The somatic (body) cells of all organisms carry a number of paired, homologous (similar) chromosomes. With few exceptions, all organisms of the same species have the same number of chromosomes in each of their somatic cells. For example, each somatic cell of a normal human being has 46 chromosomes; each of an earthworm has 32; and each of a corn plant has 20. The number of such chromosomes in each somatic cell is called a diploid, or doubled, number.

When a somatic cell divides, by a process called mitosis, each chromosome duplicates, with exact copies of the original going to each new cell. Thus, each new cell has the same number of chromosomes as the parent cell.

Sexual reproduction comes about through the union of two specialized cells, one from each parent. These cells are called gametes. A gamete, such as an egg or sperm, contains only half the number of chromosomes as a somatic cell of the same species. Therefore, the gamete is said to contain a haploid, or halved, number of chromosomes. This reduction in number occurs through the process of meiosis, or reduction division. Meiosis involves two successive cell divisions called the first and second meiotic divisions.

Special diploid cells (such as certain cells in the testes and ovaries) undergo meiosis. In these cells, the parental chromosomes duplicate. Then, homologous parental chromosomes pair up with each other and become connected in a process called synapsis. The cell then divides (the first meiotic division) a into two diploid cells with mixed parental chromosomes.

Each of these diploid cells also divides (the second meiotic division) into two parts, with each chromosome of a pair going to a separate cell. The result is four haploid cells (gametes), each of which contains half the number of chromosomes of the parent cell. Without meiosis, the chromosomes in the cells of each new individual would double in number with each successive generation.

The union of male and female gametes at fertilization is called sexual recombination. The union results in a fertilized egg called a zygote, which has the combined number of chromosomes of both gametes. The zygote, therefore, has the same diploid number as a somatic cell. The zygote develops into a new organism, producing somatic cells by mitotic cell division. As the organism matures, certain cells are set aside for reproduction. These cells are diploid but divide through meiosis, producing haploid cells (gametes).

The ways that chromosome activity affects the inherited traits of organisms have been formulated into a number of principles, or laws. Some of these principles were perceived long before anything was known about chromosomes and genes.