Genetics is the branch of biology that deals with heredity, variation, and the mechanisms by which traits are transmitted from parents to offspring. The term was coined in 1905 by British geneticist William Bateson (1861–1926) to separate a new scientific discipline from concepts that could be traced back to Hippocrates (c.460–375 BC) and Aristotle (384–322 BC). Modern genetics, also referred to as classical genetics, emerged from statistical studies of patterns of inheritance, microscopic studies of the behavior of the chromosomes, and studies of the chemical and physical properties of the genetic material.
During the 1950s the study of genetics was revolutionized by proof that DNA was the physical agent of heredity and discovery of its double-helical structure. By the end of the twentieth century, scientists had developed powerful new technologies—recombinant DNA techniques and genetic engineering—capable of manipulating and transforming genetic material.
Historical Background and Scientific Foundations
Ancient ideas about heredity can be found in myths, legends, and folklore. The biblical account of how Jacob increased the quality of his flock of sheep, for example, reflects ancient ideas about animal breeding. While tending his uncle's sheep, Jacob was allowed to keep all the spotted lambs. Such animals were usually very rare, but Jacob increased his allotment by presenting spotted designs to the best ewes.
Hippocrates argued that both parents contributed traits to their offspring, but Aristotle believed that only the male parent provided the blueprint. The mother provided “matter” and served as an incubator. If the “heat” of her uterus was insufficient, the embryo developed into a female. Aristotelian ideas about heredity were influential into the nineteenth century, when cell theory finally began to unlock the secrets of heredity and embryology.
One well known attempt to create a theoretical framework for heredity, variation, and evolution was Charles Darwin's (1809–1882) theory of pangenesis. The then-current view of inheritance claimed that each offspring was a blend of its parents' characters. This contradicted Darwin's theory of natural selection. If matings between rare variants and ordinary individuals always resulted in a blending of traits, rare variations would be diluted out in every succeeding generation.
According to the theory of pangenesis, every part of the body produced tiny particles called gemmules. These particles were collected in the reproductive organs and incorporated into the gametes. At conception, gemmules from both parents, including latent ancestral gemmules, combined to form the embryo.
Skepticism about Darwin's gemmules prompted his cousin, the Victorian polymath Francis Galton (1822–1911) to subject the hypothesis to an experimental test. If gemmules were present in the blood, a transfusion should transfer gemmules from donor to recipient. Using rabbits with different coat colors, Galton demonstrated that blood transfusions had no effect on the coat color of the offspring of transfused rabbits.
Plant Hybridization and Heredity
Traditional studies of plant hybridization were closely associated with the development of modern genetics. According to Aristotle, reproduction in plants was asexual, but evidence for plant sexuality had been established in the 1690s by natural philosopher Rudolph Camerarius (1665–1721). German botanist Joseph Kölreuter (1733–1806) later developed a technique for artificial fertilization in plants and carried out hundreds of hybridization tests.
Inspired by Kölreuter's work, German botanist Carl Gärtner (1772–1850), French botanist Charles Naudin (1815–1899), and others took up the study of plant hybridization. Many botanists had observed the phenomena now recognized as dominance and segregation, but Naudin was probably the first to suggest that ancestral traits might segregate during reproduction. Naudin and Darwin corresponded between 1862 and 1882, but Darwin did not seem to recognize the importance of Naudin's ideas. Unfortunately, Darwin was also unaware of the remarkable hybridization experiments being conducted by Gregor Mendel (1822–1884).
In 1854, after preliminary tests of 34 different strains of peas, Mendel selected 22 types for further experiments. From 1856 to 1863 he tested over 28,000 plants and carefully analyzed seven pairs of traits. The first generation of hybrids displayed the trait from only one of the parents—the dominant trait. By breeding these hybrids, Mendel proved that recessive traits were only dormant in hybrids, rather than diluted or destroyed. What is now called Mendel's first law, or the law of segregation, refers to Mendel's proof that recessive traits reappear in predictable patterns in subsequent generations. In his simplest experiments, Mendel crossed peas that differed in only one trait, but he also studied the offspring of strains that varied in two or three traits. The law of independent assortment, also known as Mendel's second law, refers to his experimental proof that the behavior of any pair of traits is independent of all other pairs of traits.
Mendel described his results in a paper entitled “Experiments on Plant Hybrids,” which was presented to the Brno Society for Natural History (1865) and published in the Society's Proceedings (1866). To reach experts on hybridization, including Karl von Nägeli (1817–1891), professor of botany at Munich, Mendel sent them reprints of his article. But Nägeli, who was regarded as an authority on plant hybridization, was not interested in Mendel's data and did not mention Mendel in his own publications. Mendel's work was neglected until the beginning of the twentieth century, when several scientists carrying out hybridization experiments independently rediscovered his laws of the segregation and independent assortment of hereditary traits, which he called “factors.”
GREGOR JOHANN MENDEL (1822–1884)
Despite the poverty into which he was born, Johann Mendel (1822–1884) became an Augustinian monk (taking the name Brother Gregor) and the abbot of his monastery at Brno in Moravia, now the Czech Republic. Respected as a teacher, naturalist, and administrator, Mendel did not become famous outside his community until 1900, when his discovery of the laws of heredity was finally recognized.
Mendel was born into a family of peasants. His teachers recognized his intelligence, but his family could barely afford to send him to school. When offered the opportunity to join the Augustinian order in Brno, Mendel gratefully accepted. The monastery was well known as a center of learning, especially natural history. Many of the monks were teachers and members of local scientific societies.
Mendel was successful as a substitute teacher, but to qualify for a regular appointment, he had to take several exams in natural history. After Mendel passed the physics and meteorology tests, but failed geology and zoology, he was sent to the University of Vienna. In addition to attending lectures in the sciences, he participated in the Zoological—Botanical Society of Vienna and published two papers in the Society's Proceedings (in 1853 and 1854.) In 1856, during his second attempt to pass the teaching license examination, Mendel experienced a severe breakdown and decided to avoid further attempts at certification.
When Mendel returned to the monastery, he taught science courses at Brno technical school, joined the local agricultural society, and began a series of experiments on pea weevils, pea hybrids, and bees. He also collected information about temperature, humidity, rainfall, barometric pressure, sunspot activity, and unusual storms. Mendel's treatment of complex weather patterns, like his approach to plant hybridization, involved the careful collection and statistical treatment of large amounts of data.
In 1868 Brother Gregor was elected abbot of the monastery. Despite his new responsibilities, he remained involved with the agricultural society, the Brno Horticultural Society, the Society of Apiculturists, and other local science groups. Unfortunately, his successors at the monastery destroyed most of his private papers after his death. Thus, we have very little direct information about Mendel's preliminary ideas, sources, and goals.
The Rediscovery of Mendelian Genetics
Attempts to explain the 40-year neglect of Mendel's work often assume that his writings had been ignored because they were published in a particularly obscure journal, but the Proceedings of the Brno Society for Natural History in which Mendel published his work were actually quite widely distributed. Several botanists included Mendel's work in their bibliographies, but ignored his conclusions. Therefore, modern or “classical” genetics began not with the publication of Mendel's papers in the 1860s, but in 1900 with the rediscovery of his laws of inheritance—segregation and independent assortment of traits—by Dutch botanist Hugo de Vries (1848–1935), German botanist and geneticist Carl Correns (1864–1935), and Austrian Erich von Tschermak-Seysenegg (1871–1962).
As they prepared to publish the results of their own experiments on hybridization and their independent discovery of the laws of inheritance, de Vries, Correns, and von Tschermak were led to Mendel's papers. Although de Vries was distressed by the need to grant priority to Mendel, Correns and von Tschermak generously suggested the use of the terms Mendelism and Mendel's laws.
British biologist William Bateson (1861–1926) also became a leading champion of Mendelian genetics. He introduced many terms now used by geneticists, including genetics (from the Greek for descent), allelomorph (which later became allele), zygote, homozygote, and heterozygote. Bateson's book Mendel's Principles of Heredity: A Defence (1902) included a translation of Mendel's paper and Bateson's assertion that Mendel's laws would prove to be universally valid.
In 1909 the Danish botanist Wilhelm L. Johannsen (1857–1927) introduced the word “gene” to replace older designations like factor, trait, and character, as well as the terms phenotype and genotype, which refer to the appearance of the individual and its actual genetic makeup, respectively. Late-nineteenth century scientists were particularly interested in linking cell theory to theories of evolution and heredity. By the early twentieth century, studies of cell structure and function, embryology, development and differentiation, and advances in histology made it possible for scientists to speculate about the physical nature and subcellular localization of the Mendelian “factors” now known as genes.
Cytology and the Structural Basis of Mendelism
As microscopes improved and cytology—the branch of biology that deals with cell formation, structure, function, and division—grew, scientists needed staining methods that would color certain cellular structures
but not others. Early cytologists used natural dyes—such as indigo, logwood, and carmine—that were used on fabrics, wood, leather, paper, leather, and so forth. In the 1850s German cytologist Joseph von Gerlach (1820–1896), professor of anatomy and physiology at Erlangen and author of the landmark text Handbook of General and Special Histology (1848), introduced one of the most successful of the early histological stains. His preparation of carmine, ammonia, and gelatin, generally called “Gerlach's stain,” helped differentiate the nucleus from other cell components.
Differential staining suggested that specific cell components had different chemical properties, as demonstrated by their ability to combine with different dyes. William Henry Perkin's (1838–1907) creation of aniline dyes in the late nineteenth century expanded microscopists' ability to see structural details in biological preparations. Perkin's discovery of mauve, the first aniline dye, in 1856 was followed by safranin, methyl violet, aniline blue, methyl green, fuchsin, crystal violet, and others. By the 1860s, aniline dyes were being used as biological stains.
By the 1880s microscopists had developed staining methods that made it possible to study the nucleus and chromosomes. Because its various components absorbed stains differently, scientists began to realize that the cell nucleus was very different from the cytoplasm. From the 1870s to the 1890s scientists made considerable progress in describing cellular components and fundamental cytological phenomena such as mitosis, meiosis, and fertilization.
One of the most influential of these researchers was German plant cytologist Eduard Strasburger (1844–1912), professor of botany at Bonn, who helped to unify studies of cytology, inheritance, and development with his monumental book On Cell-Formation and Cell-Division (1875). His studies of plant cells, however, are less well known than German cytogeneticist Walther Flemming's (1843–1905) studies of cell division in animals. Professor of anatomy and director of the Anatomical Institute at the University of Kiel, Flemming described the nuclear substance he called chromatin in the late 1870s. (The term chromosome was first used in 1888 by German anatomist Heinrich W.G. Waldeyer [1836–1921] who introduced hematoxylin as a histological stain). Flemming's 1882 work Cell Substance, Nucleus, and Cell Division established a basic framework for cell division studies.
During the 1880s, two Germans, zoologist Wilhelm Roux (1850–1924), and cytologist Theodor Boveri (1862–1905), and others became particularly interested in chromosomes, which they first saw as special bodies, stained by the new aniline dyes, that appeared within the nucleus during certain phases of cell division.
Based on the cytological studies of Flemming, Strasburger, Roux, Boveri, and others, August Weismann (1834–1914) formulated a new theoretical framework for the behavior of chromosomes and the physical basis of inheritance. In 1892 he published a landmark account of his germ-plasm theory, which states that generations are linked by a continuous line of descent from ancestral germ cells. Equally important, based on cytological studies and his own ideas about the union of the maternal and paternal germ cells, Weismann predicted and explained the necessity of a reduction of the chromosomes during germ-cell formation in order to combine genetic material.
By the end of the nineteenth century, scientists began to suspect a relationship between Mendelian “factors”—the hypothetical entities that underwent segregation and independent assortment—and the behavior of chromosomes during mitosis (cell division) and meiosis (reproductive cell formation). By 1910 studies indicated that chromosome distribution during cell division might explain Mendel's laws. This conclusion
was supported by the work of American geneticist Walter S. Sutton (1877–1916), Boveri, Nettie Stevens (1861–1912), and Edmund B. Wilson (1856–1939).
Thomas Hunt Morgan and the Chromosome Theory of Heredity
The experimental program that most successfully exploited the correlation between breeding data and cytological observation was established by American geneticist Thomas Hunt Morgan (1866–1945) and his associates, who proved that the genes of the fruit fly Drosophila melanogaster are located on the chromosomes in a specific linear sequence. Experiments subsequently demonstrated that apparent deviations from the law of independent assortment were due to linkage, that is, two factors that were carried on the same chromosome. Studies of double mutants revealed another important phenomenon called recombination, which could be explained in terms of crossing over (the reciprocal exchange of genes between chromosomes).
In 1913 Morgan's associate and fellow geneticist Alfred Sturtevant (1891–1970) constructed the first chromosome map. Within two years, Morgan and his coworkers described four groups of linked factors that corresponded to the four pairs of Drosophila chromosomes. Morgan's theory likened the arrangement of genes to beads on a string, instead of a collection of marbles in a bag. In 1926 he published The Theory of the Gene, a summation of developments in genetics since the rediscovery of Mendel's laws.
Hermann Joseph Muller and Induced Mutations
Morgan's laboratory demonstrated the value of mutants for genetic analysis, but the natural rate of mutation was very low. To overcome this obstacle, Hermann Muller (1890–1967) searched for a way to increase it. By 1927 Morgan had successfully demonstrated that ionizing radiation induced hundreds of mutations in fruit flies. After World War II, Muller studied radiation sickness and attempted to determine the rate of spontaneous and induced mutations in humans. When he received the 1946 Nobel Prize for his contributions to genetics, he used the ensuing publicity to campaign against the medical, industrial, and military abuse of radiation.
In lectures, essays, and books, Muller warned that in the absence of natural selection, undesirable genes would inevitably accumulate in the human gene pool. However, he publicly condemned the eugenics movement as a fascist perversion based on pseudoscience and racial prejudice. Eugenics, a term invented by Francis Galton (1822–1911), claimed to be the science of improving the human race. For Muller, the goal of the true science of eugenics was the admittedly distant one of using science to guide biological evolution rather than the immediate goal of purging the world of the unfit. He believed that people with bad genes should voluntarily refrain from reproducing, while those with a good genetic endowment should participate in positive eugenic programs. For example, suitable women should be artificially inseminated with sperm donated by great men.
During the 1930s Muller joined Russian botanist Nikolai Ivanovich Vavilov (1887–1943) in an attempt to combat Lysenkoism, a pseudoscientific dogma that denied the existence of genetic inheritance. At a meeting of the Lenin All-Union Academy of Agricultural Sciences in 1936, Muller compared the choice between Mendelian-Morganist genetics and Lysenkoism to a choice between astronomy and astrology.
Lysenkoism, a neo-Larmackian doctrine that claimed acquired characteristics could be inherited, was promoted by Russian horticulturist Trofim Denisovich Lysenko (1898–1976). He first gained the attention of Russian dictator Joseph Stalin (1879–1953) and the Communist Party in 1926 when he rejected genetics as bourgeois and reactionary and promoted his own neo-Larmackian version of heredity as true “Marxist” biology. In 1935, with Stalin's support, Lysenko denounced his critics as enemies of the state, including Vavilov, who was sent to a prison camp, where he died; others were summarily executed.
Lysenko retained his powerful position as director of the Institute of Genetics of the Soviet Academy of Sciences from 1940 to 1964, making it virtually impossible to teach modern genetics in the Soviet Union.
FRANCIS GALTON (1822–1911)
Victorian polymath Francis Galton (1822–1911), a cousin of English naturalist Charles Darwin (1809–1882), has been called the founder of biometry and eugenics. Galton invented pedigree analysis, twin studies, the correlation coefficient, and fingerprint analysis. He had also been convinced by statistical analyses of biographical encyclopedias that intelligence, character, and talent were the products of heredity. As a result, Galton asserted that the human race could be improved by encouraging the multiplication of superior individuals.
In 1883 Galton coined the word “eugenics,” which he defined as the science of improving the human race by applying the principles of scientific breeding. In the wake of twentieth-century atrocities committed in the name of eugenics, from American laws for the compulsory sterilization of the “feebleminded” to the murderous racist doctrines of the Nazi regime, the term later assumed an irrevocably evil connotation.
Eugenic ideas became particularly popular in the United States. In 1907 the first mandatory sterilization law was passed in Indiana, and American zoologist Charles Davenport (1866–1944) and American eugenicist Harry Laughlin (1880–1943) launched the Eugenics Records Office in New York. The two collected studies of “unfit” families and campaigned for eugenics laws and policies. By 1931 more than half the states had passed sterilization laws aimed at those considered mentally, physically, or morally unfit.
Mandatory sterilization laws were declared constitutional by the 1927 Supreme Court decision known as Buck v. Bell, in which Chief Justice Oliver Wendell Holmes, Jr. (1841–1935), declared that sterilization of those who would otherwise produce generation after generation of “imbeciles” was as justifiable as compulsory vaccination. Roughly 70,000 Americans—mostly poor female welfare recipients—were sterilized under eugenics laws. In the decade that followed, Canada, Denmark, Finland, France, Sweden, Norway, Japan, and Germany all passed sterilization laws. Indeed, the Nazi regime adopted California's 1909 sterilization law as a model.
Eugenics supporters included future Prime Minister Winston Churchill (1874–1965), Irish dramatist George Bernard Shaw (1856–1950), American President Theodore Roosevelt (1858–1919), and founder of the American birth-control movement Margaret Sanger (1879–1966), along with clergy, physicians, scientists, politicians, businessmen, and many ordinary, generally prosperous people. Many religious leaders joined the American Eugenics Society (AES). Although some Catholic clergymen supported eugenic sterilization laws, they criticized AES for its support of birth control. Eugenic societies featured Better Baby and Fitter Family contests at state fairs and other public events to showcase eugenically fit families and encourage them to produce more offspring.
Some modern ethicists warn that genetic engineering and gene therapy techniques could become part of a futuristic wave of eugenic programs. At present, however, the science of detecting genetic flaws cannot be matched by therapeutic methods.
Opposition to Lysenkoism began to grow when Nikita Khrushchev (1894–1971) became premier in 1958. With the fall of Lysenkoism, Vavilov's memory was officially rehabilitated and honored by Russian scientists.
Discovering the Chemical Nature of the Gene
Many early twentieth-century geneticists found it impossible to believe that any single particle or molecule could carry all the properties of life, but if such a molecule did exist, geneticists thought it would have to be a protein. Although scientists now know that deoxyribonucleic acid (DNA) is that genetic particle, until the 1940s it was considered an uninteresting chemical.
Johann Friedrich Miescher and the Discovery of the Nucleic Acids
In some respects, the story of Johann Friedrich Miescher (1844–1895), the Swiss researcher who discovered nucleic acids, is like Mendel's. But Mendel was later immortalized, and Miescher has been all but forgotten. He discovered nucleic acids during his studies of the chemistry of pus cells, which he took from used bandages he collected from the surgical wards of Tübingen, Germany. In the course of his research, Miescher isolated a previously unknown cell component—an organic acid with a high phosphorous content—that he called nuclein. Although at first critics dismissed nuclein as an impure preparation of proteins and phosphate salts, Miescher suggested that its presence in sperm indicated a role in fertilization. By 1900 scientists suspected that chromosomes, which had been studied by cytologists since the 1870s, might be composed of nuclein.
German zoologist Oscar Hertwig (1849–1922) proposed in 1885 that nuclein was responsible for fertilization and the transmission of the hereditary characteristics. Ten years later, Edmund B. Wilson (1856–1939) argued that since both parents contribute equivalent chromosome sets, and breeding tests indicated that the two sexes play an equal role in heredity, the physical basis of heredity must reside in the chromosomes. Since the chromosomes seemed to be made up of chromatin or nuclein, this substance might be associated with the genetic material.
Studying nuclein from thymus glands and yeast, German biochemist and future Nobel laureate Albrecht Kossel (1853–1927) proved that there were two kinds of nucleic acid. He also identified the purines and pyrimidines that are components of nuclein (adenine, guanine, thymine, cytosine, and uracil). Kossel's thymus nucleic acid and yeast nucleic acid are now known as deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Both nucleic acids contain the bases adenine, cytosine, and guanine, but in DNA the fourth base is thymine and in RNA it is uracil. DNA and RNA also differ in their sugar component, deoxyribose and ribose, respectively.
Paradoxically, further chemical studies of the nucleic acids seemed to indicate that DNA could not serve as the hereditary material. Russian biochemist and physician Phoebus Aaron Levene's (1869–1940) studies suggested that the four bases were present in equal amounts in nucleic acids from different sources. Levene's tetranucleotide theory dominated nucleic acid chemistry from the 1930s to the 1940s, before it was challenged by Austrian-born American biochemist Erwin Chargaff (1905–2002), whose tetranucleotide theory described DNA as a highly repetitious polymer that was incapable of generating the diversity essential to the genetic material.
According to Chargaff, his work on DNA was inspired by Canadian-born American bacteriologist Oswald T. Avery's (1877–1955) 1944 paper on the genetic transformation of pneumococcal bacteria. Avery and his coworkers isolated what they called the transforming factor and identified it as DNA. Avery suspected that his discovery had profound implications for genetics, but he could not rule out the possibility that his preparations contained another biologically active substance. If DNA were the transforming factor, prevailing ideas about its chemistry could not be correct.
Chargaff reasoned that DNA from different species must exhibit chemical differences. Its biological activity might depend on specific structural patterns, or sequences, but during the 1940s there were no chemical techniques that could reveal nucleic acids' subtle features. Nevertheless, using methods that first became available after World War II—paper chromatography, ultraviolet spectrophotometry, and ion-exchange chromatography—Chargaff proved that the four bases found in DNA were not necessarily present in equal amounts. Moreover, the base composition of DNA from different organs of any species were characteristic of that species. Although at the time no theory could account for Chargaff's findings about the relationship of adenine to thymine and guanine to cytosine, his base-pairing rules became famous in 1953 when DNA's double-helical structure was discovered.
The American Phage Group and the Secret of the Gene
Although studies of nucleic acids' biochemistry might have led to the DNA double helix, the successful path was the result of collaboration between geneticists and physicists. A paper entitled “On the Nature of Gene Mutation and Gene Structure” (1935) published by German-American biophysicist Max Delbrück (1906–1981) and German geneticist Karl Günter Zimmer (1911–1988) was an early example of this collaborative approach. Austrian theoretical physicist Erwin Schrödinger (1887–1962) elaborated on Delbrück's speculations about the gene molecule in What Is Life? (1944), a book that several eminent molecular biologists later credited with influencing their move from physics to biology.
One of Delbrück's key contributions to the evolution of molecular genetics was the organization of bacteriophage studies at the Cold Spring Harbor Laboratory for Quantitative Biology. According to Delbrück's disciples, this led to the emergence of the American Phage Group in the 1940s under the leadership of Delbrück, Italian-born American biologist Salvador Luria (1912–1991), and American biologist Alfred Hershey (1908–1997).
Unlike Chargaff, Delbrück and Hershey resisted the idea that DNA was the genetic material. This changed when an experiment performed by Hershey and his colleague Martha Cowles Chase (1927–2003) showed in 1952 that DNA was probably the genetic material. Scientists thought that phages might replicate by inserting their genetic material into bacteria and leaving their empty viral coats outside. To test this possibility, Hershey and Chase used radioactive sulfur to label phage proteins and radioactive phosphate to label DNA. After allowing phages to attack bacteria, the infected cultures were spun in a blender and centrifuged to separate bacteria from smaller particles. Most of the phage DNA remained with the bacterial cells while the labeled protein was released into the medium. Members of the Phage Group, including American geneticist James D. Watson (1928–), quickly accepted the concept that DNA was the genetic material.
Although evidence that DNA was the genetic material seemed persuasive, it was impossible to establish a logical relationship between its chemistry and gene behavior. This dilemma was resolved in 1953 when Watson and British biophysicist Francis Crick (1916–2004) proposed a DNA model that immediately suggested explanations for its biological activity.
The Double Helix and Molecular Biology
Despite the elegance of the double-helical model, many scientists felt that the Watson-Crick paper was rather hollow. The x-ray data available in the literature at the time were, as the authors admitted, insufficient for a rigorous test of the hypothetical structure. More exact data appeared in separate papers by Wilkins and Franklin.
It took about five years for experimental tests to prove the essential validity of the DNA replication scheme proposed in 1953. In 1957 American molecular biologist Matthew Meselson (1930–) and American geneticist Franklin W. Stahl (1929–) conducted a series of ingenious experiments that tracked parental DNA, demonstrating that each old DNA strand became associated with a newly synthesized strand.
The Genetic Code
The DNA double helix transformed the chromosome theory into the nucleic acid theory of genetics. According to the Watson and Crick's central dogma of molecular genetics, information flows from base DNA sequences through messenger RNA to the amino acid
sequences in proteins. The transfer of information from DNA to RNA is called transcription; the transfer from RNA to proteins is called translation.
During the 1950s work by Spanish-born biochemist and molecular biologist Severo Ochoa (1905–1993), American biochemist Arthur Kornberg (1918–2007), and others led to the discovery of enzymes that could synthesize nucleic acids in vitro, creating new ways to investigate DNA and RNA. Research by three biochemists, American Robert Holley (1922–1993), Indian-born American Har Gobind Khorana (1922–), and American Marshall Nirenberg (1927–) deciphered the genetic code and its function in protein synthesis. In the 1980s American chemist Thomas R. Cech (1947–) and Canadian-born molecular biologist Sidney Altman (1939–) independently demonstrated that RNA could function as a biocatalyst as well as an information carrier. Discoveries about the role of RNA molecules in the regulation of gene expression continued to emerge in subsequent decades.
Cytoplasmic Inheritance, Extranuclear Genes, and Epigenetics
Since 1900 geneticists have studied the genes in the cell nucleus and assumed that the cytoplasm could be ignored. As early as 1909, however, geneticists reported examples of non-Mendelian inheritance in plants. Green and white variegated patterns seemed to be related to the behavior of the cytoplasmic organelles known as chloroplasts.
IN CONTEXT: THE DNA DOUBLE HELIX
James D. Watson (1928–) and Francis Crick (1916–2004) won many honors for the discovery of DNA's double-helical structure, but their model was based on biochemical work and x-ray crystallographic studies performed by other scientists. In 1951, Watson, an American postdoctoral fellow, met British biophysicist Crick (1916–2004), then a graduate student at Cambridge. Two years later Watson and Crick were world famous. Both attributed their success to their special relationship: their ability to complement, criticize, and stimulate each other.
Watson began undergraduate studies at the University of Chicago when he was only 15. Rejected by Harvard and Caltech, he did his graduate work at Indiana University, a major center of genetic research in the 1940s. Distinguished members of the faculty included Hermann Muller (1890–1967), the Drosophila geneticist, American biologist Tracy Sonneborn (1905–1981), a pioneer in Paramecium genetics, and Italian-born American biologist Salvador Luria (1912–1991), who was working with phage and bacteria. Watson chose to work for Luria because he was associated with German-American biophysicist Max Delbrück (1906–1981), the hero of Austrian theoretical physicist Erwin Schrödinger's influential book (1887–1962)'s What Is Life?
Earning his doctorate at 22, Watson became a postdoctoral fellow at the Cavendish Laboratory where he discovered that Crick shared his enthusiasm for speculating about the secret of the gene.
Francis Crick grew up in Northampton, England, where his father and uncle ran a shoe factory. After receiving a degree in physics from University College, London in 1937, he began research on the viscosity of water under pressure. During World War II Crick joined the Admiralty Research Laboratory, where he worked on mine detection and development. Like other physicists who made the transition to biology, Crick said he was influenced by Schrödinger's What is Life? In 1949 Crick joined Austrian-born British biochemist Max Perutz (1914–2002) and British biochemist John Kendrew (1917–1997) at the Cavendish Laboratory to study protein structure. Although colleagues considered Crick extremely intelligent, he seemed to find it impossible to settle down and complete any research project.
In addition to working with Crick, Watson cultivated other useful relationships, including one with New Zealand—born British biophysicist Maurice Wilkins (1916–2004) at King's College, London. In contrast to Watson and Crick, Wilkins and his colleague British geneticist Rosalind Franklin (1920–1958) were unable to collaborate effectively. Indeed, Wilkins and Franklin were barely on speaking terms.
According to Crick, if he and Watson had not discovered the double helix, the structural puzzle might have been solved by Franklin, Wilkins, American theoretical physical chemist Linus Pauling (1901–1994), or by further refinements of biochemistry. Instead, working together, Watson and Crick discovered DNA's structure by a combination of guessing, model building, and the unacknowledged exploitation of Franklin's crystallographic x-ray data.
Initially Watson and Crick made many incorrect assumptions about the structure of DNA. When they finally put together Chargaff's data on the base ratios, Franklin's studies, and information about the proper forms of the purine and pyrimidine bases, they were able to build a rational, elegant, and biologically interesting model of DNA in about three weeks.
Watson's account of the discovery in his memoir The Double Helix (1968) presents many unflattering caricatures of fellow scientists, but his portrait of Rosalind Franklin was particularly vicious and, according to others who worked with her, totally false. Many readers finish Watson's memoir convinced that the expert x-ray crystallographer he referred to as “Rosy” was merely a technician who refused to take orders from Wilkins or cooperate with Watson because of her extreme feminist views. Watson's book contributed to the view that Franklin was treated unfairly. It also transformed Franklin into a powerful symbol of the difficulties often faced by women in science.
Franklin studied physical chemistry at Cambridge University, where she earned her doctorate in 1945. Because of her expertise in x-ray crystallography, Franklin was asked to join the research group at King's College in 1951. Unfortunately, rather than collaborating, Franklin and Wilkins worked separately and competitively. After leaving King's College in 1953, Franklin conducted further studies of DNA and the structure of viruses. She died of ovarian cancer in 1958, when she was only 38 years old. She did not know that Watson, Crick, and Wilkins would share the 1962 Nobel Prize for DNA, nor did she realize how her unpublished data had been used by Watson and Crick. In 2000 King's College honored Franklin and Wilkins, colleagues who had an extremely uncollegial relationship, by naming the Franklin—Wilkins Building in their honor.
During the 1950s American geneticist Ruth Sager (1918–1997), French geneticist Boris Ephrussi (1901–1979), and others were able to explain many cases of apparent cytoplasmic heredity in terms of mitochondria and chloroplasts, which were later found to contain their own DNA. Studies of mitochondrial DNA led to reports that scientists had “found” the female ancestor of modern human beings. The media immediately labeled this alleged ancestral mother of the human race “Mitochondrial Eve.”
Despite Morgan's proof that genetic inheritance trumped acquired traits, by the 1990s scientists found evidence that environmental factors like diet, stress, and
nutrition might affect the expression of genes. Epidemiological studies concerning the effects of environmental factors on human genes remain ambiguous, as do epigenetic studies of whether environmental factors can change gene function without altering the DNA sequence. One possible explanation is that certain genes might be modified by methylation or other chemical processes. Because methyl groups are derived from foods, maternal nutrition during pregnancy might affect fetal genes. For example, maternal diets low in folic acid seemed to increase the risk of abnormalities in the fetal spinal cord. Studies in laboratory animals might clarify the relationship between environmental factors and gene modification. For example, using obese yellow mice, scientists demonstrated in 2005 that adding supplements to a pregnant mouse's diet altered the gene expression in her offspring.
Demonstrating the relationship between mutant genes and defective enzymes is usually attributed to American geneticist George Beadle (1903–1989) and American biochemist Edward Tatum (1909–1975), who studied the genetic control of biochemical reactions in Neurospora (bread mold). However, in 1902 English physician Archibald Garrod (1857–1936) described the link between Mendelian inheritance and certain human genetic diseases, such as alcaptonuria, albinism, cystinuria, and pentosuria, which he called inborn errors of metabolism.
Studies of sickle cell anemia (SCA) led to the first demonstration of the relationship between a mutant gene and an abnormal protein. By analyzing the abnormal hemoglobin produced by patients with SCA, Linus Pauling proved in 1949 that hemoglobin in patients with SCA had an altered protein structure; seven years later American biochemist Vernon Ingram (1924–2006) discovered that sickle cell anemia occurs when a single amino acid in hemoglobin's protein sequence changes.
Genes for abnormal hemoglobins, which cause such diseases as SCA and thalassemia, appear to offer some protection against malaria, which is found in areas affected by the disease. In modern societies, however, the advantage enjoyed by individuals with one copy of the gene (heterozygotes) is heavily outweighed by the suffering of those with two copies of the gene (homozygotes).
In his autobiography What Mad Pursuit, Crick recalled thinking that all the interesting aspects of molecular genetics had been resolved by 1966, but the discovery of reverse transcriptase, an enzyme that can synthesize DNA from RNA, and the development of recombinant DNA technology in the 1970s began an exciting new phase of genetic research. Two American virologists, Howard Temin (1934–1994) and David Baltimore (1938–), discovered reverse transcriptase in 1970. American biochemist Paul Berg (1926–) created the first recombinant DNA molecules in 1972 when he spliced together DNA from different organisms to form a novel DNA molecule.
In 1973 two American biochemists, Stanley Cohen (1922–) and Herbert W. Boyer (1936–), announced that they had created the first recombinant organism. News of this technique spread rapidly among scientists and journalists. Within months scientists publicly called for discussions of the possible risks involved in creating recombinant DNA. Foreseeing the commercial potential of genetic engineering, Stanford University and the University of California applied for a patent on the recombinant DNA technique developed by Cohen and Boyer in 1974; six years later the U.S. Patent Office granted the first major patent of the new era of biotechnology. In 1976 Boyer, with the help of venture capitalists,
established the Genentech corporation for the commercial exploitation of recombinant DNA techniques.
As the debate about genetic engineering became increasingly volatile, Berg emerged as a leader of the movement to have scientists evaluate the potential hazards of recombinant DNA. A special committee appointed by the National Academy of Sciences published an open letter calling for a voluntary moratorium on recombinant DNA research, pending agreement on research guidelines. In 1975, over 100 molecular biologists met at a landmark conference to discuss risks and guidelines.
Debate about the potential uses and abuses of recombinant DNA, cloning, genetic engineering, and gene therapy exploded in the popular media and on college campuses during the 1970s. But by 2000 experience indicated that its benefits were much greater than the unrealistic risk assessments of the 1970s. Some observers argued that the fear of Frankensteinian biological monsters escaping from biology laboratories had been a product of media hysteria and widespread misunderstanding. The focus of attention shifted from theoretical risks to expectations of cures for cancer and genetic diseases, designer drugs, and unlimited quantities of human gene products such as insulin, growth hormone, clotting factors, and so forth.
Diseases that seemed likely candidates for gene therapy included hemophilia, adenosine deaminase deficiency (ADA), immunodeficiency diseases, and certain cancers. Some critics argued that human gene therapy was totally unacceptable, because of unresolved social, ethical, moral, and evolutionary questions, as well as the immediate risks to participants. Nevertheless, the National Institutes of Health issued thousands of grants to support gene therapy research. More than 150 biotechnology companies were founded between 1990 and 2005 and hundreds of patents for genetic technologies were granted.
Human gene therapy trials, which began in 1990, offered great promise, but 15 years later gene therapy was still an experimental procedure with uncertain benefits and demonstrable risks. In September 1999 Jesse Gelsinger, a 17-year old being treated for ornithine transcarbamylase deficiency, became the first person to die during a gene therapy trial. Shortly after Gelsinger's death an NIH advisory panel proposed more stringent rules and oversight for such treatment. Two deaths occurred in 2002 among participants in a French study involving immune deficiency diseases. Although the media initially hailed this trial as a breakthrough, the death of a third child in 2005 led to the suspension of several major gene therapy trials.
Despite the risks demonstrated by human gene therapy trials, rumors that athletes might be engaging in gene doping—the misuse of gene therapy to boost athletic performance—appeared by 2005. A German track coach was brought to trial in 2005 for allegedly attempting to purchase Repoxygen (a drug in preclinical development as a way to deliver the gene for erythropoietin to muscle cells as a treatment for muscle wasting disease) as a performance-enhancing drug. Officials at the World Anti-Doping Agency expressed concern that athletes might resort to dangerous gene doping products that promise enhanced oxygen transfer and increased muscle mass.
The International Human Genome Project (HGP), an international effort dedicated to mapping and sequencing the entire human genome, was officially launched in 1990. Although the goal seemed overwhelming, new techniques for rapid and automated sequencing emerged in the 1980s. One of the most important was the polymerase chain reaction (PCR) invented by American biochemist Kary Mullis (1944–) in 1983. PCR rapidly and selectively amplifies minute samples of DNA. Automatic DNA sequencers and other instruments for sequencing and synthesizing proteins and genes were developed by American biotechnologist Leroy E. Hood (1938–).
In 1988, James Watson agreed to serve as director of the National Center for Human Genome Research (NCHGR) established by the National Institutes of Health (NIH) to oversee the U.S. contribution to the HGP. In the same year, the international Human Genome Organization (HUGO) was established to coordinate research in various countries. Researchers expected to establish a complete map of the location, function, and exact sequence of each human gene by 2005.
Also in 1988, the consortium of scientists involved in the HGP was challenged by the private company Celera Genomics. Founded by American biologist J. Craig Venter (1946–), Celera announced that it intended to sequence the human genome by 2001. HUGO members were committed to openly publishing all data, but Celera planned to create a private database and sell the information. Somewhat surprisingly, the race between Celera and the publicly financed HGP ended with a fairly amicable announcement of simultaneous success in June 2000, when President William Jefferson Clinton (1946–) hosted leaders of the HGP and Celera at a White House press conference. In February 2001 the HGP sequence data were published in Nature and the Celera sequence data were published in Science.
When the HGP began, scientists thought the gene map would include 100,000 to 150,000 genes. Thus it was a surprise when both HGP and Celera indicated that the actual number was about 30,000 to 40,000. This relatively small number seems to support the theory that the genome is a parts list rather than a blueprint. Although many questions about the number of human genes and the role of remaining sequences remain unanswered, some research indicates that DNA regions initially characterized as “junk” may be involved in the production of RNAs that play a critical role in genetic regulation.
The availability of genomes for more and more species may also change ideas about human evolution. In 2006 scientists reported that comparisons of the genomes of humans, chimps, gorillas, and other primates suggested that the ancestors of humans and chimpanzees might have exchanged genes after the initial split between the species.
With the completion of the Human Genome Project in 2003, scientists used the data to locate, isolate, and clone specific disease genes. Such information may lead to improved diagnostic methods, new drugs, and gene therapy. Information about cancer genes, for example, has allowed individuals at risk to undergo surgical removal of organs in which cancers might occur. But the HGP did not provide the immediate medical and technological revolution that leaders of biotechnology and pharmaceutical companies had promised investors, venture capitalists, and the public. Critics argued that journalists and entrepreneurs generated unrealistic expectations, leading to excessive disappointment.
Modern Cultural Connections
Privacy Issues, DNA Kits, and Genealogy
In the United States, public concerns about various ethical, legal, and social issues led to the passage of the Genetic Privacy Act of 1994, which regulates the collection of DNA samples and the genetic information derived from them. In 1995, the Equal Employment Opportunity Commission published guidelines that extended the protections of the Americans with Disabilities Act to cover discrimination based on genetic information.
While scientists, ethicists, policymakers, and many citizens worry about issues of genetic privacy, some people eagerly share genetic information to fill in their family tree. By 2006, entrepreneurs had discovered that thousands of people were eager to buy DNA kits to help trace their ethnic ancestry. Although colleges, governments, and probate courts are unlikely to accept the results of such tests, people have used them to claim DNA-based ethnicities.
The HGP promoted the development of forensic genomics, which has been used to identify criminals and human remains and to exonerate hundreds of wrongfully convicted persons. The September 11, 2001, terrorist attack on the World Trade Center led to the largest forensic investigation in United States history. A few of the more than 3,000 victims were identified by dental records and finger prints, but in many cases DNA analysis offered the only hope of making an identification.
Databases have been established to record the DNA of criminals, but suggestions that DNA should be collected at birth for a national database raise serious questions about privacy.
Genetically Modified Organisms (GMOs)
Genetic engineering can create nutrient-dense plant varieties that are resistant to pests, parasites, and drought or extreme temperatures. Such plants, their developers claim, produce better yields under adverse conditions, cause less damage to the environment, and provide better nutrition. Nevertheless, by the end of the twentieth century, anxiety about such “frankenfoods” was increasingly prevalent in Europe, the United States, and in some other regions. This anxiety was based partly on research showing that artificially modified genes can be transferred from genetically-modified crops to wild plants, with unknown long-term consequences that, critics of the technology argue, may outweigh any benefits claimed. Critics also argue that genetically modified plants make farmers dependent on the large transnational corporations supplying copyrighted seed, diminish genetic diversity by replacing diverse traditional crops with a small number of centrally-manufactured seed types, and in some cases encourage increased pesticide application.
Defenders of GM foods argue that traditional breeding practices have always modified the genetic makeup of food species, and that genetic engineering is simply a more predictable, precise method of accomplishing the goals that breeders have worked for since agriculture and animal breeding began thousands of years ago. Critics of the technology respond that breeding allows only the step-by-step selection of preexisting, fully-functional systems of genes (most characteristics of plants and animals depend on many genes, not one), whereas genetic engineering is bound by no such constraints.
Many scientists argue that fear of genetically modified organisms is the product of public ignorance. A survey conducted by the Food Policy Institute at Rutgers University in 2004 found that more than 40% of those interviewed thought that ordinary tomatoes do not contain genes, but that GM tomatoes did; 12% thought that eating tomatoes with modified genes would modify their own genes. (However, a majority of those surveyed were correctly informed on both points.) Another survey found that more than half of those polled admitted knowing little or nothing about GM food, but almost 90% were sure that it should not be marketed until proven safe.
Europeans have demonstrated their distrust of GMOs through boycotts and bans on farming or marketing GM crops. Friends of the Earth and other organizations have campaigned throughout the European Union (EU) and via the Internet against the marketing of GMOs. Because of widespread public opposition, in 1998 Britain and the European Commission (EC) banned new genetically engineered crops. The ban was not lifted until 2004, only after the United States, Canada, and Argentina convinced the World Trade Organization that it amounted to unfair protection. European consumers, however, remain hostile to GMOs. France, Austria, and Greece have banned the use of certain GM products, despite the EC's new position. In 2005, Switzerland voted to ban the cultivation of GM crops.
In 2002, during a food shortage, Zambia's president rejected free U.S. GM corn. Among other concerns, Zambia was worried that if GM corn entered the country and transgenes were transferred to local corn crops, Zambia would lose access to European GM-free markets. Proponents of genetic modification condemn such actions as irrational—indeed, grossly unethical—refusals of obvious benefits, in this case famine relief: opponents argue that the long-term harms risked by accepting these and other GM foods outweigh the claimed benefits.
The Future of Genetics
While established genetic engineering and genome sequencing techniques have already found commercial and industrial applications, basic research into previously unanticipated aspects of DNA repair mechanisms, novel RNA molecules, and interactions between transcriptional networks and gene expression continues.
Recent studies of DNA damage and repair mechanisms suggest that certain regions of the genome may be particularly sensitive to damage induced by oxidation, ionizing radiation, and other environmental insults. This may result in mutations known as single nucleotide polymorphisms (SNPs) that are an important cause of genomic diversity. Using yeast DNA, scientists are beginning map the relationship between DNA repair and complex transcription and translation mechanisms, an approach they hope may lead to novel gene therapies.
One unexpected but exciting discovery about RNA was made in the early 1990s in studies of roundworm (Caenorhabditis elegans) larvae. Researchers discovered small RNAs (microRNAs) that bind to specific messenger RNAs and block protein synthesis. Found in many plant and animal species, some 200 human microRNAs have also been identified. RNA interference (RNAi) was also discovered in C. elegans a few years later.
Many pharmaceutical companies have been established to find and market practical applications for RNA interference, which may some day be used to inactivate undesirable genes such as those involved in obesity, schizophrenia, cancers, and viral replication. Understanding the RNA network may also have important implications for research in gene regulation and, eventually, in the battle against diseases. By 2003 scientists had determined that certain genetic diseases, including chronic lymphocytic leukemia, Prader-Willi syndrome, and Fragile X syndrome might be linked to defects in the RNA network.
Since 2000 genetic engineering has been used to alter the genome of various animal species, such as mice and primates, to provide better models for medical research. By inserting genes associated with human diseases, such as Alzheimer's disease, diabetes, sickle cell anemia, and breast cancer into animals, researchers hope to find better ways to fight these diseases.
Geneticists have also called for international cooperative efforts modeled on the successful Human Genome Project to study human cancers (the Human Cancer Genome Project) and patterns of human genetic variability (the International HapMap Project). Advocates of such large-scale projects believe that better knowledge of the range of genetic variability in human populations will identify patterns of variants that contribute to disease.
Both critics and advocates of genetic engineering predict that emerging technologies will be used to redesign humans and transform human evolution. Although a few people may be able to exploit controversial new reproductive techniques, population geneticists generally agree that transforming the gene pool shared by six billion humans is quite a different matter.
See Also Biology: Cell Biology; Biology: Concepts of Heredity and Change Prior to the Rise of Evolutionary Theory; Biology: Developmental Biology; Biology: Evolutionary Theory; Biology: Genetics, DNA, and the Genetic Code; Biology: Genetics: The Human Genome; Biology: Ontogeny and Phylogeny.
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