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 then 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 to be implanted into each Space Marine. These organs are usually produced in vats from extracted progenoids, but other methods can be used in a pinch, and, of course, the Sinew Coils aren't an actual organ, so they can be produced much more readily, like any other cybernetic implant.

The Organs

Oddly, most 40K fluff never refers to most of these organs, if any, especially when situations come up where these organs might have been especially useful. The worst offenders are usually the sensory ones; the Occulobe, Lyman's Ear, and Neuroglottis, taken together, ought to make them notice things normal humans can't, especially under low-light conditions, but the fluff usually has them only picking up the standard level of detail.

1. The Secondary Heart is pretty self-explanatory. It lets a Space Marine get his blood flowing at a faster rate through his body and probably to the more critical bits, 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 Guilliman, 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 an Astartes); otherwise, it just sits there awaiting activation.
2. The Ossmodula makes a Space Marine's skeleton grow and incorporate some ceramic dietary supplements into itself and fuses his ribs together into a solid structure. The Tyrant guard's fused ribcage hints that the Hive mind has incorporated Space marine DNA into-*BLAM* Heresy! Ironically, the fusing of the rib cage would be considered a bad thing, as the Space Marine would not be able to breathe properly, but since this is 40K, little things like biology can and will be ignored if the end result sounds sufficiently cool, then again the Multi lung (see bellow) may help with this. A (decidedly) secondary effect of a fused rib cage would be a loss of flexibility. This bit sounds like it was tailor-made for Ultramarines since the vast majority of them walk around like they've got a broom handle shoved up their collective asses anyway.
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 by improving its ability to carry oxygen. It also turns it bright red.
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 is the case with normal humans. Yes, marines have in-built anti-toxin organs, but Dark Eldar and Tyranid poisons don't give a fuck about that, and can usually bypass that.
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 making it possible for them not lose awareness, even after like 2 weeks without sleep. It also makes the brain susceptible to hypno-therapy, which is a fancy word for "brainwashing" the Initiates while they are asleep. As a nasty side effect, it allows Chaos marines to re-brainwash loyalists they captured alive (or better: in suspended animation). Unsurprisingly to anyone familiar with Chaos, this trick doesn't work in reverse, which might just go to show that the Chaos Marines might be closer to the true human nature than the loyal Marines... or more likely that Chaos is corrupting as fuck.
7. The Preomnor is like a bird's crop, in that it stores food before passing it to the stomach, except that the Preomnor also decontaminates its contents as well. 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 somehow takes knowledge from the brain tissue that a Space Marine can eat. Interestingly, Space Marines are pretty reluctant to actually eat brains, as the process is physically extremely unpleasant, leaves them in a vulnerable torpor whilst they process the information, and is one of the very few things that will make a Space Marine vomit afterwards. In fact, it is not just the brain, since some Astartes rituals, if not chapter names, originated from this; the Blood Angels and their descendants are especially famous for blood drinking and rip and tear in general; this is actually because they receive knowledge and visions from it. What? You thought the Blood Angels had visions of Sanguinius only to have a grimdark, mystical aura? Additionally, the memories they absorb become very hard to forget (brain washing technologies can help). Now just add some chaos to that mix and you've got heresy brewing...
9. The Multi-Lung... do I have to spell it out? It's an additional lung! It also has a lot of filters and the like, 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. Just how does this third lung fit in with the other two Marines are born with and all the other organs implanted later? Very carefully, that's how.
10. The Occulobe gives Space Marines super-vision, and allows their eyes to adjust to changes in light almost instantly, or at least in the fluff.
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, probably because it's often delivered by a hammer to the head.
12. The Sus-an Membrane lets them enter a state of Suspended animation when they are critically 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 has entered sus-an, rather than died. In fact, on most campaigns that's what happens to your characters. It should be noted, marines cannot leave suspended animation state at will, and can only be "awakened" through the use of specific drugs. Basically, the 40k version of LSD.
13. The Melanochrome implants into the lymph nodes, and makes Space Marine skin like transition lenses: it darkens in response to radiation, rapidly producing reinforced melanin that protects against both UV and gamma. On the plus side, they tan really fast, but the fact that they pale right back up once they're inside armour (unless they are Salamanders) is not that 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 unaffected by most poisons short of what the Dark Eldar or Tyranids can produce. It also means they can't get drunk, though the Space Wolves found a way around that by creating Mjöd, an extremely toxic drink made of local plants of Fenris, that temporarily neutralizes the Oolitic kidney's ability to filter toxins. Another organ which ceases to work on the tabletop, as Space Marines have absolutely identical reactions to Poison as ratlings, ogryn, and standard humans. Of course, considering that the poisons you find on the tabletop are Tyranid, Dark Eldar, and Nurgelitle shit, we can probably write this one off.
15. The Neuroglottis implants into the upper nasal passage, drastically improving both smell and taste; lets a Space Marine identify chemicals and track adversaries by taste. Blood-hound much?
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. Remember that awesome scene where Guilliman boxes Word Bearers in space? Yep, he did it while covered in that thing.
17. The Betcher's Gland gives them acid spit, which is probably the only bonus for going to combat without a helmet. 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. In any event, Space Marines on the table-top do not have access to acidic spit, unlike e.g. Tyranids. (Or maybe they do; space marine ancients get a lot of attacks for someone with their hands full.)
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 the organs above. 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 (or vat-grown bodies similar to the ones they use for Servitors), 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 armour, making them effectively one being. It also serves as back-up armour, providing a level of protection similar to a flak jacket, in case a marine somehow ends up running around naked. Space Marine Scouts don't have this yet, because of 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 did the foot-slogging hive tyrants survive anything beforehand. As clearly established by how humans perform in Power Armor, this organ does nothing on the table-top; Sisters of Battle, for example, get just as much (or more - they can lift a lascannon by themselves, unlike humans in mere carapace armour) benefit from Power Armor as Space Marines do.
20. The Canis Helix is not an organ, but considered nonetheless to be part of the gene-seed; it appears to be a genetic cocktail brewed by the Wolf Priests of the Space Wolves, and while it induces a level of mutation beyond that seen in any other chapter, Russ' gene-seed organs appear to have a 100% rejection rate in humans who do not first take the cocktail.

Primaris Organs

After Roboute Guilliman told Belisarius Cawl to make a next generation Space Marine, he ended up creating Marines that had an enhanced version of the Astartes Gene-Seed, which contained an extra three organs (possibly due to taking elements of the Custodes or even the Primarch creation process). While some have expressed concern of the project, some due to worries about how some of the mutations of some of Chapters with more exotic mutations and some due to the belief that the Emperor's work shouldn't be tampered with, Cawl claims that the Primaris Gene-seed is more stable than the regular Astartes gene-seed, with an estimated deviancy of .001% per generation. How true this claim might be is up for debate, but Cawl has created Space Wolves successor chapters that haven't immediately turned into Wulfen so it may have some truth in it.

As of recent fluff updates/leaks, the new organs are as follows.

1. The Sinew Coils essentially modify the muscle strands and makeup of the Primaris Marines, buffing their pecs and biceps to the 10th degree. Whilst one would assume it would resemble something from Inspector Gadget's coil legs, the truth is....it is pretty much like that in function. These are reinforced coil-cables made of durable supermetals that are capable of contracting with incredible strength, making Primaris stronger and tougher than others. Even outside of their power armor, swinging a sword at a Primaris might just bounce off his hot and sweaty muscles because of these coil cables, and it amplifies his strength to the point where he can crush a human skull with his bare hands or bite through metal cable like it were jerky.
2. The Magnificat may sound a bit lame, but its function is actually what makes the Primaris 'bigger, better, faster and stronger' then regular Astartes. It produces a mix of hormones that greatly enhance the effects of all of the other genetically engineered organs that float around in the Primaris. So every Primaris gets more bang for his genetic buck. Intriguingly, it's stated to be only half of another super-organ dubbed the Immortis Gland originally made for the Primarchs, but Cawl was only able to find the materials and genetic blueprints needed to make the right half; the information needed to make the left half had seemingly been erased. The function of the complete Immortis Gland is unknown, but given that its other name is "The God-Maker" and the fact that only the Primarchs are known to have the organ, it stands to follow that it is likely what amplifies their abilities to the level of demigods.
3. The Belisarian Furnace is an organ that is named after the Galaxy's least humble Archmagos. This one is a last-chance fail-safe kind of thing. The Belisarian Furnace is an organ that activates when a Primaris is on death’s door, giving them a great burst of super-adrenaline (not the actual term), but helping to keep them alive even at great cost, so that a Primaris can take out as many of the enemy as possible even as his flesh and bone rapidly regrow themselves. This allows fan-fic writers to create Hollywood-esque 'final pushes' without making it look stupid, which should make for some amazing last stands.

Implantation

Not everyone is compatible with implantation; potential candidates are screened for tissue compatibility, and kept young, as being post-pubescent has a marked increase in the tissue rejection rate (and being actually elderly makes it even worse); even so, not all candidates survive the implantation process. One of the most obvious limits of thier design is that the Emperor never solved the problem of making them compatible with female hosts (although some argue that this was deliberate, nobody can be entirely sure), so the very first screening step is usually by gender. Furthermore, not all organs can be implanted at once, as some prepare the body for others, and the body needs time to adapt to and accept the implants. The maximum speed process (assuming the soon-to-be Marine will be a Primaris) is:

1. If you are the Space Wolves, the Canis Helix.
2. Secondary Heart, Ossmodula (bone growth), and Biscopea (muscle growth).
3. Sinew Coils (tendon/ligament reinforcement), Magnificat (enhances Ossmodula and Biscopea), and Belisarean Furnace (combat stimms, attaches to hearts).
4. Haemastamen (improved red blood cells) and Larraman's Organ (improved platelets).
5. Catalepsean Node (brain implant: sleep like a dolphin).
   • If this is an actual Space Marine, begin hypnotherapy/brainwashing only after this implant.
6. Preomnor (gullet), Omophagea (flatworm stomach), and Multi-lung (third lung).
7. Occulobe (improved visual cortex).
8. Lyman's Ear (replacement outer, middle, and inner ear).
9. Sus-an Membrane (brain implant: self-induce coma).
10. Melanochrome (lymph implant for radiation resistance).
11. Oolitic Kidney (enhanced liver/kidney) and Neuroglottis (improved inner nose).
12. Mucranoid (secretable space suit).
13. Betcher's Gland (extremely acidic saliva).
14. Both Progenoids (for reproduction).
15. Black Carapace (for improved power armour compatibility).

That being said, it is possible for the Primaris organs to be implanted later in an existing Space Marine, functionally converting them into a Primaris Marine.

Tabletop Effect

Most of the organs have no visible effect in the game, at least if the in-game rules are an indication; here are how the organs seemingly manifest (or not). For some reason, Space Marines aren't any faster at running than an unaugmented human and are slower than fast humans, such as Sisters of Silence. Most likely, this lack of impact is either for balance purposes or because it can't be properly represented using in-game means.

• Has an effect, at least in aggregate:
  • Secondary Heart, Ossmodula, Biscopea, Haemastamen, Larraman's Organ, Multi-Lung, Oolitic Kidney, Mucranoid: +1 Strength and Toughness, relative to a normal human.
  • Sinew Coils, Magnificat, Belisarean Furnace: +1 Wound and Attacks.
  • Catalepsean Node + Hypnotherapy: And They Shall Know No Fear, +1 Ld.
  • Progenoids: These are given as the reason an Apothecary loses his turn if he fails a resurrection roll, as he is busy collecting them.
• Effects limited to fluff:
  • Occulobe: Demonstrably ignored; with or without his helmet on, a Space Marine's vision is never considered any better than an unaugmented humans, even when distance or lack of light are explicitly causing vision problems.
  • Lyman's Ear: Demonstrably ignored under the 8th edition rules; Space Marines behave exactly like unaugmented humans when exposed to sonic weaponry, when needing to notice things they can't see, or when in need of balance. In previous editions, this would presumably have contributed to a higher Initiative stat.
  • Neuroglottis: Demonstrably ignored, as noted above with the Lyman's Ear, as this would predominantly serve to supplement that, given some tabletop rule for Space Marines actually having improved senses.
  • Melanochrome: Demonstrably ignored; Space Marines respond to photonic radiation (usually provided by the Adeptus Mechanicus) exactly like unaugmented humans do.
  • Black Carapace: Demonstrably ignored; many human factions have access to Power Armour, particularly Sisters of Battle, and none have ever been shown to be even slightly more encumbered by Power Armour than Space Marines are.
  • Preomnor, Omophagea, Sus-an Membrane, Mucranoid, Betcher's Gland: These have no in-game effect, but would typically not be expected to, as a Space Marine with his suit on and participating in a battle would not have call to use them, although the Betcher's Gland could be implemented as a very, very short range weapon, in principle.
• Alternate Take: The Occulobe, Lyman's Ear, and Neuroglottis could grant the +1 Initiative marines enjoy(ed prior to 8th). The Melanchrome also likely adds to toughness.

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, 'cause he's the motherfucking Emperor).

• Note: Like Mitochondria (the powerhouse of the cell), the implanted organs carry their own DNA, cloned from the Primarchs, thus turning the marines into chimeric humans. So yes indeed, bio-borgs, but ones that show characteristics of DNA foreign to the host's body. Example: is there a specific organ that gave Sigismund "a patrician face that echoed the same austere lines as his liege lord, Rogal Dorn"? Let us be reminded, Sig was terran, and Dorn grew up on another planet (he was made by the Emperor in a Petri dish, does that count as being born on Terra too?). Who knows what he looked like when he was mortal, he now looks like a son of Dorn. In fact, most Astartes end up looking similar to their parent Primarchs, like Horus "little Horus" Aximand looking like Horus "fucking Horus" Lupercal. Of course, this is because GW thinks it is bioengineering, but at least it's "Human Chimeras" instead of "Humans with furry creatures", that'd be Heresy, right Emps?
  • RIGHT
  • As it happens, for Sigismund, the answer is yes - the Ossmodula would alter the shape of his skull, and the Biscopea the arrangement of his facial muscles - but the fact remains that GW routinely treats gene-seed as if it were gene therapy, rather than implants.

The Tithe and New Foundings

As part of the deal in Guilliman's Codex Astartes, the Astartes were afforded almost complete autonomy from the Administratum, which means they don't have to pay Imperial Taxes except for a tithe of 5% of their gene-seed which they must send to Mars.

This has a few important purposes:

1. To monitor the genetic purity of the Space Marine Chapter as a whole and make certain a chapter is not suffering heretical mutations.
2. To create new foundings of Space Marines whenever the High Lords of Terra deem it appropriate to do so.
3. To act as an emergency reserve of gene-seed in the unlikely event that the Chapter's existing gene-seed stocks are lost or tainted.

Note that even though the Adeptus Mechanicum will have stockpiles of a chapter's genetic material, it is not frequently used as a contingency for a space marine chapter if things get a bit hairy. Several times in the fluff a Chapter has faced extinction due to high losses (see Black Consuls, Celestial Lions, Lamenters, Marines Errant, Scythes of the Emperor yadda yadda yadda) but they'd never thought to ask Mars for their gene-seed back. The only known instance of it happening is in the 8th Edition Death Guard Codex, where the Minotaurs were forced to undergo a mission to Mars when a Death Guard assault on their fortress-monastery resulted in the poisoning of their gene-seed stocks.

It's also a bit uncertain how chapters with particularly negative defects manage to keep their genetic condition concealed, if the government has access to their core materials. Some chapters get around it by sending only the "purest" gene-seed to Mars and so come out of the inspection smelling of roses, but when the condition is widespread (as it is with the Blood Angels) it probably only highlights how little the Mechanicus actually understand about biology.

It's also important to note that each chapter's gene-seed is kept in isolation. i.e: Just because a chapter claims descent from the Ultramarines does not actually make their gene-seed the same as their forefather. It's basic evolutionary biology that when a population splits and develops elsewhere it starts forming its own traits advantageous to its circumstances and environment. Hence the Mortifactors gene-seed will largely be very different from the Howling Griffons, despite sharing a common ancestor. So rather than being genetic "brothers" as they would be within a chapter, different space marine chapters sharing a lineage would be more like nephews or cousins.

When the time comes around to found new chapters, the adepts will look for favourable traits that they want to replicate, although the Ultramarines are cited as being the most utilised source for new foundings, thus any new chapter of Ultramarine descendants would likely be second generation Ultramarines and be quite close to their forefather genetically at least until the chapter adapted to its founding homeworld or particular circumstances. But that doesn't prevent those descendants themselves from being used as templates in later foundings, so after several generations you could end up with something completely different from the original baseline stock.

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 - mutations typically get worse as you make copies of copies. Further mutations have been introduced, of course, in foundings featuring heavy tampering by the Adeptus Mechanicus (see the discussion of the 21st founding, below).

No Known Mutations

Alpha Legion

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

• Were you really expecting known mutations, or anything else, from this Legion???

In all seriousness; like much of the Alpha Legion, we don't know all that much about their gene-seed. Whatever idiosyncrasies or flaws they have are kept under very tight lock and key. And with their nature of running as individual operatives in and out of the Eye of Terror, to what degree their gene-seed is tainted by the Warp or other means varies from Marine to Marine. Though their knack for secrecy, paranoia, and tendency to subvert the enemy instead of direct combat is probably a notable trait. Outside of occasional Warp-corruption, we assume it's possibly one of the most stable gene-seeds, probably as much as the Imperial Fists or Night Lords. Something of note; they often use any mutations that they have as a scare tactic, hiding them and then revealing them on the battlefield in order to shock the opponent. They also use hypnosis and deep-indoctrination more than other chapters, so they may have an altered sus-an and/or catelapsean.

Dark Angels

No defects, unless you consider their rampant paranoia and secrecy STAUNCH LOYALTY AND HONOUR as such. 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" THE MOST LOYAL CHAPTERS DUE TO THE PURITY OF THEIR GENE SEED. Although that would be more credible if they had ever been anything less than a Legion.

Death Guard

• Emphasis here is on known mutations - there's no actual evidence of Death Guard gene-seed having any problems at all, because we've never gotten any fluff covering any attempts to use the gene-seed from a Death Guard, even though one would assume they'd have the most deeply fucked up Progenoids of any Legion.

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, apparently made with the gene-seed reclaimed by the Plague Surgeons. 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.

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. It also doesn't help how they tend to be total pricks at times.

Night Lords

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. There is one defect on Curze's Geneseed, and that is the Occulobe defect, causing them to have black pupils&sclera, and astonishing sight in the dark, even more than a normal space marine. Although, there is a bad side of this effect; Light hurts their eyes like helL, and blinds them for a short time. Also, Curze's gene-seed may have 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. 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.

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.
  • Also worth noting is that the Ultramarines, in one of their bro-iest moves, "adopted" a fair number of loyalists from traitor legions after the heresy. There is no real way of telling if an Ultramarines "successor" might be from the relatively stable stock of World Eaters or Iron Warriors. In most cases only a single high-ranking archmagos knows the truth, so even the chapter itself is in the dark about its history.

White Scars

• No noted defects.
  • Tendency to do whatever they fancy.
  • 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.

World Eaters

I know what you're thinking, but it's caused by an additional cybernetic implant - the Butcher's Nails - rather than anything in the gene-seed itself. 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. Hell, they were known to be total bros before all the trouble with nails. And now they are little more than Teamkilling Fucktards.

Minor Mutations

These chapters may have cosmetic or easily-fixed mutations.

Emperor's Children

• Implicitly defective and/or enhanced Lyman's Ear, and possibly Occulobe and Neuroglottis.
  • 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 reason. Millennia of drugs from both inside and outside 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 even crazier. Even before the Heresy, they were more sensitive to external stimuli, like sound, implying a defective Lyman's Ear with reduced ability to filter out sounds the Marine doesn't want to hear, and potentially similarly defective Occulobe and Neuroglottis.

Iron Warriors

• Implicitly defective Ossmodula, but implicitly enhanced Progenoids:
  • They have the lowest gene-seed rejection rate, which enabled them to replenish their huge number of casualties with an even higher number of recruits.
  • Minor Ossmodula mutation sometimes causes limbs to twist and deform as the marine grows really old (we're talking hundreds of years to millennia) - this is usually dealt with by replacing limbs with bionics, or cutting them off and placing the cripple into a Dreadnought, Helbrute, 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? It's been confirmed Perturabo and Mortarion worked together on the obliterator virus) 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 skinless abomination/mutant, they will normally emerge a genetically stable, gene-seed uncorrupted skinless 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.

Luna Wolves/Sons of Horus/Black Legion

• Implicitly defective Ossmodula/Biscopea, causing cosmetic similarity.

No real defects, but the tendency for members to look more like their Primarch is more pronounced (and possibly more frequent) than it is for members of other legions to resemble their own respective Primarchs. "Little Horus" 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 were 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 faction), 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 with the only real requirement being loyalty to Abbadon, it's not always easy to tell which Black Legionnaires are descended from Horus and which are from other Primarchs.

Salamanders

• Unique Melanchromic Organ, causing their skin to simply always be black instead of waiting for radiation to happen to fix it, and glowing red eyes.
• Possibly altered Mucranoid - high degree of heat resistance.
• Possibly altered Biscopea or Lyman's Ear - tend to be somewhat slower than other marines, although sometimes it's blamed on the somewhat high gravity on Nocturne, since they can still run at the same level as any other chapter. (Way back when, they had -1 Initiative.)

While slightly dysfunctional, their gene-seed is pretty stable. Would probably be, like the 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 while they claim no official kinship with any successors, there are at least a couple of examples with similar unit markings and tactical dogma.

Word Bearers

• Implicitly defective Catalepsian Node.

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.

Major Mutations

These Chapters feature at least one problem as least as bad as missing an entire organ, scaling up to the worst issue, where they go World Eaters-level crazy without needing any implants to get there (Blood Angels and Space Wolves).

Blood Angels

• Occasional bouts of temporary 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. Meaning that he quite literally hit Sanguinius so hard that his grandchildren's-grandchildren's-ad-infinitum-children still feel it.

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.

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).
• Occasional bouts of suicidal rage (Luckily for them this is not permanent unlike a certain other chapter.)

While their primary gene-seed is decently stable, when under extreme stress a percentage Raven Guard warriors will lapse into bouts of suicidal rage known as Ash Blindness or Sable Brand. This is rarely permanent, but occurred with enough frequency that Corax would organize these warriors into the Moritat. Their missing and malfunctioning geneseed organs is a result of Corax, out of desperation following the Drop Site Massacre, delving 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 during 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, a few 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.

Space Wolves

They actually have two different implanted DNA sources: the second is the Canis Helix, which makes them viking werewolves (long canines, tough skin, awesome sense of smell, animalistic RAEG in combat) or, if one is really unfortunate, into an ordinary werewolf (the ones they ride at that), 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. Of particular note are the Wulfen, who are Space Wolves who happened to be in the Warp for over ten thousand years. Instead of being mutated into unmentionable gribbly beasts, they managed to turn into literal werewolves with savage strength, meaning there's a possibility the gene-seed has a factor that lets them resist the influence of Chaos.

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!.

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 boys to implant them with their oh-so-rare gene-seed to ensure the 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. It is known that Magnus the Red used his knowledge of Biomancy to turn his mentor Amon into an Astartes despite his old age, so its possible this technique was passed down to his Legion.

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 hair gel, 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 referred to as "unavailable voice actor syndrome" that results in their voices changing every few years.

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 and Night Lords 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, it's 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. 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 the 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'. He also has access to a small quantity of what is alleged to be the blood of the Primarch Sanguinius. Why did he get that last part? To clone the Emperor. Never let it be said that this fuck, sick as he may be, lacks ambition.

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.

It is also noted that Gene-seed can be decoded, but it requires eons of knowledge about biology, medicine, and so on. Only the Emperor or AdMech, with their advanced equipment, is able to do so (and even the AdMech couldn't fully work it out). The hard part is to decode the "impossibly complex" amino acid chains of the gene-seed and extract the necessary information encoded within the Gland's stored zygotes. Arik Taranis was able to decode it using the knowledge gleaned by just watching the Emperor work, and remade it to fit in his Thunder Warrior body.