Genetics and Genetic Engineering – Finals/ 8/7/2009 10:58 Page 1 CHAPTER 1 THE HISTORY OF GENETICS Science seldom proceeds in the straightforward logical theories held that information from the parent’s heart, lungs, manner imagined by outsiders. and limbs was transmitted directly from these body parts to —James D. Watson, The Double Helix: A Personal Account of the create the offspring’s heart, lungs, and limbs. Discovery of the Structure of DNA (1968) Genetics is the biology of heredity, and geneticists are Particulate theories were attempts to explain observed the scientists and researchers who study hereditary pro- similarities between parents and their children. One rea- cesses such as the inheritance of traits, distinctive charac- son these theories were inaccurate was that they relied teristics, and diseases. Genetics considers the biochemical on observations unaided by the microscope. Microscopy instructions that convey information from generation to (the use of or investigation with the microscope) and the generation. recognition of cells and microorganisms did not occur until the end of the seventeenth century, when the British Tremendous strides in science and technology have naturalist Robert Hooke (1635–1703) first observed cells enabled geneticists to demonstrate that some genetic varia- through a microscope. tion is related to disease, and that the ability to vary genes improves the capacity of a species to survive changes in Until that time (and even for some time after) heredity the environment. Even though some of the most important remained poorly understood. During the Renaissance (from advances in genetics research—such as deciphering the about the fourteenth to the sixteenth centuries), preforma- genetic code, isolating the genes that cause or predict tionist theories proposed that the parent’s body carried susceptibility to certain diseases, and successfully clon- highly specialized reproductive cells that contained whole, ing plants and animals—have occurred since the mid- preformed offspring. Preformationist theories insisted that twentieth century, the history of genetics study spans a when these specialized cells containing the offspring were period of about 150 years. As the understanding of genetics placed in suitable environments, they would spontaneously progressed, scientific research became increasingly more grow into new organisms with traits similar to the parent specific. Genetics first considered populations, then indi- organism. viduals, then it advanced to explore the nature of inheri- The Greek philosopher Aristotle (384–322 BC), who tance at the molecular level. was such a keen observer of life that he is often referred to as the father of biology, noted that individuals sometimes EARLY BELIEFS ABOUT HEREDITY resemble remote ancestors more closely than their immedi- From the earliest recorded history, ancient civilizations ate parents. He was a preformationist, positing that the male observed patterns in reproduction. Animals bore offspring parent provided the miniature individual and the female of the same species, children resembled their parents, and provided the supportive environment in which it would plants gave rise to similar plants. Some of the earliest ideas grow. He also refuted the notion of a simple, direct transfer about reproduction, heredity, and the transmission of infor- of body parts from parent to offspring by observing that mation from parent to child were the particulate theories animals and humans who had suffered mutilation or loss of developed in ancient Greece during the fourth century BC. body parts did not confer these losses to their offspring. These theories posited that information from each part of the Instead, he described a process that he called epigenesist,in parent had to be communicated to create the corresponding which the offspring is gradually generated from an undif- body part in the offspring. For example, the particulate ferentiated mass by the addition of parts. Genetics and Genetic Engineering 1 Genetics and Genetic Engineering – Finals/ 8/7/2009 10:58 Page 2 Of Aristotle’s many contributions to biology, one of ories of biogenesis when he posited in 1858 that cells repro- the most important was his conclusion that inheritance duce themselves. involved the potential of producing certain characteristics Improvements in microscopy and the increasing study rather than the absolute production of the characteristics of cytology (the formation, structure, and function of cells) themselves. This thinking was closer to the scientific reality enabled scientists to identify parts of the cell. Key cell of inheritance than any philosophy set forth by his predeces- components include the nucleus, which directs all cellular sors. However, because Aristotle was developing his theories activities by controlling the synthesis of proteins, and the before the advent of microscopy, he mistakenly presumed mitochondria, which are organelles (membrane-bound cell that inheritance was conveyed via the blood. Regardless, his compartments) that catalyze reactions that produce energy enduring influence is evident in the language and thinking for the cell. Figure 1.1 is a diagram of a typical animal cell about heredity. Even though blood is not the mode of trans- that shows its component parts, including the contents of mission of heredity, people still refer to ‘‘blood relatives,’’ the nucleus, where chromosomes (which contain the genes) ‘‘blood lines,’’ and offspring as products of their own ‘‘flesh are located. and blood.’’ Germplasm Theory of Heredity One of the most important developments in the study of Studies of cellular components, processes, and functions hereditary processes came in 1858, when the British natural- produced insights that revealed the connection between ists Charles Darwin (1809–1882) and Alfred Russel Wallace cytology and inheritance. The German biologist August (1823–1913) announced the theory of natural selection—the Weissmann (1834–1914) studied medicine, biology, and idea that members of a population who are better adapted to zoology, and his contribution to genetics was an evolutionary their environment will be the ones most likely to survive theory known as the germplasm theory of heredity. Building and pass their traits on to the next generation. Darwin pub- on Darwin’s idea that specific inherited characteristics are lished his theories in On the Origin of Species by Means of passed from one generation to the next, Weissmann asserted Natural Selection (1859). His work was not viewed favor- that the genetic code for each organism was contained in its ably, especially by religious leaders who believed it refuted germ cells (the cells that create sperm and eggs). The pres- the biblical interpretation of how life on Earth began. Even ence of genetic information in the germ cells explained how in the twenty-first century the idea that life evolves gradually this information was conveyed, unchanged from one gener- through natural processes is not accepted by everyone, and ationtothenext. the dispute over creationism and evolution continues. In a series of essays about heredity published between 1889 and 1892, Weissmann observed that the amount of CELL THEORY genetic material did not double when cells replicated, sug- gesting that there was some form of biological control of In 1665, when Hooke used the microscope he had the chromosomes that occurred during the formation of the designed to examine a piece of cork, he saw a honeycomb gametes (sperm and egg). His theory was essentially cor- pattern of rectangles that reminded him of cells, the cham- rect. Normal body growth is attributable to cell division, bers of monks in monasteries. His observations prompted called mitosis, which produces cells that are genetically scientists to speculate that living tissue as well as nonliving identical to the parent cells. The way to avoid giving off- tissue was composed of cells. The French scientist Rene´ spring a double dose of heredity information is through a Dutrochet (1776–1847) performed microscopic studies cell division that reduces the amount of the genetic material and concluded in 1824 that both plant and animal tissue in the gametes by one-half. Weissmann called this process was composed of cells. reduction division; it is now known as meiosis. In 1838 the German botanist Matthias Jakob Schleiden Weissmann was also the first scientist to successfully (1804–1881) presented his theory that all plants were con- refute the members of the scientific community who believed structed of cells. The following year the German cytologist that physical characteristics acquired through environmental Theodor Schwann (1810–1882) suggested that animals were exposure were passed from generation to generation. He also composed of cells. Both Schleiden and Schwann theor- conducted experiments in which he cut the tails off several ized that cells were all created using the same process. Even consecutive generations of mice and observed that none of though Schleiden’s hypotheses about the process of cell for- their offspring were born tailless. mation were not entirely accurate, both he and Schwann are credited with developing cell theory. Describing cells as the A FARMER’S SON BECOMES THE basic units of life, they asserted that all living things are FATHER OF GENETICS composed of cells, the simplest forms of life that can exist Gregor Mendel (1822–1884) was born into a peasant independently. Their pioneering work enabled other scientists family in what is