Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 8
Текст из файла (страница 8)
This branch ofgenetics is known as genetic engineering. Genetic engineering has had an enormous impact ingenetic research, particularly in our ability to understandConnection Balancing ActThomas Hunt Morgan 1913Columbia University, New York,New YorkGenetics and cell biology have both advanced with surprising rapidity in recent years. Hardly a weegoes by without a new discovery of notable importance being reported in the pages of Science or Naturor some other major research journal. Nontechnical accounts of new discoveries are regularly reportethe popular press and on television. We are in the midst of a knowledge explosion—doubtless youremember being told this before. We are so often reminded that we live in a fast-paced world and shoulbe proud to be speeding along.
But hit the brakes, and pause for a moment, to reread the first sentenis an almost direct quotation of the words that Thomas Hunt Morgan wrote to introduce his first boogenetics. This was in 1913. Morgan was one of the pioneers of modern genetics, and genetics in 1913 wapoised for truly spectacular advances. He could scarcely have imagined what modern genetics woullike—how much we would know about some things, how little we would know about others; how powerfuthe methods would be in some ways, how limited they would be in others. Morgan did see one thingclearly. It was that the key to understanding biology is to maintain the right balance among differenof studying organisms—through genetics, cell biology, molecular biology, biochemistry, biophysics,developmental biology, neurobiology, evolutionary biology, and ecology. Maintaining the right balanfor today's students has been our primary goal in writing this book.Two lines of research have developed with surprising rapidity in recent years.
Their development haindependent, but at many stages in their progress they have looked to each other for help. The study ocell has furnished some fundamental facts connected with problems of heredity. The modern study oheredity has proven itself to be an instrument even more subtle in the analysis of the materials of the gercells than actual observations on the germ cells themselves. The time has come, we think, when a fato recognize the close bond between these two modern lines of advance can no longer be interpreted awise or cautious skepticism. An anarchistic spirit in science does not always mean greater profundity, nois our attitude toward science more correct because we are unduly skeptical toward every advance. Tmaintain the right balance is the hardest task we have to meet.
What we most fear is that in attempting tformulate some of the difficult problems of present-day interest we may appear to make at timesunqualified statements in a dogmatic spirit. All conclusions in science are relative and subject to chafor change in science does not mean so much that what has gone before is wrong, as the discovery obetter strategic position than the one last held.Source: Heredity and Sex. NY: Columbia University Press.gene expression and its regulation in plants and animals. Topics previously unapproachable suddenlybecame amenable to experimental investigation.
Currently, genetic engineering is providing us with netools of great economic importance and of value in medical practice. Current projects of great interesinclude the genetic modification of plants and domesticated animals and the production of clinically acsubstances.Beginning in the 1980s came the new emphasis on genomics, the application of recombinant DNAstrategies to the study of whole genomes (the totality of genetic information in an organism) rather thasingle genes. The complete set of DNA instructions has been determined by direct DNA instructions habeen determined by direct DNA sequencing in a number of viruses, cellular organelles such asmitochondria, several bacteria, and the yeast Saccharomyces cerevisiae.
Programs are also underway tdetermine the complete DNA sequence of other model organisms. (In genetics, a model organism isspecies that is studied as an example to learn basic principles that we hope will be applicable to otherorganisms.) Just on the horizon is the capability of determining the complete DNA sequence in the humagenome. The availability of genomic sequences opens up new approaches for genetics because it turnsubject on its head. Instead of starting with a mutant organism that has some physical abnormality,attempting to identify the gene responsible, and determining the DNA sequence, one can now start witDNA sequence that has already been determined and try to learn what the gene does.By far the greatest practical influence of genetics has been in the fields of medicine and agriculture.
Therhave been many important contributions to modern clinical practice, and progress is accelerating becausthe increased emphasis on genomic analysis. Genetic experiments have revealed thousands of new genetimarkers in the human genome and have given us new methods for the detection of mutant genes—noin affected individuals but also in their relatives and in members of the population at large.
These methave given genetic counseling new meaning. Human beings are at risk forPage xxiiiany of several thousand different inherited diseases. Married couples can be informed of thepossibility of their producing an affected offspring and can now make choices between childbearingand adoption. Consider the relief of a woman and man who learn that they do not carry a particulardefective gene and can produce a child without worry.
Even when an offspring might be affectedwith a genetic disorder, techniques are available to determine if a fetus does, in fact, carry a mutantgene.In agriculture, studies of the genetic composition of economically important plants have enabledplant breeders to institute rational programs for developing new varieties. Among the moreimportant plants that have been developed are high-yielding strains of corn and dwarf wheat,disease-resistant rice, corn with an altered and more nutritious amino acid composition (high-lysinecorn), and wheat that grows faster, allowing crops to be grown in short-season regions such asCanada and Sweden.
You will be introduced to the techniques for developing some of these strainsin this book. Often new plant varieties have shortcomings, such as a requirement for increasedamounts of fertilizer or a decreased resistance to certain pests. How to overcome these shortcomingsis a problem for the modern geneticist, who has the job of manipulating the inherited traits. Geneticengineering is also providing new procedures for such manipulations, and quite recently there havebeen dramatic successes.A few words about the book. Each chapter contains two or three Connections set off in specialboxes.
Each connects the material in the text to the real world of genetics outside the classroom.Some of the Connections are excerpts from classic papers, including Mendel's paper. Others arevery recent, such as the paper that reports the cloning of an adult sheep. Some of the Connectionsraise issues of ethics in the application of genetic knowledge, social issues that need to be addressed,issues related to the proper care of laboratory animals, or other matters. We have included aConnection in this Introduction to give you a taste. For an appreciation of genetics in a broadhistorical context related at many points to contemporary research and social and ethical issues, weurge you to connect with the Connections.
There is a complete listing, chapter by chapter, of all theConnections in the Table of Contents. Following the Table of Contents is a complete list of all thematerial excerpted, shown in chronological order.Each chapter comes with a set of Internet Exercises, called GeNETics on the web, which willintroduce you to the genetic resources and information that can be accessed through the Internet.These are important because genetics is more richly represented on the Internet than any other fieldof biology. Each exercise uses a key word in describing an issue or a problem.
The key words aremaintained as hot links at the publisher's web site (http://www.jbpub.com/genetics) and are keptconstantly up to date. Each exercise comes with a short written component that your instructor maywish to assign. We urge you to go through the web exercises even if they are not assigned, as theywill help you to become familiar with some of the extraordinary resources that are out there. Weshould mention two special types of exercises.
One is the mutable site in which the site and theexercise are changed frequently. You can check back on a mutable site that you have exploredbefore, and there will be a good chance that it will have changed in the meantime. The other specialsite is the PIC site, which connects you to a genetics site chosen for its visual appeal.As a pedagogical aid, important terms are printed in boldface in the text.
These terms are collectedat the end of each chapter in a section entitled Key Terms. You should know their meaningsbecause they form the basic vocabulary of genetics. If necessary, you can look them up in theConcise Dictionary of Genetics at the back of the book. Each chapter also includes a Summary atthe end of the text. Sample problems are worked in the section titled Guide to Problem Solving.Each chapter ends with a fairly large collection of problems. These are of three types:Review the Basics problems ask you to restate genetic principles or definitions in your own wordsor to apply elementary principles.Analysis and Applications problems require you to apply several concepts in logical order andusually to do some numerical calculation.
(The calculations use only simple arithmetic, so there isno reason to be intimidated even if higher mathematics is not a comfortable part of your repertoire.)Challenge Problems are similar in nature but a little more difficult because you may need toanalyze some data to solve the problem.It is essential that you work as many of the problems as you can, because experience has shown thatpractice with problems is a good way to learn genetics and to identify particular points or conceptsthat have been misunderstood. Sometimes it is not even necessary to solve a problem completely butonly to read the problem and decide whether you could solve it if asked to do so. The Answers to allof the problems, and full explanations, are given at the back of the book. A problem will be moreuseful to you if you take a fair shot at it before turning to the answer.
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