What is epigenetics, and how much of the hype is true?

Epigenetics is the study of changes in gene expression that occur without altering the DNA sequence. It is how a single fertilized egg's descendants become neurons, muscle cells and blood-vessel cells: as the zygote divides, the daughter cells develop into different cell types by activating some genes while inhibiting others. Mechanisms include DNA methylation and histone modification, and methylated cytosines in the promoter regions of genes often repress them.

Identical twins show the effect. They were epigenetically indistinguishable during their early years, while older twins had remarkable differences in their DNA methylation and histone acetylation.

Then come the big claims. After the Dutch hunger winter of 1944 to 1945, children of women who were pregnant during the famine were more susceptible to diabetes, obesity, cardiovascular disease and other health problems, and grandchildren of women who were pregnant with girls were smaller at birth and had more health issues later. That suggests damage or epigenetic changes to the ova developing in the female fetus. But the health risks in these first two generations are a known phenomenon called fetal programming, caused by exposure to harmful environmental factors in utero.

True inheritance across generations is much harder to establish. In mammals, epigenetic marks are erased after fertilization and again in the developing germ cells, and verification of transgenerational inheritance in mammals remains highly controversial.