Complex sentence: Studies of the third generation are important because DES might be associated with epigenetic changes, which involve changes to the way genes behave (not involving the DNA itself) that may be heritable from one generation to another.
Split into simpler sentences: Studies of the third generation are important because DES might be associated with epigenetic changes. Epigenetic changes involve changes to the way genes behave (not involving the DNA itself) that may be heritable from one generation to another.