Three generations at once are exposed to the some environmental conditions (diet, toxics, hormones, etc.). In order to provide a convincing case for epigenetic inheritance, an epigenetic change must be observed in the 4th generation.

There are multiple periods during which environmental exposures could affect the F₁ individual’s methylation status, potentially affecting the F₁ phenotype. The first window is during F₀ (parental) germ cell development when methylation is reprogrammed following imprint erasure in the father’s sperm (solid blue line) and the mother’s egg (solid red line) The second window is post-conception, during F₁ embryonic development, when all but imprinted genes are demethylated, with the male germ line (dashed blue line) demethylating more quickly, followed by the female germ line (dashed red line). Imprinted genes (purple line) maintain their methylation marks throughout this reprogramming, allowing for the inheritance of parental-specific monoallelic expression in somatic tissues throughout adulthood [203]. All of the non-imprinted genes are subsequently remethylated once the embryo reaches the early blastocyst stage. During the gonadal sex determination of the F₁ embryo, primordial germ cells undergo epigenetic reprogramming, where parental imprinting is erased, as the germ cells of the F₁ individual mature (solid light blue or pink line). To determine whether epigenetic alterations are transmitted transgenerationally, the F₃ generation must be studied (see text and Fig. 2) [7]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)