During embryonic development in humans and other mammals,
sperm and egg cells are essentially wiped clean of chemical modifications to
DNA called epigenetic marks. They are then held in reserve to await
fertilization.
In flowering plants the scenario is dramatically different.
Germ cells don’t even appear until the post-embryonic period – sometimes not
until many years later. When they do appear, only some epigenetic marks are
wiped away; some remain, carried over from prior generations – although until
now little was known about how or to what extent.