Identical twins begin from the same zygote, the same combination of egg and sperm. Therefore they have identical DNA, giving them the same genetic makeup. Yet identical twins do not remain identical throughout life. Because they don’t, researchers study the differences that monozygotic twins develop to learn more about epigenetics.
The field of epigenetics concerns itself with changes in appearance or function that are not caused by changes in the DNA sequence. Twin studies prove that organisms can vary even though their DNA does not.
The epigenome
Monozygotic, identical, twins inherit the same DNA, the same genome and also inherit the same epigenetics, the epigenome. The genome is the set of instructions for the development and functioning of the cell, spelled out in the DNA. But the epigenome is a second set of instructions that also controls cell function, by turning certain genes, certain bits of DNA, on or off.
The chemical instructions that control gene function are called epigenetic tags. These tags enable each cell to specialize, to become a particular cell in the heart or the skin, for instance, by developing and functioning in specific ways. In effect, the epigenetic tags tell the undifferentiated cells of the zygote what role to play in the growing body.
Epigenome effects
However, epigenetics can also change DNA function when an organism is exposed to environmental influences. Toxins may cause epigenetic instructions to activate certain genes, or to inactivate others. Diet, exercise, and stress can each have an effect on an organism’s epigenome, whether to promote the organism’s health or to harm it. They are factors that can cause twins to look different and behave differently as they mature.
People who smoke for years, other things being equal, look older than people who quit or don’t start. The toxins inhaled with cigarette smoke cause harm to the functioning of every cell. Severe stress is also aging. Twin studies demonstrate this, and so do pictures of American presidents taken when they enter office and four years later. It’s a stressful job, and the epigenome transmits the aging effect of the pressure to every cell.
So far, science has found that genetics has a strong effect on some traits, such as height, a weaker effect on some others, such as measured IQ, and a very variable effect on still others, such as susceptibility to multiple sclerosis. The untangling of the effects of heredity and environment, though still in its beginning stages, already owes a great deal to the families that contribute their time and energy to participating in twin studies.
