In order to understand why the laws of inheritance work as they do, scientists had to look into the interior of cells. Cells are the building blocks of life, and all living things-from bacteria to human beings-are composed of them. The number of cells varies greatly from organism to organism: A bacterium has just one cell; an average-sized adult human has between 60 trillion and 100 trillion. Most cells are too small to be seen with the naked eye and must be viewed under a microscope. In the human body, almost all cells vary between 1/25,000 of an inch and 1/125,000 of an inch in diameter, although there is a nerve cell in the upper leg that, while extremely thin, is several feet long. Other cells are larger. A hen's egg, for example, is a single cell, and the largest cell of any organism on Earth is the...