Two great influences, acting together throughout this time, have made the sheep into what they are today. These influences are genetics and environment. Genetics is the science of passing on of traits from parents to offspring. This change tends to be permanent. For example, through genetic improvements, the sheep has changed from one with a hairy coat to one with the potential to produce up to 10 pounds of clean fine wool annually.
Environment is all of the outside forces that act on an organism.
Sometimes it is hard to tell where the effects of heredity end and those of environment begin. For example, heredity determines the mature body size of a sheep. But factors in the environment, such as the feed availability and the climate, also affect the growth. In other words, heredity controls what the sheep can become, but what the sheep does become often depends on environment as well. Those concerned with sheep production as a means of providing for, or contributing to, their livelihood should have some type of mating and selection program.