Ja'far was the son of Abdallah ibn Muhammad al-Abbasi al-Hashimi (the future caliph
al-Mansur) in 742/743. When Ja'far was eleven or twelve year old, his father became caliph. His mother was Arwa. She was al-Mansur's first wife.
Arwa also known as Umm Musa, whose lineage went back to the
kings of Himyar. Her father was Mansur al-Himyari. She had a brother named Yazid. Arwa had two sons, Muhammad (future Caliph
al-Mahdi) and Ja'far. She died in 764. The two sons of Arwa, Ja'far and Muhammad were regarded as his heirs after the removal of
Isa ibn Musa as heir. However, Ja'far died during his father al-Mansur's reign. His brother, Muhammad became sole candidate to the throne. One of his al-Mansur's concubines was a Kurdish woman. She was the mother of al-Mansur's son Ja'far. Al-Mansur named this son after his elder son. After Ja'far's death, his father took care of his young children. When his father, al-Mansur died. His brother caliph
al-Mahdi took responsibility for his children. Al-Mahdi married his son and heir to Ja'far's daughter
Zubaidah. ==Relatives==