The first undoubted evidence about DNA being the genetic material came from :
Transformation of a rough strain of Streptococcus pneumoniae to the smooth strain.
Fredrick Griffith has successfully demonstrated the phenomenon of transformation in bacteria. It was one of the classic experiments where non virulent strain bacteria were transformed into virulent strain due to a"transforming principle". This transforming principle was a chemical substance released by the dead cells of smooth-coated non virulent bacteria. It was later proved by Avery, MacLeod and McCarty that it was DNA that was the transforming principle that was transferred from S-strain to R-strain. Hence, the transformation is regarded as the first undoubted evidence that DNA is the genetic material.