Epigenetics refers to shifts in gene expression that occur without changes to the DNA sequence. Some epigenetic changes are an aspect of cell function, such as those associated with aging. However, environmental factors also affect the functions of genes, meaning people's behaviors affect their genetics. For instance, identical twins develop from a single fertilized egg, and as a result, they share the same genetic makeup. However, as the twins age, their appearances may differ due to distinct environmental exposures. One twin may eat a healthy balanced diet, whereas the other may eat an unhealthy diet, resulting in differences in the expression of their genes that play a role in obesity. Nutritional epigenetics is the study of how your diet, and the diet of your parents and grandparents, affects your genes. The dietary choices a person makes today affects the genetics of their future children. A ... study in sheep showed that a paternal diet supplemented with the amino acid methionine given from birth to weaning affected the growth and reproductive traits of the next three generations. Methionine is an essential amino acid involved in DNA methylation, an example of an epigenetic change. These studies underscore the enduring impact parents' diets have on their children. They also serve as a powerful motivator for would-be parents and current parents to make more healthy dietary choices, as the dietary choices parents make affect their children's diets.