who was the biological father of Pandu, father of Pandavs ??

In the Mahabharata, Vichitravirya was the legal father of Pandu but died without children. Through the niyoga tradition (where a brother is summoned to produce children for his deceased brother's widow), Vyasa (also called Krishna Dvaipayana) fathered Pandu (and his blind brother Dhritarashtra). So while Vichitravirya was Pandu's 'official' father, Vyasa was his biological father. Santanu was Vichitravirya's father, and Dushyant was an earlier king in the lunar dynasty.