The Devil Gene is a genectic abnormality isloated in the people of the Mishina bloodline and is mostly used by the likes of Kazuya Mishima. While the exact specifications of this gene are never fully truly explained, it is generally assumed that the gene allows a person to have a symbiosis or a parasite/host type relationship with a malignant spirt referred to as a devil, as well as having it control one of the other. The exact requirements of these transformations are never fully revealed. The spirits can take over the person at any given time during the person's lifespan, even while their in the womb of their mother or just after death.
The people who currently carry the Devil Gene are Kazuya Mishima, his son, Jin Kazama, and Azazel. Heihachi Mishima doesn't seem to possess the Devil Gene in his blood like his son and grandson. The concept of the Devil Gene was not thought of until Tekken 4, which then somewhat reconnected the pervious three canonical games. Prior to this, the concepts of Devil was entirely spirtual. The classic and orginal concept, before the coming of a Devil Gene, was not entirely scrapped though, and has in some cases been incorporated in to the new continuity. Kazuya had his encounter with the devil, later to be know as Devil by name, when he was five years of age. Heihachi, sickened by his son's weakness, threw him off a cliff in order to have him climb back up, but because of the Devil Gene, Kazuya was able to climb back up the cliff with ease.
This could also mean that the Devil Gene can heal a Mishima when they are close from dying, if they are knock out, they may be in a coma because of the Devil Gene in their blood. After the events of Tekken 2, Heihachi threw Kazuya's body into a volcano. This caused Devil's soul to split into two parts, one that stayed with Kazuya, and another that went into Jin Kazama, who was still unborn. It was warded off by his mother, Jun Kazama, and was unsuccessful in controlling Jin's body. However, fifthteen years later, it had return, branded Jin Kazama's left arm with a mark and (likely) brainwashed him (as seen in the Tekken 3 opening).
The common Denominator
Interestingly enough, while Heihachi Mishima does not have the powers of the Devil Gene like, he has been the catalyst whose actions have resulted in the Devil Gene possession in the family of the Mishima clan. He staged a coup d'etat against Jinpachi Mishima and imprisoned him under Hon-Maru, which resulted in Jinpachi being possessed by a monster (though this demonic possession has nothing to do with the Devil Gene). Heihachi was also responsible for throwing Kazuya off a cliff when Kazuya was five, causing the Devil Gene to awaken, several years later, after the second iron fist tournament, Heihachi and his Tekken Force soldiers shout Jin near his death after the conclusion of Tekken 3. This cause the Devil Gene to awaken inside his body and partially transfromed into his Devil from.
Activation of the Devil Gene
It is not truly explained how the Devil Gene is activated, but the main series of the games seem to suggest that it can be activated consciously, subconsciously, or based on external stimuli. Regardless of which, the effects are clear, devil side of a Mishima becomes more dominant, to the point of changing their physical appearance of the host, who in turn becomes much more powerful in a physical sense. Kazuya performs a conscious activation of his Devil Gene between the events of Tekken 4 and Tekken 5, during the attacks of Hon-Maru by the Jack robots. Jin seems to be able to activate his Devil Gene consciously by the events of Tekken 6 without turning into a devil like his father Kazuya. Jin's first transformation into his Devil from at the end of the events of the third tournament wad arguably a subconscious activation of hid Devil Gene, likel part of the hatred he felt for his grandfather, Heihachi Mishima, Jin also transformed into his devil from unwillingly again during the events of Tekken 5, after his defeat from Howarang because of stress, when Kazuya was thrown down into a mountain, enraged by Kauya's subconscious mind, and thus gaining ultimate power to rule the world. Jin partially transforms into his Devil from at the end of the fourth tournament due the Devil Gene's attempts at forcing him to (as seen in Jin's Tekken 4 ending).
Suppression of the Devil Gene
Aside from the suppression of the Devil Gene through sheer force of will. As is implied by the method of conscious, and to some extent subconscious activation, and to some extent subconscious activation, there are some external factors that can supasd the Devil Gene as well. The exact nature of these things are. However not fully explained, often not at all, so it remains unknown how they are able to suppress the Devil Gene. It is only known that they are able to do so.
Before the concept of a Devil Gene was created in the main series of Tekken, the original concept was spiritual, Jun Kazama seemed to possess the power to suppress the Devil Gene inside Kazuya. It was explained as having been because of her serene and pure being. However, when the Devil history was reconnected and the Devil Gene concept created from then on in the main series, the influence of the Kazama's became genetic as well: their bloodline, besides Jun Kazama, having been a special case. At the conclusion of Tekken 4, the effects of the bloodline is shown as the Devil form of Kazuya tries to transform Jin into his own Devil form, by using his own rage that he has for his Mishima family, he fails however, and theorizes that it is because of the Kazama blood flowing inside Jin's body. Asuka Kazama, in her ending during the events of Tekken 5, also has the ability to suppress the Devil Gene of Jin, however, it is unknown just how strong an effect she has on the Devil Gene.
Materials While it is not known how, the Mishima Zabatsu were able to create chains of a materials that is not known, to suppress the Devil Gene. Angel Before the concept of the Devil Gene, Angel, was able to suppress the Devil Gene of Kazuya Mishima, Angel has all but vanished from the Tekken storyline, as it is unknown whether she is still able to suppress Devil or if she still even exist.
In Tekken 4, when the G corporation discovered Kazuya's body, they discovered strange malignant cells within Kazuya's body that they revered to as the Devil Cells, they used his body to further their research and commenced a project known as the (Devil-Human) Integration Program. Using his DNA, they attempted to combine the Devil cells and Human cells, however, when combining the two types of cells into one, the Devil cells destroyed the human cells, making it impossible to create a Devil Human, a viable life from similar to that of Kazuya's genetic make up. The G Corporation used Julia's GENOCELL Program and even discovered the the GENOCELL can suppress the Devil Cells, allowing the Human Cells to combine with the Devil Cells.