Accumulating evidences suggests that the relationship between heredity and the environment is bidirectional. Genes affect people's behavior and experiences, but experiences and behavior also affect gene expression. Epigenesis refers to development resulting from ongoing, bidirectional exchanges between heredity and all levels of the environment.

Some of the most interesting research on epigenetics has examined the effect famine on health in later life. Let's consider this research to learn more about the science of epigenetics.

The Dutch Hunger Winter took place in the mid-1940s. The people of Netherlands were under German occupation during this time toward the end of WWII. At this time the population of Netherlands was about 9 million. The Germans blocked the shipment of food into the country restricting access to food for to about half of the country during a harsh winter. An estimated 22,000 individuals died and many other suffered ill effects as a result of the famine for many years.

Researchers decided to look at the effect of the famine on adults that were in utero at the time of the famine. During the famine pregnant mothers were estimated to consume only 400-800 calories a day. Researchers found that the birth weight of the children born to mothers experiencing famine conditions varied based on when their mother's experienced such conditions.

"Women exposed to the famine during mid- to late-gestation had babies with significantly reduced birth weights. Babies whose mothers were exposed only during early gestation had normal birth weights; however, they grew up to have higher rates of obesity than those born before and after the war and higher rates than those exposed during mid- to late-gestation...Those who were exposed to the famine only during late gestation were born small and continued to be small throughout their lives, with lower rates of obesity as adults than those born before and after the famine....Only in those exposed to famine during early gestation is there a significant impairment in a test of selective attention at ages 56-59 years."

Famine research conducted in China also found metabolic differences in people born during famine conditions, but noted the effects not only in those born during the famine, but among their children a generation later as well.These findings support the idea of a bidirectional relationship between our genetics and our environment that is notable not only for us, but for future generations as well. In other words, the environment affected the genetics of children born to women during a famine and to the grandchildren of women pregnant during the famine. Famine (an environmental factor) influenced genetic development.

Let's look more closely at prenatal development.