In most animals the so-called Weismann barrier sequesters the germ line very early and then scrubs its epigenome twice (during primordial-germ-cell formation and again post-fertilization). Those genome-wide demethylation waves act like a factory reset, so environmentally induced marks almost never persist beyond F2 or F3, and when they do (e.g. in C. elegans small-RNA cascades or a handful of mouse diet-stress studies) the effects are usually weak or transient.