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 does what it says on the tin. 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).
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, which, among other things, turns it bright red.
5. The Larraman's Organ creates Larraman's Cells, which are like super-platelets; they make wounds rapidly scab over.
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 an in low-oxygen atmospheres.
10. The Occulobe gives Space Marines super-vision.
11. The Lyman's Ear gives them super-hearing and makes them immune to motion sickness.
12. The Sus-an Membrane lets them enter a state of Suspended animation when they are hurt, or starving, or just bored.
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.
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.
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, and they are removed after a few years of maturation, or after their death, whichever comes first. Each harvested progenoid is a bunch of DNA and chemical stuff that when implanted in a suitable environment grows into more organs. 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 spend a decade or more in a Scout Company.

Funny how most 40K works never refer to most of these, if any.

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.

White Scars

No defects.

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 so, with the momentary exception of the Wolf Brothers, no effort to found a successor has succeeded.

The 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.

Imperial Fists

• Missing Betcher's Gland (no acid spit)
• Missing Sus-an Membrane (no suspended animation)

Blood Angels

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 and Red Thirst; a berserk rage where they believe themselves to be Sanguinius during his final battle, and a thirst for blood, respectively.

Iron Hands

No defects.

Ultramarines

No defects.

Thousand Sons

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.

Luna Wolves

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.

Word Bearers

The Word Bearers suffer a unique "quirk" in their gene-seed, resulting in a psychologically obsessive loyalty to anything. The Word Bearers in turn thus have a much stronger sense of loyalty to their battle-brothers and Primarch than other Chaos Space Marines, but this loyalty also borders on fanaticism.

Salamanders

• Malfunctioning Melanchromic Organ (Salamanders are all black, all the time, and have glowing red eyes)

Raven Guard

• Missing Mucranoid (always pale white)
• Missing Betcher's Gland (no acid spit)

Corax made use of forbidden techniques to accelerate gene-seed maturation, to try and boost his Legion's numbers after their losses of the Horus Heresy, but his experiments resulted in the creation of monsters.

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.
• Indestructability: The Sons of Antaeus are much more tough than normal marines, and nobody knows why.