Complex sentence: The fad faded in the 1930s due to its lack of scientific basis and ultimately the discovery of DNA in the following decades which it later became clear had a vastly more complex and important role in both heredity generally and personality specifically.
Split into simpler sentences: The fad faded in the 1930s due to its lack of scientific basis. Ultimately, the discovery of DNA in the following decades made it clear that DNA instead had an important role in both heredity generally and personality specifically.