from other avenues. For example, the proscription against interbreeding may stem from the same appearance-based psychology and prejudice from whence it comes in our world, or it might stem from an inherent tendency towards instability and uncontrolled violent behavior. Prior to the increased availability of birth control abstinence would have been the only way to ensure that pregnancy did not result from an interspecies marriage and, as a result, that villages were not overrun by hybrid children with incredibly poor self control and violent outbursts. Equally possible, though as yet there's even less direct evidence for it, is that a hybrid human-Wesen or dual-Wesen hybrid is capable of greater ability and greater control over their woge. Though we have only one example of a hybrid exhibiting such control (Renard, who only experiences woge when under prolonged exposure to a psychoactive potion or when deliberately self-inducing) there is plentiful evidence to suggest that hybrids of real-world species display certain traits more strongly than either parent, e.g. the size of a liger or the intelligence of a mule. If it were the case that a Wesen-human or Wesen-Wesen hybrid maintains greater control over woge and any abilities that result, that would be a cause for alarm among the factions which seek to control Wesen, and thus a proscription would most likely be enacted.
We've also been exposed to an array of simplistic medical data regarding the Wesen, thanks at least in part to Rosalee's profession as an apothecary and starting, in fact, with her introduction. Apparently a mold that is toxic to humans is only intoxicating to Wesen, which is neither surprising nor remarkable; any pet lover knows that certain foods are toxic to pets and there are several plants whose toxicity is dependent on how much of which part of the plant one consumes. That said, though, the difference in drug interaction implies some basic similarities within Wesen biology as to enzymes, digestion, nutrient intake, and so on, particularly emphasizing that these are ways in which all Wesen are different from humans. Which really prompts one to wonder about all the books in Rosalee's shop, and if the solution that she concocted for Hank and Wu's poisoning and Juliette's drugging was designed for humans or Wesen, or if in those cases it didn't matter. This difference between humans and Wesen hasn't been brought up again so far, although she is depicted as having quite an extensive library, so it's possible that some Wesen at some point wrote down a number of antidotes for humans specifically so as to help a friend. We also have the example of the Yellow Plague, which causes similar symptoms in different species of Wesen but as far as we were able to see, didn't affect humans at all.
I have no explanation for the various archaic medical equipment Rosalee uses when it would be just as easy and probably more sanitary to use modern equipment, but we can also put that down to the fact that magic clearly exists, too. Because that must be considered as well.
At the crossroads of biological traits all Wesen share and the magical aspects of the show sits, firmly planted on a seat of narrative necessity, the woge. It is essential that some external trait or aspect exists, first of all, so that the audience knows the person in the picture is Wesen and not human. With vampires it's easy enough, all they have to do is show fang, and with shapeshifter species one of the easiest and most common ways to indicate 'shapeshifter' is via the eyes. For Wesen, they go one step further (presumably since the various species would have different ocular traits) and give a full-face morph, which Monroe indicates is called 'woge' at least by the German-speaking set of Wesen. While physical shifts in appearance are a common and well-documented method in nature for driving away predators, the most common involve coloration changes or presentation of physical attributes (skin flaps, spines) that were previously lying flat against the body. Spontaneous hair growth or extension of spines is not likely, especially given that the hair and spines would then have to retract. At a guess, then, the woge is equal parts narrative conceit and magical manifestation of the inward self, fitting more with the fantasy aspects of Grimm than the urban part of urban fantasy fiction. The one aspect of woge that might exist in a more tangible, real-world sense is some form of temporarily increased strength or speed, faster reflexes. This could be a Wesen version of the adrenaline surge, fight or flight response causing various body changes to increase the chances that an individual organism will survive whatever is threatening it. Human beings, even, have this response, so there's no reason a Wesen wouldn't have it either, and since woge is described as being something that happens when the Wesen is experiencing high emotional output, they would most likely go together.
We haven't yet seen a Wesen get hospitalized for anything more significant than some cuts and bruises and a broken limb or two, so it's hard to say what medical complications might arise there. But it's almost certain that some portion of their blood, either the type or perhaps some strangeness regarding the rh factor, is different from the human norm. Which brings up the question, then, would a Wesen be able to donate blood to a human, or vice versa? Is it like the problem of the universal donor, who can give to anyone but only receive of the same type? What about bone marrow? Organ donation? Can Wesen donate organs across species lines or can, say, an Eisbieber only donate a kidney to another Eisbieber? If so, I would be fascinated to know what kind of organ donation network the Wesen community might have amongst itself. Is it run by ethical Geier? Fandom, get on this, too.
All right, speaking of Geier and organ dealing, let's go back to that. Monroe states that human organs, dried and powdered or otherwise prepared for ingestion, does have an effect on Wesen physiology. Apart from this giving the carnivorous or omnivorous and more predatory Wesen a good excuse to go after humans for food, particularly the wendigo, there is at least some real-world precedence for this even if it's not the direct type-to-type correlation that human organs seem to have for Wesen (e.g. Monroe's discussion of testicles and Wesen erectile function). Livers, for example, contain various nutrients that are highly beneficial. Certain types of animal muscle protein are believed by some to increase the capacity for muscle gain in the person ingesting. Depending on the enzymatic differences to humans as mentioned above with drug interactions, it's possible that the breakdown of the purpose-oriented material in each organ enhances the corresponding organ in the Wesen who ingests it. It'd be a hell of a weird adaptation, but not the strangest one nature has coughed up.
In the interests of full disclosure, I have to say, none of my various areas of special concern involve a biology degree of any kind. A couple years worth of biology courses as part of overall curriculum requirements, yes, and a couple decades worth of experience researching anything and everything in the interests of fleshing out stories. If you happen to be a biologist and would like to correct me on something or offer insight, feel free! My appetite for learnings is vast and not picky. But this completes a series of ramblings and speculation, as well as a compilation of the information we have learned from the show and what I've been able to extrapolate from it, on the subject of the science in the fiction of Grimm.