Kekkei Genkai

are abilities passed down genetically in specific clans. It's possible for a ninja to have more than one of these abilities. Kekkei genkai abilities that work via the user's eye are called dōjutsu. Other kekkei genkai include mixing one type of elemental chakra with another, creating a new one unique to the users, which is usually impossible for normal ninja.

Kekkei genkai and their related techniques cannot be taught to or copied by others. However, kekkei genkai can be given to others. Dōjutsu, or other kekkei genkai isolated to a single organ, can be transplanted into the body of another, as was done with transferring Obito's Sharingan to Kakashi. Use of these organs requires much more chakra than someone born with the kekkei genkai. Chakra elements can also be passed down to people outside of the clan, such as Yamato going through Orochimaru's DNA re-modification to acquire the Wood Release technique. Yamato's skill it is not nearly as powerful as the original practitioner, Hashirama Senju.

Trivia

 * Kekkei genkai can be preserved through the creation of Human Puppets. This allows them to retain the special traits of the users when they were still alive.
 * When a person is brought back to life via the Summoning: Impure World Resurrection technique, they also retain any kekkei genkai they possessed before their death.
 * A Kekkei genkai should not be confused with Hiden jutsu. Hiden jutsu are exclusive to a certain person or clan, but only because they have kept others from learning how to perform those jutsu.

List of Kekkei Genkai
Crystal Release - Guren

Inspiration
The concept of special ninja techniques limited by bloodline inheritance may have been inspired in part by the works of Futaro Yamada, which strongly influenced later depictions of ninja in fiction. In particular,, published in 1959, featured a pair of warring ninja clans that, through selective breeding, had produced ninja with a variety of deformities, mutations, and special abilities, including even dōjutsu.