One of the most basic characteristics of life is the ability to reproduce.  The offspring of any organism closely resembles their parent or parents. But it does not mean we are exactly the same. Many unicellular organisms and a few multicellular organisms can reproduce identical clones of themselves through binary fission.  But others reproduce regularly by sexual reproduction.  Sexual reproduction is the production of two haploid cells and their fusion to form a single, unique diploid cell.  Through multiple mitotic cell divisions, this diploid cell will develop into an adult organism. Haploid cells, a part of the sexual reproductive cycle, are produced by a type of cell division called meiosis. Sexual reproduction, specifically meiosis and fertilization, introduces variation into offspring.

Figure 1. Each of us, like these other large multicellular organisms, begins life as a fertilized egg. After trillions of cell divisions, each of us develops into a complex, multicellular organism. (credit a: modification of work by Frank Wouters; credit b: modification of work by Ken Cole, USGS; credit c: modification of work by Martin Pettitt)