The ability to reproduce is a basic characteristic of all organisms: Hippopotamuses give birth to hippopotamus calves (a); Joshua trees produce seeds from which Joshua tree seedlings emerge (b); and adult flamingos lay eggs that hatch into flamingo chicks (c). However, unlike the organisms shown above, offspring may or may not resemble their parents. Generally resembling parents as opposed to the offspring having similar features to the parents, represent a finer detail in reproduction. Heredity (meaning heir, heres in Latin) is significant in this process because of the transmission of traits from one generation to the next generation, which has the additional benefit of introducing variation. Also, reproduction in some species follows a circuitous path. For example, in the case of most insects, such as butterflies (with a complete metamorphosis), the larval forms rarely resemble the adult forms.
The study of the science of heredity and inherited variation is at the core of our understanding of how chromosomes pass from parents to offspring in sexually reproducing organisms. Although, many unicellular organisms and a few multicellular organisms can produce genetically identical clones of themselves through asexual reproduction, many single-celled organisms and most multicellular organisms reproduce regularly using another method—sexual reproduction. This highly evolved method involves the production by parents of two haploid cells and the fusion of two haploid cells (fertilization, the fusion of the sperm and egg, in the picture below) to form a single, genetically recombined diploid cell—a genetically unique organism. Haploid cells that are part of the sexual reproductive cycle are produced by a type of cell division called meiosis. Sexual reproduction, involving both meiosis and fertilization, introduces variation into offspring that may account for the evolutionary success of sexual reproduction. The vast majority of eukaryotic organisms, both multicellular and unicellular, can or must employ some form of meiosis and fertilization to reproduce.
In most plants and animals, through thousands of rounds of mitotic cell division, diploid cells (whether produced by asexual or sexual reproduction) will develop into an adult organism.