Gene-seed

In Warhammer 40,000, humans are transformed into Space Marines by, among other things, implanting them with a series of 19 organs that dramatically alter and enhance their biology. The organs' DNA strings, from which they are vat-grown, are collectively known as "gene-seed."

Origins

Gene-seed is a vital part of the Space Marine program, and is as old as the Astartes themselves. When the God-Emperor of Mankind was still conquering Terra, many of the techno-barbarians' armies had augmented human warriors; the Emperor realized that he would need vast armies of super-soldiers for his coming Great Crusade, so he embarked on a project to create a mass-producible means of enhancing humans. This project resulted in the twenty Primarchs, and from their genes, he created the gene-seed organs.

The Organs

1. The Secondary Heart is pretty self-explanatory. It lets a Space Marine get more blood flowing through his body, which lets his muscles work harder and longer, and it gives him a backup just in case one fails (i.e. gets shot, stabbed, ripped out by a Dark Eldar Archon or what have you). The Secondary Heart is not beating at all times. It only activates during times of stress (bear in mind that a twenty mile run is not considered stressful to Astartes) otherwise it just sits there awaiting activation.
2. The Ossmudula makes a Space Marine's skeleton grow and incorporate some ceramic dietary supplements into itself and fuses his ribs together into a solid structure.
3. The Biscopea triggers muscle growth. These first three organs are often implanted together, as they collectively prepare the Initiate's body for further augmentation. Around this time, the Initiate gets put on a diet containing extra nutrients to fuel his new growth.
4. The Haemastamen enhances a Space Marine's blood to better accommodate the Space Marine's nifty set of new organs. It also turns it bright red. A mutant candy-red blood if you will
5. The Larraman's Organ creates Larraman's Cells, which are like super-platelets; they make wounds rapidly scab over. This also can work against them, sealing poisoned wounds with poison still within before the bloodstream wash most of it out like it normally happens with normal humans. Yes, marines have in-built anti-toxin organs, but Dark Eldar and Tyranid poisons didn't give a fuck about it.
6. The Catalepsean Node messes with the body's sleep cycle, letting Space Marines get away with four hours of sleep every day, or even less, and allowing them to not sleep for like 2 weeks without losing awareness. It also makes the brain susceptible to hypno-therapy, which is a fancy word for brainwashing Initiates while they are asleep.
7. The Preomnor is like a bird's crop, in that it holds food before passing it to the stomach, except that the Preomnor also decontaminates its contents before passing them on. In Ian Watson's Space Marine novel, Initiates celebrated getting this organ by eating the most disgusting things they could until they finally provoked a vomit reflex.
8. The Omophagea is one of the odder organs. It connects to the stomach and the spinal cord, and it somehow takes knowledge from the brains that a Space Marine eats.
9. The Multi-Lung...do I have to spell it? It's another lung! It also has a lot of filters and such, and can function underwater and in low-oxygen atmospheres. In other words, it allows the Space Marine without a helmet to breathe under water and in toxic air.
10. The Occulobe gives Space Marines super-vision, and allows their eyes to adjust to changes in light almost instantly.
11. The Lyman's Ear gives them super-hearing and makes them immune to motion sickness and concussion; however, in-game, they have no particular immunity or even resistance to concussion.
12. The Sus-an Membrane lets them enter a state of Suspended animation when they are hurt or starving. This serves the purpose of conserving a mortally wounded marine until his body is recovered and healed by apothecaries. So each time you remove a marine model on the tabletop, chances are fluff-wise it enters sus-an, rather than dies for good. It should be noted, marines cannot leave suspended animation state at will, and can only be "awakened" through the use of specific drugs.
13. The Melanchromic Organ makes Space Marine skin like Transition lenses; it darkens in response to radiation. On the plus side, they tan really fast; the fact that they pale right back up once they're inside is not so great.
14. The Oolitic Kidney detoxifies their blood, like their regular kidneys, but this one does it really fast. Also, it controls some of the other organs' hormones. As a result, Space Marines are incapable of ingesting poison unless with the help of Chaos or the Dark Eldar. It also means they can't get drunk, though the Space Wolves found a way around that by creating Mjod, an extremely alcoholic drink made using the local plants of Fenris that temporarily neutralizes the Oolitic kidney's ability to filter toxins.
15. The Neuroglottis lets a Space Marine identify chemicals and track adversaries by taste.
16. The Mucranoid gives Space Marines super-sweat that protects them from heat and cold, and can even be hardened into a vacuum-proof shell.
17. The Betcher's Gland gives them acid spit. The acid within varies depending on the source, from simply reacting only to organic tissue to fully dissolving ceramite. Some Chapters, such as the Blood Angels and the Iron Snakes, consider using the Betcher's Gland in combat to be dishonourable.
18. The Progenoids provide no benefit to the Space Marine, but are universally considered the most important organs, as they are the keys to making more of these organs. Each Space Marine starts with two, one in the neck and one in the chest, and they are removed after their death. The neck-based progenoid is ready for harvesting five years after implantation, while the chest-based progenoid is ready after ten. Each harvested progenoid is a bunch of DNA and chemical stuff that when implanted in a suitable environment grows into more organs. Chapters that took severe losses and could not reclaim the gene-seed of their brothers basically take random people (with an affinity to the Astartes gene-seed), implant them with all space marine organs, wait ten years and slaughter them to harvest the gene-seed. Sometimes, the term "gene-seed" is taken to refer to these glands specifically.
19. The Black Carapace interfaces a Space Marine with his armor, making them effectively one being. Space Marine Scouts don't have this yet; under the Codex Astartes, they don't get it (and thus an expensive suit of power armor) until they've spent a decade or more in a Scout Company. Tyranids nicked this for their tyrant guard which makes you think how foot-slogging hive tyrants survived anything beforehand

Funny how most 40K works never refer to most of these, if any. Especially when situations come up that these organs were specifically designed for. Even in the game, marines with two hearts, super-fast-clotting blood and three lungs still has only one wound, somehow. A marine can survive a heart being ripped out, but a lucky shot can kill it? Are all enemies headshotting snipers?(you know the answer, quit whining)

Gene-seed vs. Genetic Engineering

A lot of people (including, it must be admitted, Games Workshop itself) often think of gene-seed as a genetic modification program, but this is inaccurate. Gene-seed does fall under the blanket of "bio-enhancement" or "bio-engineering" - that is, the augmentation of the physical form. However, gene-seed is based on the physical implantation of artificially designed and engineered organs, whereas genetic engineering would modify the DNA of the aspirants to naturally develop the various traits. In essence, Space Marines are what Rifts would call "bio-borgs" - organisms biologically modified by artificial biological implants. Fleshy cyborgs, essentially. This is an important difference, as it allows the Space Marines to wriggle around that pesky "thou shalt not modify the holy human form" law of the Imperium. From a genetic standpoint, a Space Marine is probably closer to human than most abhumans are, and so they fulfill the letter of the law (if not its spirit- but hey, the Emperor made the rules, so he can break them as much as he wants).

Mutation

As each new set of gene-seed comes from a Space Marine, requires many years to mature, and many more to be used, there is a lot of room for things to go wrong. This is especially true when desperate circumstances (the impending Great Crusade, or a Chapter being dangerously under-strength) cause Space Marines to skimp on the safety checks and wait periods in order to accelerate Chapter growth. Over the millennia, many flaws have found their way into into the gene-seeds of each Legion, and some of their successors have even more.

Dark Angels

No defects. Ties with Ultramarines for "most stable source", so the limited number of Dark Angels successors is usually put down the Inquisition's limited knowledge of the Fallen Angels. That and the fact that due to successor chapters often working with/take orders from the Dark Angels has lead to the Inquisition to fear the Angels and successors to be "legion building".

Emperor's Children

An accident during the early days of the raising the Legions resulted in only two hundred Space Marines of the III Legion by the time the Emperor found Fulgrim and Chemos, forcing the Emperor's Children to work alongside the Luna Wolves for the early decades of the Great Crusade; due to Fulgrim's high standards of perfection being passed on the Legion's apothecaries, Emperor's Children Aspirants had to be nothing less than physically perfect, resulting a largely stable gene-seed, including forbidding psykers to join the ranks. By the 41st millennium, however, the Emperor's Children's gene-seed is fucked up beyond all reasonability. Millennia of drugs from both inside and out the Warp have damaged it greatly. While the relatively few sane tech-priests left and Dark Mechanicus working with them can and do make new Marines (thus proving their Marines can and do make new gene-seed), the chemical and other material abuse render each generation further crazy. Even before the Heresy, they were more sensitive to external stimuli like sound.

Iron Warriors

• Innate familiarity with machinery, due to millennia of work and cybernetic implants
• Noted expertise with problem-solving, especially in pragmatic methods
• Minor Ossmundula mutation sometimes cause limbs to twist and deform as marine grows really old (we're talking hundreds of years to millenias) - this is usually dealt with by replacing limbs with bionic or cutting them off and placing the cripple into a Dreadnought, Hellbrute, or some weird custom made dreadnought-esque walker, since due to their specialty and organization only high-ranking officers can survive for so long, and they surely ain't going to suffer from the "Crazed" rule.

Especially noteworthy for the Iron Warriors considering they reside in the Eye Of Terror, there has been little mutation in their gene-seed as well. Those that do suffer mutation simply replace their mutated limbs with cybernetics. However, there may be some unknown issue which caused them to be afflicted with the Obliterator Virus (or was it they who made it?) more easily - the first ones originated from them.

However, this is rendered null by the fact they are the sick fucks behind the Daemonculaba. Due to its nature, the women themselves may have the gene-seed organs implanted (among other things), or at least the DNA required to be able to quickly grow the organs into a child stuck in their wombs. While the poor, innocent, scared kid inside can become a fleshless abomination/mutant, they will normally emerge a genetically stable, gene-seed uncorrupted fleshless adult SPHESS MAHREEN with the mind of that same scared child. Suffice to say, this is a sick way of getting to FEMALE SPHESS MAHREENS, and is DOUBLE HERESY.

White Scars

• No noted defects.
• Mongolians, but this just makes them better.

Possibly corrupted slightly, due to them fighting the Dark Eldar for so long, that there HAS to be some sex/hybrids in there SOMEWHERE. Don't even ask about the White Scars prisoners in Commorragh.

Space Wolves

They actually have two different implanted DNA: the Canis Helix, which makes them viking werewolves (long canines, tough skin, vertical pupils, awesome sense of smell, animalistic RAEG in combat) or, if one is really unfortunate, into an ordinary werewolf (aka Wulfen), and their super-unstable geneseed, which can't function without pre-implanted Canis Helix and has been found incompatible with any populace outside of Fenris. For the most part, they get around this by just founding tons and tons more Space Wolves than the Codex Astartes would normally permit, meaning that they still tend to have tons of chapters' worth of marines on call.

While many factions within the chapter are proud of how "potent" their gene-seed is, plenty of Space Wolves have constantly labored to stabilize their genome in order to found successors to cordon off the Eye of Terror, and crusade against the Thousand Sons, a plan called "the Sons of Russ." Once, they got really close, which Magnus the Red didn't like, so he took his Marines and bumped off the Wolf Priest in charge (and generally wrecked things for the Space Wolves), using some Tzeentchian trickery to get an opening. In short, he got shit done!. But considering just how needlessly cruel and dickish the Space Wolves have been to the Thousand Sons (how exactly do you NOT even doubt or question an order to destroy a valiant and loyal Legion just because your boss feels pissy at their boss?) since day one, many people think this was a rightly delivered boot in the ass to the Furries In Power Armor. Even if Black Library disagrees.

Imperial Fists

• Missing Betcher's Gland (no acid spit)
• Missing Sus-an Membrane (no suspended animation)
• Possibly increased pain tolerance and/or a tendency towards masochism.

Despite missing two organs, the Imperial Fists gene-seed is considered to be amongst the most stable and most manly sources. It is the second most desirable source of gene-seed for Foundings. Second to the Ultramarines, of course.

Night Lords

Despite what you may think, no defects! The Night Lords are officially the most stable of the Traitor Legions, a trait often put down to the fact that they rarely dwell in the Eye of Terror. That, and they don't even like Chaos or the gods. What little mutations there are are usually upon their Sorcerers...what ones have survived do not have them anymore either. While there are some implications that the Legion's exceptional night vision is part of the gene-seed, it is important to note that Night Lords' eyes are extremely sensitive to light largely due to an environmental condition because of the nocturnal conditions of Nostramo, and not due to an inherent flaw in Konrad Curze's gene-seed.

Also Curze's gene-seed has passed onto psykers within his Legion his "gift" of seeing the future in the worst way possible, and as scenarios that become self-fulfilling prophecies. EDIT: In one case, it was found out that the Primarch's "gift" was actually the side effect of a marine's body slowly rejecting the gene-seed. Prior to receiving this gift, he never demonstrated any psychic capability. Even so, this situation is rare.

Blood Angels

• Occasional bouts of bloodthirsty, genocidal rage, otherwise known as the Red Thirst.
  • Occasional bouts of permanent, bloodthirsty, hallucinatory, genocidal rage, better known as the Black Rage.
• Extreme longevity.

Stable, but flawed. Prior to the Horus Heresy, Sanguinius discovered that some of his sons were beginning to suffer the Red Thirst, where they would descend into a thirst for blood, whether human or xenos, in a blind rage. If that wasn't bad enough, as a result of Sanguinius's death at the hands of Horus imprinting itself on their genetic memory (or something), all Blood Angels and their successors have the Black Rage; a berserk rage where they believe themselves to be Sanguinius during his final battle.

Iron Hands

• No defects.
• Noted predilection towards machinery and a disdain towards flesh.

And just to make sure, if there are any, they follow the Bible's advice on offending organs and replace them with holy machinery instead. Would probably be a very desirable source for new chapters if not for the absolute devastation they suffered in the Drop Site Massacre leaving them barely recovered millennia later and needing all the gene-seed they can get for themselves. Because, you know, even though there has been more than enough time for more than twenty other foundings of whole, brand new chapters, and some of those foundings likely making new foundings of their own, the Iron Hands still haven't had enough time.

World Eaters

Pretty stable for a Chaos-heavy traitor legion, so long as the Butcher Surgeons don't fuck up the extraction process. Nothing particularly mutated about the gene-seed either except for minor things like increased adrenaline, etc; but during training Angron had the Legion's neophytes undergo cybernetic conditioning like the surgery he received as a gladiator (referred to as the Butcher's Nails) that increased aggression even further, making an already testosterone-fueled warrior even more aggressive and unstable. While not technically part of their gene-seed, it has been carried on as a tradition by all the "successors" that resulted from the Legion's fracturing after the Battle of Skalathrax.

Ultramarines

• No official defects. Considered the most stable and preferred source of gene-seed for new Foundings.
  • Due to a lack of any real interesting or identifying traits in and of itself, discussions for reclassifying the Ultramarine gene-seed from the most stable source to the most generic source were in the works. These talks were surreptitiously shut down by Inquisitor Warden for offending his sensibilities.

Thousand Sons

• Increased chance of bringing out psychic powers, as well as amplified psychic power.
• Recipients subject to a high rate of mutation, called The Flesh Change; unstable.

The most unstable genome in the Space Marine legions, the Thousand Sons suffer from the Flesh-Change, a problem that caused Thousand Sons to mutate spontaneously. Magnus the Red fixed it, with some "help" from Tzeentch. After the Horus Heresy, the flesh-change came back with a vengeance. Ahzek Ahriman tried to fix it with his Rubric, and he did end up stopping the flesh-change... by turning all non-psyker Thousand Sons into dust; sorcerers also had their powers enhanced in addition to no longer suffering ill-effects from the Flesh-Change. So while theoretically, the Sorcerers of the long suffering Legion can provide gene-seed, in practice, it's corrupted so often that few will ever make new Marines. Thousand Sons specifically hunt for healthy and stable psyker boysto implant them with their oh-so-rare gene-seed to ensure recipient would become a sorcerer. If "Masters, bidding!" TS character words to be trusted, Thousands Sons found a way to implant their progenoids into grown up men (so they can teach and test them for longer before implantation), and perform a scaled down Rubric on them to prevent the Flesh Change..

Death Guard

Pre-heresy, the Death Guard had a much stronger constitution than most Legions, leading to their role as the toxic environment specialists. Post-heresy, however, the Death Guard are so diseased that implanting Aspirants seems more or less impossible - however, there have been new Death Guard legionaries, who are not defectors from other Legions or renegade Chapters. This means that its possible that gene-seed is used in the Legion to make new Marines. How it functions still no one knows, though we suspect it has something to do with Nurgle and that he has necromantic powers of sorts, who can raise his fallen disciples back from the dead.

Another possibility is that they're creating new marines using another chapter's/legion's gene seed and working their Nurgle magics on them to turn them into full-fledged Plague Marines. Not a complete impossibility given how the Black Legion is still able to do it.

Luna Wolves/Sons of Horus/Black Legion

No real defects, but members have a tendency to look more like their Primarch than members of other legions. Aximand was the most notable example of this, but by no means the only one. This gives extra meaning to the "Sons of Horus" thing. In fact, before the Luna Wolves was renamed the Sons of Horus, Luna Wolves that resembled Horus in appearance were known as "Sons of Horus". Ten thousand years in the Eye of Terror has left the Legion's gene-seed more unstable. However, we do know that new Black Legionaries are inducted constantly (seeing as they are the primary generic Chaos Space Marines), so its a good bet a large part of theirs is still usable and that generations of children for millennia to come will be torn away from their parents and families, unfortunate enough to live in the Eye Of Terror, to be inducted into the Legion. Furthermore, given that they frequently "recruit" from other chapters and legions, it's not always easy to tell which Black Legionnaires are descended from Horus and which are from other Primarchs.

Word Bearers

The Word Bearers suffer a unique "quirk" in their gene-seed, resulting in a psychologically obsessive loyalty to something, be it a man, a religion, or even an ideal. The Word Bearers in turn thus have a much stronger sense of loyalty to their battle-brothers and Primarch than other Chaos Space Marines, helping them to maintain cohesion and avoid treachery, but this loyalty also borders on fanaticism. Otherwise, while given to some instabilities, they have been able to maintain strength.

Salamanders

• Malfunctioning Melanchromic Organ (between Vulkan's personal blackness and Nocturne's volcanoes, Salamanders are all black, all the time, and have glowing red eyes)
• Possibly altered Mucranoid - high degree of heat resistance.
• Possibly altered Biscopea - tend to be somewhat slower than other marines. (Way back when, they had -1 Initiative.)

While slightly dysfunctional, their geneseed is pretty stable. Would probably be, like the Blood Angels Iron Hands, a common source of new chapters if not for, again, their extreme decimation during the Drop Site Massacre. They've since recovered, perhaps a little quicker than the Iron Hands (possibly due to being a little closer to the center and being able to evacuate more marines), and may have contributed to a few successors over the millennia.

Raven Guard

• Malfunctioning Melanchromic Organ (their skin is always pale and their eyes are always dark. Anti-Salamanders, basically.)
• Missing Mucranoid (no super-sweat)
• Missing Betcher's Gland (no acid spit)

While their primary gene-seed is decently stable with a few deletion mutations, Corax, out of desperation following the Drop Site Massacre, delved into use of forbidden lore and archeo-tech from days long past to accelerate gene-seed maturation, and try to boost his Legion's numbers after their losses of the Horus Heresy. He was successful, until the Alpha Legion decided to make the whole project go FUBAR. The subsequent 'Marines', if they can be called that, were numerous, but were barely controllable monsters and/or would simply die off as a result of the extreme imbalances in their dispositions. Presumably, few out of them would even have functioning Progenoids, let alone usable gene-seed. Due to the law of averages though, at least a FEW would have had to been stable enough, and is being worked on secretly by the Raven Guard Apothecaries and associated Mechanicus Biologicans.

Otherwise, despite being pretty fucked up by the Drop Site Massacre, they've mostly recovered (thanks in no small part to having the only loyalist Primarch to get away from the Massacre in one piece) and do still contribute to successors here and there.

Alpha Legion

No such Legion exists, therefore no such gene-seed exists.

Blood Ravens

• A mutated Catalepsean Node - gives them perfect memory recall, but unable to enter REM (deep) sleep
• Unknown mutation that increases psychic ability and occurrences
• Small tendency towards mutations (hurr sounds like any other Legions we know of?)
• Horrendously massive tendency to develop kleptomania.
• The pores in their scalps secrete a substance not all too dissimilar to hairgel, often resulting in a higher tendency to develop hair-etical hair compared to other chapters.
  • Alternatively, the pores on their scalps may prevent hair from growing altogether, giving the marine Astartes-pattern baldness. It has been long thought to be a terrible weapon to behold.
• Unknown mutation that results in their voices changing every few years.

Blood Drinkers

• They suffer from a literal blood thirst, and must drink blood. This is often a fantastic party trick.
• Standard Blood Angel problems too, though the blood chugging seems to stabilize them. (Mwuhahahaha!)

The Flawless Host

• Possibly an offshoot of the Emperor's Children, these psychos are so hopped up on combat and other (read, Warp derived, impossible) drugs that they make normal Emperor's Children drug use look tame - however, damage from things like extreme sounds/etc doesn't seem as prevalent. Little comfort. Despite all this, and defying any sense of physics or reality, their Progenoids still seem to manufacture new gene-seed.

The Cleaved

A Chaos Space Marine warband unique in the fact that they constantly ooze some oily liquid from broken joints and seals within their armor. Have an abominably high threshold of pain tolerance, and can take huge amounts of damage that would fell a normal Space Marine.

A Note On That Sick Fuck Fabius Bile

Fabius Bile is fucking insane. I mean shit, his labcoat is made of human skin - normally only Space Wolves wear clothing made of that stuff. While originally the Chief Apothecary of the Emperor's Children, he is now a freelancer. His goal even before the Heresy was to find and unlock the secret to the creation of the Space Marines. In the Warp, he provides a valuable service to the Chaos Legion warbands and their auxiliaries of billions more regular Chaos humans - while often millions strong even now, the warbands still need to replenish their numbers. Considering how many die and how often they are away from any place where they can produce more, that's a lot. Even with slave raiding into the Imperium, child abduction both in and out, breeding centers (stop touching yourself) and many other creative ideas, its still not enough. Fabius provides services of human cloning, along with gene-seed production, among his many skills. As he is not a super-devotee of Chaos, the gene-seed he has and makes is some of the least corrupted found within the Eye Of Terror. Though of course, he fucks this all up by hopping up his own warband's clones with the same psycho drugs and shit that make monsters out of men. Lore hints that his genetic tinkering is far more than we have been lead to believe - much of his 'changes' to even standard human genome are genetic and now passable by both men and women. Even now the Imperium is trying to hunt and eradicate what they call his 'New Men'.

21st Founding

Over the millennia, further mutations have afflicted the many Successor Chapters, but the 21st or "Cursed" Founding deserves a special mention, because a bunch of tech-priests decided that they wanted to try and fix the flaws, but failed so hard they actually created some new ones.

• Bursting into Flames: Yes, the Flame Falcons really were covered in fire. The Flame Falcons thought it was a blessing from the God-Emperor of Mankind, but the Inquisition disagreed, and sent the Grey Knights to wipe them out.
• Bone Growths: Due to a malfunctioning Ossmudula, the Black Dragons suffer a bone solid growth to harden and extend from their forehead and forearms during times of stress. The ones on the forearm can be used as weapons against unarmored opponents, so they decided to coat them in adamantium. So in essence, they're space-Wolverine-after-the-bone-claws kind of guys. Issue is that this is a very visible mutation, and as this is the grimderp Imperium, mutations from the sacred human form are EVUL, even if they are fucking awesome.
• Indestructibility: The Sons of Antaeus are much tougher than normal marines, and nobody knows why. Some speculate something about a super-reinforced skeleton, some say it's Nurgle's doing, etc. The only real truth to it is that there's some sort of mutation present in the gene-seed.

A Note On Implant Organs/DNA Elsewhere

While the SPHESS MAHREENS are the most widespread users of artificial glands, organs, and such, they are not the only ones. Inquisitors likely have access to organs or other artificial mutations they require thanks to their influence in the Imperium. Depending on what world they hail from and its beliefs, Imperial Guard regiments and PDF may also have artificial organs, or even whole new ones derived from millennia of mutations, which of course has certain advantages. Furthermore, Warp Exposure can result in a change of the functions of organs - a simple example of this is the Cadians, who nearly universally have powerful, violet colored eyes. Gland Warriors were an Adeptus Mechanicus project to help legions of Guardsmen heading into battle against a Tyranid hive fleet. They were to do battle in an incredibly toxic and hard environment, and a variety of experimental organs and DNA strands were utilized - including organs that would secrete stims, painkillers, and various medical drugs, more powerful natural air filters, and the like. While only three survived the whole campaign in the end, the AdMech was pleased and took the three to be debriefed.