The final proof for DNA as the genetic material came from the experiments of:

While Griffith discovered "transformation" and Avery et al. identified the "transforming principle" as DNA, the final/unequivocal proof that DNA is the genetic material came from Alfred Hershey and Martha Chase's experiment ($1952$) using radioactive isotopes ${}^{32}\text{P}$ and ${}^{35}\text{S}$ on bacteriophages.