Kekkei Genkai

are abilities passed down genetically in specific clans. Kekkei Genkai and their related jutsu can't be taught or copied by others, but it is possible to confer the ability to another person if it's localized in a particular organ like the eyes via organ transplant, like Obito passing his Sharingan eye to Kakashi, although Kakashi has to use extra chakra. 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 ninjas. These traits can also be passed down through people outside of the clan, such as Tenzo going through Orochimaru's DNA re-modification to acquire the Wood Release, though his is not nearly as powerful as the original practitioner, Hashirama Senju. Also, Kekkei Genkai can be preserved through the creation of Human Puppets, where they retain the special traits of the users when they were still alive. It's also possible for a ninja to have more than one Kekkei Genkai.

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.

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.