Whose? Dessen and deren
Genitive relative pronouns express possession. Use dessen for a masculine or neuter owner and deren for a feminine or plural owner. Their form follows the owner, not the gender of the thing possessed.
Understand the idea
Dessen and deren express a possessive relationship inside a relative clause. Their form follows the antecedent that owns something, not the gender of the possessed item. They do not behave like an ordinary article that changes to match the next noun.
Build it step by step
Use dessen for a masculine or neuter singular antecedent, and deren for a feminine singular or plural antecedent. Then build the rest of the relative clause and place its finite verb at the end. The possessed noun phrase still has its own role in that clause; the relative possessive form identifies the owner.
See it in a sentence
Das ist der Mann, dessen Tochter hier arbeitet.
Das ist das Kind, dessen Vater Arzt ist.
A useful memory aid
Draw a small arrow from dessen or deren back to the owner. Choose its form from that antecedent before reading the following noun.
A common trap
Do not choose dessen merely because the possessed noun is masculine. A woman's son still uses deren when the woman is the antecedent.