Recently, I was having a discussion on what Varnas, Karna and Satyavati should belong to.
We all know Karna was born from Kunti and Surya but was adopted by Adhiratha Nandana. Therefore, he was born as a Kshatriya but was adopted by Suta (product of Brahmin mother and Kshatriya father) parents. Of course, his identity was not known to people.
Satyavati and his twin brother Matsya, on the other hand, was born as the product of King Uparichara Vasu and Adrika, an Apsara cursed by a Brahmana to turn into a fish. She was found by a fisherman (who are generally considered Shudras) who was a subject of Uparichara Vasu incidentally. The fisherman approached the King and narrated the tale of their miraculous birth. In the end, Uparichara Vasu took Matsya while Satyavati was adopted by the fisherman. I'm guessing Uparichara Vasu was aware that Matsya and Satyavati were his biological son and daughter.
However, Karna and Satyavati are not the subject of my question but rather how adoption generally affects one's Varna. If a boy of certain Varna is adopted by a couple of a different Varna, would he be considered a member of his biological Varna or the Varna of his foster parents? Would the knowledge of his birth i.e. if the identity of his biological parents and their Varnas were known, have any affect on determining his Varna? Do scriptures have anything to say regarding this? Perhaps, Nandapandita's Dattakamimamsa (a treatise on adoption written in the sixteenth century) has the definite answer to this question?