Parent isotope, radiogenic daughter কী?

In a decay system used for dating, the parent isotope is the radioactive isotope whose decay serves as the clock, and the daughter is the isotope its decay produces. The radiogenic daughter, written $D^*$, is the part of the daughter made by the decay of the parent inside the sample since the clock started; the rest, $D_0$, was there from the start. The number of atoms of the daughter measured today is $D = D_0 + D^*$.