Joining segment (J)
A small DNA segment that links genes in order to yield a functional gene encoding an immunoglobulin.
Jumping genes
See transposable elements.

Juvenility
Early phase of development in which organism is juvenile and incapable of sexual reproduction.

Juvenile in vitro embryo technology (JIVT; JIVET)
A technology involving collection of immature ova form young animals, in vitro maturation of those ova, in vitro fertilization, and then transfer of the resultant embryos into recipient females. A method of achieving rapid generation turnover.