Why are there no female Space Marines?
Have come across some theories which range from the models not being up to scratch, to lore that Space Marines are almost not human, but none of these give a definitive answer.
There are female Imperial guardsmen and there are Sisters of Battle (one of the reasons the sculptures not being up to scratch is a terrible argument).
Anyone have any ideas?
The in-universe answers I have seen are twofold:
Firstly the Primarchs were all male; space marines are created using their gene seed and it reacts poorly to being implanted into a female.
As others have pointed out, from Lexicanum we have:
They must be male because the zygotes are keyed to male hormones and tissue types.
Secondly (and possibly most importantly) the Imperium of Man is a place where tradition and ritual are treated as important above all other things. Space marines were male when the Emperor created them, so they will ALWAYS be male. To even think otherwise is heresy.
From an out-of-universe perspective; Games Workshop first created the Space Marines in the 80s to appeal to 13 year old boys. Back then the idea of girl space army men was probably not in vogue.
it reacts poorly to being implanted into a female : This sounds like you've seen examples. Is there a source for the positive assertion? +1 for the tradition though.
The Imperium has actually lost the technology required to create new gene-seed, especially on the level of a Primarch. Many people have tried, but have failed. The closest that anyone got to creating new gene-seed was actually the Space Wolves, but Magnus attacked Fenris and destroyed the research lab there - forever keeping the Space Wolves from creating any successor chapters. They made one before, but that chapter was corrupted by Magnus himself.
The Emperor did know of Alpharius Omegon, remember that he created both of them. It may have not been planned for the embryo to split, but that's what it did and it had to have happened before Chaos threw the Primarch's across the galaxy. So, the Emperor was aware of the twins but kept it secret for unknown reasons. I believe in one of the books in the Horus Trilogy it briefly discusses when the Emperor found the Primarchs and knew them both to be twins.
Woman would not be ideal candidates to be Space Marines for several reasons.
The closest you'll get to female Space Marines are the Sisters of Battle.
The Adeptus Astartes are the Space Marines, and are (so far as I know) all male.
But don't despair for the women, they are not left out of the never-ending war. The Adepta Sororitas or Sisters of Battle is the women's division of the forces of the Emperor. The Sisters tend to operate more as an arm of the Inquisition than in "traditional" theatres of operation. Check the wikia link for more in-depth information on this chapter.
I also like the comment @Pureferret has that the geneseeds for the Adeptus Astartes are taken from the Primarchs and thus are inherently male DNA, and thus may be incompatible. I'm no biologist, but this does sound like it makes some sense too.
Another way to think about it is that the gene-science used to create the Primarchs and marines was exceptionally difficult. Neither the Emperor nor a similar powerbroker ever succeeds in replicating the same Primarch -> Geneseed relationship again.
Given how precarious the geneseed mutations of male candidates were found to be, it stands to reason that there simply was not enough expertise to then also create a similar process for female marines. Even before the Heresy it's well known among the Primarchs, Malcador and the Emperor that several legions possess unstable geneseeds. It's even hinted that at least one of the two lost legions suffered the fate of expungement due to faulty geneseed. Hence, a process as finicky, and frankly error-prone, as the creation of a space marine probably never gets to the point where the powers that be consider expanding to female subjects.
Also note that the Emperor's Custodians are also all male. It could simply be that the gene enhancement techniques developed during Terran unification focused only on male physiology too. There was not a need for more warriors so much as an efficient and trusted cadre of super-human ones, the Adeptus Custodes. If so, again the need for investing research and development into female enrollment is not needed when a small number of elite male warriors would suffice.
Just to help with this all of the gene-seed was originally crafted in the 30th Millennium by the Emperor of Mankind in his Himalayan gene-laboratories deep beneath the Himalayan Mountains for this purpose using the genomes of each of the 20 Primarchs as a template. (Here is a little real science for you) In May of 2017 an article was published that "out of 20,000 genes looked into 6,500 of them are expressed differently in men and women." So that is more than 25 %. (back to the game) Since the First Founding, the Chapters of the Adeptus Astartes have lost the ability to engineer new gene-seed or implant organs from scratch. Now using the gene-seeds they are stuck with some chapters are lucky to have a 1 in 10 success rate other 1 in 100 and that is with males who's genomes are most likely to be compatible with the original gene seeds.
So there you have it even if you just did it on the math alone the ones that have a 1 in 10 using males(10% success) would be lucky to get a 3 in 40 using females(7.5%) and the ones with 1 in 100(1% success) would be lucky to get a 3 in 400(0.75%) using females so from a pure math position you go with the highest success rate. Not to mention if you go from a science stand point of the 6.5 K genomes that are different between males and females how much do you want to bet that there aren't a good chunk of those that are inherently different in the genders that are required to make it work so now you drop the success rate from 3 in 40 (7.5 %) success rate to 0.00000000000000000000000000000000001 % success rate or 1 out of 10000000000000000000000000000000000 because you will have that one freak of nature that is a female but possesses all of the required missing male genomes in there body as a mutation.
Sorry to say it but men and women are not identical that is why some medicine works better for women than men and vice versa, same for ethnicity, it is no one trying to be mean except maybe mother nature. So until someone succeeds in replicating the lost technology sorry it is not going to happen and it is still not going to happen because the Emperor (may his will guide my words) based his research off of the aborted Primarch project.
Now the Primarch project led to the creation of the Primarchs who were the twenty genetically-engineered "sons" of the Emperor of Mankind, and the genetic "fathers" of the Space Marine Legions. The Emperor used his own DNA in their creation. It is said they were created with the Emperor with the aid of forbidden sciences and arcane lore derived from the Warp. The Emperor utilized dark powers to construct the Primarchs. It is speculated the Primarchs were imbued with energies and knowledge that the Emperor gained in a Warp Portal on Molech.
So in order to rediscover the technology you will have to uncover the forbidden sciences and arcane lore in the warp which is deadlier now in 40 K than it was in 30 K, do this without the Emperors help of course because he is in no condition to go there, return back and get DNA from the Emperor which should be fun if any of it is still viable and you can get to the golden throne, genetically alter his DNA to give birth to a Female Primarch without corrupting it. Then use the genome of that Primarch to create new Gene Seeds, which by the way is also lost technology and needs to be rediscover then you can make a female space marine. Have fun. By the way the above is only about a 10th of what would have to be done if that. There was a lot more to making the Primarch's alone than what I have above the, the lore in Warhammer is so in-depth it makes real life seem simple so it takes quite a bit to alter Canon lore without alienating your current ensured fan base to such a degree that you lose all of them for an unknown fan base you may possibly gain but are not assured to gain.
well it would seem to the average lore follower that all the Primarchs were male, however, upon closer inspection of the history of the ultra marines, there is a segment that many of the lore followers overlooked. the two lost Primarch's, whose statues were eventually removed from the palace on terra were never really discussed in the series by the loyalists, but instead were mentioned by the horus traitors. it's important to note that gender was not mentioned for these two primarchs, secondly the primarch of the alpha legion had a twin that emperor didn't even know about! so there was a slim chance of the two being females?= female space marine gene seed? however, because horus mentioned them at all, it's a safe bet that they were already killed off by the forces of chaos, but what's stopping the imperium of man from creating a new set of female primarchs, or even just one, so that even the lowly female guardsman could become a space marine. and wage all out war against the enemies of the empire?
Because of how they were engineered only men can accept the gene-seed of the male Primarchs. If there were female Primarchs this would have been possible, but because they do not exist this is not possible.
Do note that in one of the recent Horus Heresy novels Malcador the Sigilite mentioned to Rogal Dorn that he once advised the Emperor to make some of the Primarchs female because they would lack the rivalries and proneness to infighting common amongst brothers, and that some sisters thrown into the mix would lessen this risk. Rogal Dorn was not amused by this, and even His Imperial Majesty was royally flabbergaste that Malcador suggested this with a straight face.
What is known of Space Marines gene-seed transformation are the following:
As Space Marines, Thunder Warriors, and Custodes are not all the same thing, and the Emperor specifically was talking about Space Marines, then it can be assumed that Thunder Warriors and Custodes (the most powerful of the three), can include women and intersex people.
It should be noted this is quite literally the only thing women can't do in the Imperium. They can join any other branch of the military and attain any government or order position or anything in the professional world besides being a Space Marine and all humans of the Imperium are citizens holding equal rights.
This is assuming of course that aspect of Imperial law, of equality, is actually enforced throughout all the planets of the Imperium, but I suspect that where the Adeptus Sororitas,the Adeptus Mechanicus and the better groups of the Imperial Guard, Knights, Ordo Tempestus, Imperial Navy, Adeptus Astartes and the Arbites are, it'll be enforced. I put the Tech-Priests up there because they would find sex and gender equality illogical, so they have that to be thanked for despite their other shortcomings in human right.
The nature of gene seed aside, It has to do with logistics and statistics. There are roughly 1 million space marines at the time of the 41st millennium, roughly one marine for each planet in the Imperium. Assuming the average planets has the same population as earth today, that means that there's 1 space marine for roughly every 7 billion mortal humans. Now when it comes to picking recruits to become space marines, that means that in order to be selected, you have to be the strongest, fastest, biggest human out of 7 billion humans, and that person is always going to be a man, without fail. Hell, even out of a group of 100 humans, 50 men and 50 women, the strongest person in that group of 100 is going to be a man virtually every time. Even if Gene seed were compatible with female anatomy, no female recruit would ever be able to compete against the male recruits at such a staggeringly high level of physical performance and human ability.
Tldr: in order to even be considered for recruitment into the astartes, a person has to be the strongest human out of a group of (at least) 7 billion humans (reflecting the ratio of space marines to mortal humans), and that person is always going to be a man.
There are currently no known female Space Marines, mainly due to the genetic problems mentioned by others. That being said, the possibility of that changing some point in the future has opened up a bit with the introduction of Primaris Space Marines. The reason I think this is that, if the gene seed can be manipulated to make primaris marines, it could possibly be altered further in the future.
Well the gene problem only explain why since all primarch are male space marine are male.. By the way "creating" a space marine does include base upgrade but the entire process is centered around transforming the aspirant into something reassembling the primarch. That why the sons of horus ressemble him à lot, why most thousands sons are psyker and why the blood angel can still feel the death of their primarch. So in all that technicality we aren't talking about WHY all the primarch are men. Because, in my opinion, primarch where made by the Emperor "in his image" using his gene and each represent a part of the emperor. An aspect of his human psyche. The ultramarine being his pride, the lunar wolves his ambition the thousands sons his desire for knowledge and power The space wolve being his animalistic nature ect ect The primarch are more of a deformed copy of himself than of a real sons. That is a reason why he never really treat them as sons, why he has complete confiance but also don't estimate them much.
