Unequivocal proof that DNA is the genetic material was first proposed by:

While Griffith discovered transformation and Avery et al. showed DNA was the transforming principle, the unequivocal proof came from the $1952$ experiment by Alfred Hershey and Martha Chase. They used radioactive isotopes (${}^{32}P$ and ${}^{35}S$) to track DNA and protein in bacteriophages infecting bacteria.