The first truly scientific study of heredity was conducted by Gregor Mendel (1822–1884), an Austrian monk. He published his findings in 1866. The principles he established—segregation and dominance, and independent assortment—became the basis for the science of genetics.

Mendel's work was neglected until 1900, when a number of biologists rediscovered his work and his papers were republished.

In 1906, Thomas H. Morgan and his associates at Columbia University began a long series of genetic studies on fruit flies, which produce a new generation every two weeks. These studies resulted in an understanding of linkage, sex determination, and sex linkage, and in the mapping of gene arrangements on chromosomes. One of Morgan's associates, H. J. Muller, showed that mutations could be produced by X rays.

In the 1940's and early 1950's, biologists discovered that the nucleic acid DNA (and RNA in some viruses) was the carrier of the hereditary pattern. In 1953, the structure of DNA was discovered by Francis H. C. Crick, a British molecular biologist, James D. Watson, a United States biochemist, and Maurice H. F. Wilkins, a British biophysicist. In 1957, Arthur Kornberg, a United States biochemist, discovered the enzyme that synthesizes DNA. By 1966, biologists knew which nucleotide sequences specify the genetic code for which amino acids, the compounds that make up proteins. In 1977, Richard J. Roberts and Phillip A. Sharp, United States biochemists, discovered that some genes have DNA segments with no apparent function; these segments are called introns. Introns occur between exons, the DNA segments that specify the genetic code for proteins. Researchers soon discovered that most genes are made up of introns and exons.

In the mid-1970's, scientists developed genetic engineering, a process in which some DNA is taken from cells of one organism and combined with DNA in the cells of another. Genetic engineering is used in the manufacture of a number of substances, including certain drugs and hormones. It is also used to produce desired characteristics in crops, livestock, and laboratory animals. In an experimental technique called gene therapy, human genes are manipulated to treat or cure genetic diseases.

In a process called cloning, genetically identical copies of individual organisms are produced. Since ancient times, cloning has been practiced with plants, as in growing a new plant from a cut stem. It has also been known that simple animals could be cloned; for example, when a worm is cut into pieces, each piece develops into a new worm.

The first vertebrates (animals with backbones) to be cloned were frogs, in the 1950's. Mammals were first cloned in the 1980's. In the principal technique used to clone animals, the nucleus of a cell of an embryo is removed and inserted into a fertilized egg whose nucleus has been removed. The egg develops into an animal genetically identical to the embryo. In this way, many identical individuals can be made from a single embryo. Laboratory mice and certain livestock are the animals most commonly cloned today.

In the 1980's, biochemists developed a process for readily duplicating segments of DNA. This process, called the polymerase chain reaction, or PCR, makes it possible to produce very large amounts of DNA from samples that would otherwise be too small to analyze for their DNA. The polymerase chain reaction is widely used in a number of biological fields, including forensic pathology and studies of evolution.

In 1990, an international undertaking called the Human Genome Project was begun to locate the positions of all of the genes on the human chromosomes. The project is also concerned with mapping these genes; that is, determining each gene's sequence of nucleotides. Among the genes that have been located and mapped are those for muscular dystrophy, Huntington's disease, cystic fibrosis, colon cancer, and amyotrophic lateral sclerosis. In 1998, scientists completed mapping the genetic sequence of the roundworm Caenorhabditis elegans, the first full mapping of a multicellular organism. In 2001, the first nearly complete mapping of a major food crop (rice) was announced.

By 2000 a number of countries, including Iceland and the United Kingdom, had begun projects to collect DNA samples from citizens and link this genetic information with their health records, information about their lifestyles, and their family health history.