The article doesn't really explain it, but looking at the image provided, I think I understand. A girl is born with all her eggs already, so the egg germ cell t

The article doesn't really explain it, but looking at the image provided, I think I understand. A girl is born with all her eggs already, so the egg germ cell that becomes the grandchild is already present inside the grandmother while she is pregnant with the mother. In this case, the genes being methylated are in the cell which will go on to become the grandchild. Changes themselves are not being passed down through generations, per se, but are being effected in the 3rd generation at the instant of the trauma itself.