New Thinking About Genetics

Britannica Educational Publishing, Kara Rogers

• Publisher: Britannica Educational Publishing
• Year: 2010
• ISBN:
  9781615301690
• Topic: Biological Sciences
• Subtopic: Genetics & Genomics
• Index: Biological Sciences

Debates on nature versus nurture rage on and the question of identity looms large for almost everybody at some point in their lives. Thanks to continuing developments in genetics research, some of those questions can be put to bed—or at least, better understood. Moreover, studies of the genome can yield a number of possibilities in improving health and curing disease. This book explores all aspects of this rapidly evolving field by introducing the science of genetics as well as detailing the controversies and implications for future studies.

CHAPTER 1
THE CODE OF LIFE

Since the dawn of civilization, humankind has recognized the influence of heredity and has applied its principles to the improvement of cultivated crops and domestic animals. A Babylonian tablet more than 6,000 years old, for example, shows pedigrees of horses and indicates possible inherited characteristics. Other old carvings show cross-pollination of date palm trees. Most of the mechanisms of heredity, however, remained a mystery until the 19th century, when genetics as a systematic science began.

Genetics arose out of the identification of genes, the fundamental units responsible for heredity. Genetics may be defined as the study of genes at all levels, including the ways in which they act in the cell and the ways in which they are transmitted from parents to offspring. Modern genetics focuses on the chemical substance that genes are made of, called deoxyribonucleic acid, or DNA, and the ways in which it affects the chemical reactions that constitute the living processes within the cell. Gene action depends on interaction with the environment. Green plants, for example, have genes containing the information necessary to synthesize the photosynthetic pigment chlorophyll that gives them their green colour. Chlorophyll is synthesized in an environment containing light because the gene for chlorophyll is expressed only when it interacts with light. If a plant is placed in a dark environment, chlorophyll synthesis stops because the gene is no longer expressed.

Genetics as a scientific discipline stemmed from the work of Gregor Mendel in the middle of the 19th century. Mendel suspected that traits were inherited as discrete units, and, although he knew nothing of the physical or chemical nature of genes at the time, his units became the basis for the development of the present understanding of heredity. All present research in genetics can be traced back to Mendel’s discovery of the laws governing the inheritance of traits. The word gene, coined in 1909 by Danish botanist Wilhelm Johannsen, has given genetics its name.

Genetics forms one of the central pillars of biology and overlaps with many other areas such as agriculture, medicine, and biotechnology.

ANCIENT THEORIES OF PANGENESIS AND BLOOD IN HEREDITY

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Although scientific evidence for patterns of genetic inheritance did not appear until Mendel’s work, history shows that humankind must have been interested in heredity since ancient times. Curiosity must first have been based on human family resemblances, such as similarity in body structure, voice, gait, and gestures. Such notions were instrumental in the establishment of family and royal dynasties. Early nomadic tribes were interested in the qualities of the animals that they herded and domesticated and, undoubtedly, bred selectively. The first human settlements that practiced farming appear to have selected crop plants with favourable qualities. Ancient tomb paintings show racehorse breeding pedigrees containing clear depictions of the inheritance of several distinct physical traits in the horses. Despite this interest, the first recorded speculations on heredity did not exist until the time of the ancient Greeks; some aspects of their ideas are still considered relevant today.

Hippocrates (c. 460–c. 375 BCE), known as the father of medicine, believed in the inheritance of acquired characteristics, and, to account for this, he devised the hypothesis known as pangenesis. He postulated that all organs of the body of a parent gave off invisible “seeds,” which were like miniaturized building components and were transmitted during sexual intercourse, reassembling themselves in the mother’s womb to form a baby.

Aristotle (384–322 BCE) emphasized the importance of blood in heredity. He thought that the blood supplied generative material for building all parts of the adult body, and he reasoned that blood was the basis for passing on this generative power to the next generation. In fact, he believed that the male’s semen was purified blood and that a woman’s menstrual blood was her equivalent of semen. These male and female contributions united in the womb to produce a baby. The blood contained some type of hereditary essences, but he believed that the baby would develop under the influence of these essences, rather than being built from the essences themselves.

Aristotle’s ideas about the role of blood in procreation were probably the origin of the still prevalent notion that somehow the blood is involved in heredity. Today people still speak of certain traits as being “in the blood” and of “blood lines” and “blood ties.” The Greek model of inheritance, in which a teeming multitude of substances was invoked, differed from that of the Mendelian model. Mendel’s idea was that distinct differences between individuals are determined by differences in single yet powerful hereditary factors. These single hereditary factors were identified as genes. Copies of genes are transmitted through sperm and egg and guide the development of the offspring. Genes are also responsible for reproducing the distinct features of both parents that are visible in their children.

PREFORMATION AND NATURAL SELECTION

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In the two millennia between the lives of Aristotle and Mendel, few new ideas were recorded on the nature of heredity. In the 17th and 18th centuries the idea of preformation was introduced. Scientists using the newly developed microscopes imagined that they could see miniature replicas of human beings inside sperm heads. French biologist Jean-Baptiste Lamarck invoked the idea of “the inheritance of acquired characters,” not as an explanation for heredity but as a model for evolution.

Lamarck lived at a time when the fixity of species was taken for granted, yet he maintained that this fixity was found only in a constant environment. He enunciated the law of use and disuse, which states that when certain organs become specially developed as a result of some environmental need, then that state of development is hereditary and can be passed on to progeny. He believed that in this way, over many generations, giraffes could arise from deerlike animals that had to keep stretching their necks to reach high leaves on trees.

British naturalist Alfred Russel Wallace originally postulated the theory of evolution by natural selection. However, Charles Darwin’s observations during his circumnavigation of the globe aboard the HMS Beagle (1831–36) provided evidence for natural selection and his suggestion that humans and animals shared a common ancestry. Many scientists at the time believed in a hereditary mechanism that was a version of the ancient Greek idea of pangenesis, and Darwin’s ideas did not appear to fit with the theory of heredity that sprang from the experiments of Mendel.

THE WORK OF GREGOR MENDEL

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Before Gregor Mendel, theories for a hereditary mechanism were based largely on logic and speculation, not on experimentation. In his monastery garden, Mendel carried out a large number of cross-pollination experiments between variants of the garden pea, which he obtained as pure breeding lines. He crossed peas with yellow seeds to those with green seeds and observed that the progeny seeds (the first generation, F₁) were all yellow. When the F₁ individuals were self-pollinated or crossed among themselves, their progeny (F₂) showed a ratio of 3:1 (3/4 yellow and 1/4 green). He deduced that, since the F₂ generation contained some green individuals, the determinants of greenness must have been present in the F₁ generation, although they were not expressed because yellow is dominant over green.

Gregor Mendel (1822–1884) pioneered the type of scientific experimentation that is used in modern genetics. Here he can be seen working in his laboratory. Authenticated News/Hulton Archive/Getty Images

From the precise mathematical 3:1 ratio (of which he found several other examples), he deduced not only the existence of discrete hereditary units (genes) but also that the units were present in pairs in the pea plant and that the pairs separated during gamete formation (gametes are mature sexual reproductive cells). Hence, the two original lines of pea plants were proposed to be ΥΥ (yellow) and yy (green). The gametes from these were Υ and y, thereby producing an F₁ generation of Υy that were yellow in colour because of the dominance of Υ. In the F₁ generation, half the gametes were Υ and the other half were y, making the F₂ generation produced from random mating 1/4 Υy, 1/2 ΥΥ, and 1/4 yy, thus explaining the 3:1 ratio. The forms of the pea colour genes, Υ and y, are called alleles. An allele is one member of a pair of genes located in a particular place on a particular chromosome. The dominant allele determines hereditary variation in an organism.

Mendel also analyzed pure lines that differed in pairs of characters, such as seed colour (yellow versus green) and seed shape (round versus wrinkled). The cross of yellow round seeds with green wrinkled seeds resulted in an F₁ generation that were all yellow and round, revealing the dominance of the yellow and round traits. However, the F₂ generation produced by self-pollination of F₁ plants showed a ratio of 9:3:3:1 (9/16 yellow round, 3/16 yellow wrinkled, 3/16 green round, and 1/16 green wrinkled; note that a 9:3:3:1 ratio is simply two 3:1 ratios combined). From this result and others like it, he deduced the independent assortment of separate gene pairs at gamete formation.

Mendel’s success can be attributed in part to his classic experimental approach. He chose his experimental organism well and performed many controlled experiments to collect data. From his results, he developed brilliant explanatory hypotheses and went on to test these hypotheses experimentally. Mendel’s methodology established a prototype for genetics that is still used today for gene discovery and understanding the genetic properties of inheritance.

THEORETICAL INTERPRETATION

Cross of a purple-flowered (shown here in gray) and a white-flowered strain of peas. R stands for the gene for purple flowers and r for the gene for white flowers. Encyclopædia Britannica, Inc.

Mendel related his results to the cell theory of fertilization, according to which a new organism is generated from the fusion of two cells. Mendel proposed that in order for a cross-pollinated hybrid to produce pure breeding forms of both the dominant and the recessive type, the hybrid must form germ cells bearing the potential to yield either type. Thus, there had to be some temporary accommodation of the two differing characters in the hybrid as well as a separation process in the formation of the germ cells (in this case, the pollen and egg cells). This has since been described as the law of segregation, or the doctrine of the purity of the germ cells. Since one pollen cell fuses with one egg cell, all possible combinations of the differing pollen and egg cells would yield just the results suggested by Mendel’s theory.

Mendel first presented his results in two separate lectures in 1865 to the Natural Science Society in Brünn. His paper Experiments on Plant Hybrids was published in the society’s journal, Verhandlungen des naturforschenden Vereines in Brünn, the following year. It attracted little attention, although many libraries received it and reprints were sent out. The tendency of those who read it was to conclude that Mendel had simply demonstrated more accurately what was already widely assumed—namely, that hybrid progeny revert to their originating forms. They overlooked the potential for variability and the evolutionary implications that his demonstration of the recombination of traits made possible. Most notably, Swiss botanist Karl Wilhelm von Nägeli actually corresponded with Mendel, despite remaining skeptical as to the significance of his results and doubting that the germ cells in hybrids could be pure.

REDISCOVERY

In 1900, Dutch botanist and geneticist Hugo de Vries, German botanist and geneticist Carl Erich Correns, and Austrian botanist Erich Tschermak von Seysenegg independently reported results of hybridization experiments similar to Mendel’s, though each later claimed not to have known of Mendel’s work while doing their own experiments. However, both de Vries and Correns had read Mendel earlier—Correns even made detailed notes on the subject—but had forgotten. De Vries had a diversity of results in 1899, but it was not until he reread Mendel in 1900 that he was able to select and organize his data into a rational system. Tschermak had not read Mendel before obtaining his results, and his first account of his data offers an interpretation in terms of hereditary potency. He described the 3:1 ratio as an “unequal valency” (Wertigkeit). In subsequent papers he incorporated the Mendelian theory of segregation and the purity of the germ cells into his text.

HOW THE GENE IDEA BECAME REALITY

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In Great Britain, biologist William Bateson became the leading proponent of Mendel’s theory. Around him gathered an enthusiastic band of followers. However, Darwinian evolution was assumed to be based chiefly on the selection of small, blending variations, whereas Mendel worked with clearly nonblending variations. Bateson soon found that championing Mendel aroused opposition from Darwinians. He and his supporters were called Mendelians, and their work was considered irrelevant to evolution. It took some three decades before the Mendelian theory was sufficiently developed to find its rightful place in evolutionary theory.

The distinction between a characteristic and its determinant was not consistently made by Mendel or by his successors, the early Mendelians. In 1909, Danish botanist and geneticist Wilhelm Johannsen clarified this point and named the determinants genes. Four years later, American zoologist and geneticist Thomas Hunt Morgan located the genes on the chromosomes (threadlike strands made of tightly compacted DNA and proteins), and the popular picture of them as beads on a string emerged. This discovery had implications for Mendel’s claim of an independent transmission of traits, for genes close together on the same chromosome are not transmitted independently.

Moreover, as genetic studies pushed the analysis down to smaller and smaller dimensions, the Mendelian gene appeared to fragment. Molecular genetics has thus challenged any attempts to achieve a unified conception of the gene as the elementary unit of heredity. Today the gene is defined in several ways, depending upon the nature of the investigation. Genetic material can be synthesized, manipulated, and hybridized with genetic material from other species, but to fully understand its functions in the whole organism, an understanding of Mendelian inheritance is necessary. As the architect of genetic experimental and statistical analysis, Mendel remains the acknowledged father of genetics.

EARLY MOLECULAR GENETICS

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In 1908, British physician Archibald Garrod proposed the important idea that the human disease alkaptonuria, and certain other hereditary diseases, were caused by inborn errors of metabolism, providing for the first time evidence that linked genes with molecular action at the cell level. Molecular genetics did not begin in earnest until 1941 when American geneticist George Beadle and American biochemist Edward Tatum showed that the genes they were studying in the fungus Neurospora crassa acted by coding for catalytic proteins called enzymes. Subsequent studies in other organisms extended this idea to show that genes generally code for proteins. Soon afterward, American bacteriologist Oswald Avery, Canadian American geneticist Colin M. MacLeod, and American biologist Maclyn McCarty showed that bacterial genes are made of DNA, a finding that was later extended to all organisms.

DNA AND THE GENETIC CODE

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A major landmark was attained in 1953 when American geneticist and biophysicist James D. Watson and British biophysicists Francis Crick and Maurice Wilkins devised a double helix model for DNA structure. In this model, DNA can be likened to a spiraling staircase or a twisting ladder. The DNA double helix consists of two intertwined sugar-pho...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Contents
• Introduction
• Chapter 1: The Code of Life
• Chapter 2: The Physical Basis of Heredity
• Chapter 3: DNA as the Agent of Heredity
• Chapter 4: The Study of Genetics
• Chapter 5: Genetic Engineering
• Chapter 6: Genetics Applied to Plants and Animals
• Chapter 7: Human Genetics
• Chapter 8: Genetic Diseases of Humans
• Glossary
• For Further Reading
• Index