The Human Body


Gene, basic unit of heredity found in the cells of all living organisms, from bacteria to humans. Genes determine the physical characteristics that an organism inherits, such as the shape of a tree’s leaf, the markings on a cat’s fur, and the color of a human hair (see Heredity).

Genes are composed of segments of deoxyribonucleic acid (DNA), a molecule that forms the long, threadlike structures called chromosomes. The information encoded within the DNA structure of a gene directs the manufacture of proteins, molecular workhorses that carry out all life-supporting activities within a cell (see Genetics).

Chromosomes within a cell occur in matched pairs. Each chromosome contains many genes, and each gene is located at a particular site on the chromosome, known as the locus. Like chromosomes, genes typically occur in pairs. A gene found on one chromosome in a pair usually has the same locus as another gene in the other chromosome of the pair, and these two genes are called alleles. Alleles are alternate forms of the same gene. For example, a pea plant has one gene that determines height, but that gene appears in more than one form—the gene that produces a short plant is an allele of the gene that produces a tall plant. The behavior of alleles and how they influence inherited traits follow predictable patterns. Austrian monk Gregor Mendel first identified these patterns in the 1860s (see Mendel’s Laws).

In organisms that use sexual reproduction, offspring inherit one-half of their genes from each parent and then mix the two sets of genes together. This produces new combinations of genes, so that each individual is unique but still possesses the same genes as its parents. As a result, sexual reproduction ensures that the basic characteristics of a particular species remain largely the same for generations. However, mutations, or alterations in DNA, occur constantly. They create variations in the genes that are inherited. Some mutations may be neutral, or silent, and do not affect the function of a protein. Occasionally a mutation may benefit or harm an organism and over the course of evolutionary time, these mutations serve the crucial role of providing organisms with previously nonexistent proteins. In this way, mutations are a driving force behind genetic diversity and the rise of new or more competitive species that are better able to adapt to changes, such as climate variations, depletion of food sources, or the emergence of new types of disease (see Evolution).