In the simplest type of reproduction, a one-celled organism divides into two new cells and the offspring closely resemble the parent. In other types of reproduction, however, the process that provides for the passing on of traits from one generation to the next is not always apparent. This is especially true of sexual reproduction.

The offspring of some organisms that reproduce sexually do not at first even resemble their parents. Tadpoles do not look like frogs, nor caterpillars like butterflies. Eventually, however, they reach a stage in their development where they look like their parents and other adults of their species. It is apparent, therefore, that what they have inherited is a growth pattern.

All organisms inherit a pattern of growth. Sexually reproduced organisms inherit a pattern from each parent, and the combined patterns determine how the organism will develop. The information necessary to produce these patterns is found in the hereditary, or genetic, material that exists in all living cells. This material is usually found in a cell's nucleus, where it is arranged into tiny bodies called chromosomes. Each chromosome contains several types of proteins and deoxyribonucleic acid (DNA). DNA consists of two strands, each composed of a long chain of subunits called nucleotides, of which there are four kinds. Inheritable traits are determined by DNA segments, called genes, that contain specific sequences of the four kinds of nucleotides. The body, or somatic, cells of all organisms carry a number of paired, homologous (similar) chromosomes. Sexual reproduction occurs with the union of two specialized cells, called gametes, one from each parent. A gamete, such as an egg or sperm, contains only half the number of chromosomes as a somatic cell of the same species. The process by which gametes are produced is called meiosis (mī-ω'si$s).

Only certain cells (such as those in the testes and ovaries) undergo meiosis. In meiosis the homologous chromosomes duplicate, then pair up and interconnect in a process called synopsis. The cell then divides into two, with each chromosome of a homologous pair going to a new cell. The result is four gametes, each containing half the number of chromosomes as the the parent cell. .