gene
n. the basic unit of heredity, responsible for storing genetic information and transmitting it to subsequent generations. The observable characteristics of an organism (i.e., its phenotype) are determined by numerous genes, which contain the instructions necessary for the functioning of the organism’s constituent cells. Each gene consists of a section of DNA, a large and complex molecule that, in higher organisms, is arranged to form the chromosomes of the cell nucleus. Instructions are contained in the chemical composition of the DNA, according to the genetic code. In classical genetics, a gene is described in terms of the trait with which it is associated and is investigated largely by virtue of the variations brought about by its different forms, or alleles. At the molecular level, most genes encode proteins, which carry out the functions of the cell or act to regulate the expression of other genes. A
minority encode vital components of the cell’s protein-assembling apparatus, such as ribosomes. Recent advances in genetic technology and the work of the Human Genome Project have done much to illuminate the mechanism of gene action and have pinpointed genes associated with various inherited diseases. See also dominant allele; recessive allele.