Recent studies highlight that fact.
[M]ethylation — a chemical alteration that turns off genes — occurs most often near, but not precisely within, the DNA regions on which scientists have typically focused their studies. . . . Methylation is just one of many epigenetic signals — chemical changes to DNA and its associated proteins — that modify gene activity without altering the genetic information in the genes. Methylation and other epigenetic signals help guide stem cells as they develop into other type of cells. . . .
Epigenetic patterns established in the early embryo are carried throughout life with some differences introduced by the environment and others by random chance and error in replicating the patterns as the person develops. DNA is reproduced with high fidelity — mistakes happen in about one in a million bases — but the process of reproducing epigenetic patterns in dividing cells is rather more error-prone, with one in a thousand epigenetic marks going awry.
Identical twins are similiar not just because they have the same DNA, but also because the have similiar methylation patterns, which are believed to result from sharing the same embryo, rather than from having the same DNA.
According to the author of a recent study of methylation patterns in twins:
[T]he methylation patterns in monozygotic twins were more similar than for dizygotic twins, fraternal twins who develop from two separate eggs. And the group found that the amount of variation between monozygotic twins correlates with the time the embryo split. . . . [T]he similarity between monozygotic twins results not from shared DNA sequences but from having come from the same embryo. “We don’t see any reason to think that the DNA sequence makes up the epigenetic profile[.]
Epigenetic traits are believed to be particularly important in explaining some kinds of mental illness and the long lasting effects of some kinds of drug abuse.
Similarly, exposure to certain hormones in utero is believed to impact who the sexuality and gender specific traits of a person, including sexual orientation, develop, influencing how the basic XY or XX (or in some cases more complex gene combinatations) express themselves in a person.
One interesting line of research looks at the ratio of index finger length to ring finger length (sometimes called a person's digit ratio and the number of girls and boys born to one's mother prior to your own birth as a stable markers for this kind of in utero exposure to hormones, and has found a great many statistically significant correlations that suggest that hormone exposure in utero can have a meaningful impact on the kind of person that one grows up to be in life. Many of the traits in question appear to go to some version or other of what a lay person might call a person's temperment. Digit ratio studies are cheap, easy and non-invasive to conduct with large sample sizes, making it fairly easy to conduct studies that can capture even modest statistical significance related to it. To the extent that these produce consistent results and that the trait is, as hypothesized, as marker for a person's in utero experience, they have the potential to provide more than pop psychological insight.
For example, one simple study done in 2002 which compared digit ratio (measured on a photocopy of the person's hand without that individual present and without knowing how a questionaire administered at the same time was answered) and self-identification as a "butch" or "femme" lesbian on a questionaire, produced a statistically significant (p score .04) relationship in the expected direction, with women who identifying as "butch" having digit ratios more similar to that of men than that of other women.
Also interesting is the fact that one's "epigenome" can change during a person's life, renewing interest in the ability of people to biologically adapt, by means other than simple natural selection, that were mostly rejected when the mechanism of core DNA was better understood.
These are fascinating findings because the standard model for epigenetics has been that DNA marks are immutable once set down as part of development. The new findings … strongly support the emerging idea that the epigenome is dynamically regulated over the lifetime of a person, perhaps in response to environmental signals, life experiences and as part of the normal aging process.
For example, extended exposure to severe stress might cause a person's epigenome to change permanently in a way that makes that person vulnerable to depression. Or, drug use might change one's epigenome in a way that creates an ongoing need for the drug.