가후
According to the "Xin Tang Shu", Jia Xu descended from the royal family of the Zhou dynasty. His ancestor was Gongming (公明), the youngest son of Shuyu (a son of King Wu of Zhou). Gongming was enfeoffed by King Kang of Zhou as the Count of Jia (賈伯) and given the lands around Linfen as his fief, which was named "Jia". His descendants adopted "Jia" as their family name. Jia Yi, a notable scholar-politician who lived in the Western Han dynasty, was an ancestor of Jia Xu. Jia Xu's father, Jia Gong (賈龔), served as a General of Light Cavalry (輕騎將軍) in the Eastern Han dynasty and moved his residence to Wuwei Commandery, where Jia Xu was born. Jia Xu had an elder brother, Jia Cai (賈綵).