Genetics is the study of heredity. It is an ancient discipline. At least 4000 years ago, in Sumeria, Egypt and other parts of the world, farmers recognized that they could improve their crops and their animals by selective breeding. Their knowledge was based on experience and they recognized that many features of plants and animals were passed on from generation to generation. Furthermore, they were aware that desirable traits - such as size, speed and weight of animals could sometimes be combined by controlling mating, and that in plants, crop yield and resistance to arid conditions could be combined by cross pollination, The ancient breeding programs were not based on experimental studies because nothing was known of genes or any of the principles of heredity.