A Special Relationship
The special relationship between twins allows researchers to examine the differences between genetic and environmental influences over both physical and mental health, as well as traits and behaviors, in the general population. Identical or monozygotic twins develop from the same fertilized egg, and are the same sex sharing 100 percent of their genes. Some of their similarities provide evidence of a high degree of genetic influence. Meanwhile, fraternal or dizygotic twins develop from two different eggs fertilized by separate sperm cells; they generally share only about 50 percent of their genes. Therefore, in that sense, fraternal twins are more like typical brothers and sisters. Similarities found between sets fraternal twins suggest more environmental influences than genetic. The information gathered from twin studies lays the groundwork for future genetic research, including the degree to which any and all aspects of life are determined by genetics, the location of specific genes, and the prevention and treatment of diseases and disorders.