Parents have known for thousands of years that physical characteristics—such as curly hair or brown eyes—seem to run in the family. These characteristics are called inherited traits, and the transmission of traits from generation to generation is called heredity. Farmers realized the power of inherited traits very early on—thousands of years ago, when plants were first being cultivated and animals were first being domesticated. When selecting individuals to breed, a farmer would choose the stock with the best traits—the strongest oxen, the fastest horses, or the plants with the largest fruit—in the hope that their offspring would have the same characteristics.
By the nineteenth century, it still wasn't known exactly how those traits were passed from one generation to another. Gregor Mendel, now known as the father of genetics, studied the hereditary patterns of pea plants. Starting in 1856, he spent years performing experiments and observing traits like flower color, seed shape, and stem length. For example, Mendel bred a pea plant that produced only green peas with another that produced only yellow peas. By doing this, he created a hybrid generation of pea plants, one that produced only yellow peas. Then he took some of those hybrid plants and bred them with each other. The green trait reappeared in approximately a quarter of the peas in the next generation. This showed Mendel that individuals can pass a characteristic to the next generation even if they do not express that trait. These observations led Mendel to conclude that every trait is controlled by "factors." Each individual receives two of these factors for each trait—one version from each parent. Sometimes, one version is expressed, and at other times, the other version is expressed.
Meanwhile, British scientist Charles Darwin had also been observing traits and experimenting. Darwin was puzzled by physical variations that had been absent in previous generations but suddenly appeared in a new generation of an organism. He proposed that the characteristics of a species change over time through a process he called "natural selection." Individuals that are not well adapted to their environment die young and do not have a chance to reproduce. Other individuals are better adapted, with features that make them more competitive in the struggle for survival and mates. These features are known as adaptations. Darwin observed examples of adaptation in a group of birds known now as Darwin's finches. Originally all one species, the finches had spread to different islands within the Galapagos Archipelago. Over time, the beaks of these finches changed, becoming suited to whatever food was available in each area. As a result, the one species had branched out into 14 species. Darwin’s great insight—that over many generations, species can change dramatically to better fit their environments—is known as the theory of evolution. He published his ideas in 1859 in a groundbreaking book called On the Origin of Species. But Darwin still did not understand the biology behind this process.
The modern science of heredity is known as genetics. In the decades following the work of Mendel and Darwin, scientists continued to study heredity. By the 1940s, scientists had deduced that at the center of the puzzle is a molecule that they called deoxyribonucleic acid (DNA). DNA contains the biological set of instructions that make each species unique. What makes an elephant an elephant? DNA. What makes a tulip a tulip? DNA. Minor changes in DNA, called mutations, lead to changes in the structure and behavior of organisms and, over millions and millions of years, lead to new and different life forms. This is the process behind Darwin's theory of evolution.
Although researchers were relatively sure that DNA was the key to inherited traits, they didn't know what it looked like. In 1953, the scientific community was racing to discover its structure. One team, at Cambridge University, was led by James Watson and Francis Crick. Researchers Maurice Wilkins and Rosalind Franklin, at King's College in London, were also working on the problem. Franklin used x-rays to make images of a DNA sample, and she suspected that the structure was a helix shape. Her partner showed Franklin's research to Watson and Crick. The Cambridge team solved the rest of the puzzle. DNA was helix-shaped, but it was a double helix—a double-stranded, twisted molecule. This discovery won Watson, Crick, and Wilkins the 1962 Nobel Prize for Physiology or Medicine. (Franklin had died of illness in 1958.) The work of these two teams allowed other scientists to unravel the mystery of DNA.
DNA is found in the nucleus of every cell. It is organized into structures called chromosomes. Chromosomes are made up of genes. Human chromosomes have tens of thousands of genes, and all of these genes together—known as a genome—control an organism's characteristics.
Further discoveries in the twentieth century paved the way for amazing advances in genetics. In the 1970s, "genetic engineering" became possible. Scientists learned how to take a gene from one organism and combine it with the DNA of another. For example, a gene that keeps flounder from freezing in cold water was cut from the chromosome of the fish and inserted into a tomato's DNA. This change resulted in a tomato less vulnerable to frost. Genes have been inserted into many other crops to make the plants resistant to disease or to increase production. Although genetically modified crops are becoming quite common, many believe that they pose a significant danger to the environment and human health because their long-term effects are not known.
Promising—and somewhat less controversial—advances have also been made in the area of human genetics. In the 1980s, scientists developed methods to compare the DNA sequence of different individuals. DNA left behind at the scene of a crime has been used as evidence in court, both to prosecute criminals and to set free people who have been wrongly accused. DNA sequencing techniques are also useful in the field of medicine. Studying families with rare genetic disorders has allowed doctors to trace the genetic basis of disease through generations. This kind of genetic tracking helps doctors to predict the likelihood of a person getting a disease and to diagnose it—although not to cure the illness. However, some genetic diseases can now be treated by replacing damaged genes with healthy ones, a practice called gene therapy. In many cases, it's not possible or ethical to experiment on humans, so scientists often rely on other organisms, ones that reproduce rapidly such as mice or fruit flies, to learn more about the effects of gene therapy.
The twenty-first century continues to be a time of technological advancement in genetics. The first milestone of the twenty-first century was the completion of the Human Genome Project in 2003. Researchers now know the DNA sequence of all human genes, and are making a detailed map of each chromosome. Doctors are able to take a sample of a person's DNA, scan it, and determine his or her genetic risk. New mysteries and concerns will undoubtedly arise as scientists continue to explore genetics. But their discoveries will also answer many questions about the biology of the world's species and the code of life.