Genetics is the science of heredity and variation in living organisms.Knowledge that desired characteristics were present at birth has been implicitly used since prehistoric times for improving crop plants and animals through selective breeding. However, the modern science of genetics, which seeks to understand the mechanisms of inheritance, only began with the work of Gregor Mendel in the mid-1800s.
Mendel observed that inheritance is basically a discrete process with specific traits that are inherited in an independant manner. These basic units of inheritance is now known as "genes". In the cells of organisms, genes exist actually in the structure of the molecule DNA and the information genes contain is used to create and control the components of cells. Although genetics plays a large role in determining the appearance and behavior of organisms, it is the interaction of genetics with the environment an organism experiences that determines the ultimate outcome. For example, while genes play a role in determining a person's height, the nutrition and health that person experiences in childhood also have a large effect.