《现代遗传学》课程教学资源（参考书籍）【美】D. Peter Snustad、Michael J. Simmons《Principles of Genetics（SIXTH EDITION）》

8 Chapter 1 The Science of Genetics Gene HBB Human Chomosome 11 a888aag8月88g 日e因88日- DNA GGsoGGGc 9AAA7 Transcription mRNA &月6H6 c h u G G u G&eeUe 日g8UUo日8UBg- Untranslated Translation Triplet codons specifying amino acids Translation region start codon stop codon STEo 2 Translation Amino acids Polypeptide Lys Tyr His (Termination) Met) 3 Removal of terminal methionine (Met) (Lys 144 145 146 Human B-globin polypeptide FIGURE 1.7 Expression of the human gene HBB coding for the B-globin polypeptide of hemoglobin.During transcription (step 1),one strand of the HBB DNA (here the bottom strand shown highlighted)serves as a template for the synthesis of a complementary strand of RNA.After undergoing modifications,the result- ing mRNA(messenger RNA]is used as a template to synthesize the B-globin polypeptide.This process is called translation (step 2).During translation each triplet codon in the mRNA specifies the incorporation of an amino acid in the polypeptide chain.Translation is initiated by a start codon,which specifies the incorporation of the amino acid methionine [met),and it is terminated by a stop codon,which does not specify the incorpo- ration of any amino acid.After translation is completed,the initial methionine is removed (step 3]to produce the mature B-globin polypeptide. in their proteome.One reason for the large size of the human proteome is that a particular gene may encode several different,but related,polypeptides,and these polypeptides may combine in complex ways to produce different proteins.Another reason is that proteins may be produced by combining polypeptides encoded by different genes.If the number of genes in the human genome is large,the number of proteins in the human proteome is truly enormous. The study of all the proteins in cells-their composition,the sequences of amino acids in their constituent polypeptides,the interactions among these polypeptides and among different proteins,and,of course,the functions of these complex molecules-is called proteomics.Like genomics,proteomics has been made possible by advances in the technologies used to study genes and gene products,and by the development of computer programs to search databases and analyze amino acid sequences. From all these considerations,it is clear that information flows from genes,which are composed of DNA,to polypeptides,which are composed of amino acids,through

8 Chapter 1 The Science of Genetics in their proteome. One reason for the large size of the human proteome is that a particular gene may encode several different, but related, polypeptides, and these polypeptides may combine in complex ways to produce different proteins. Another reason is that proteins may be produced by combining polypeptides encoded by different genes. If the number of genes in the human genome is large, the number of proteins in the human proteome is truly enormous. The study of all the proteins in cells—their composition, the sequences of amino acids in their constituent polypeptides, the interactions among these polypeptides and among different proteins, and, of course, the functions of these complex molecules—is called proteomics. Like genomics, proteomics has been made possible by advances in the technologies used to study genes and gene products, and by the development of computer programs to search databases and analyze amino acid sequences. From all these considerations, it is clear that information fl ows from genes, which are composed of DNA, to polypeptides, which are composed of amino acids, through T A T A C G C G T A C G C G C G A T A T A G T C G C A G T C G C G C G C T A T C A G T A A T A T A T A T A T A T G C G C G AA A C C C U G G U G U C CC A G AA G U AA A UU C C A Human Chomosome 11 Gene HBB Untranslated region Met Met Val His Leu Val His Leu Lys Tyr His Translation start codon Translation stop codon (Termination) Triplet codons specifying amino acids Human β-globin polypeptide DNA mRNA Polypeptide 1 STEP 2 STEP 3 STEP 1 2 3 144 145 146 Transcription Translation Removal of terminal methionine (Met) Amino acids Lys Tyr His FIGURE 1.7 Expression of the human gene HBB coding for the -globin polypeptide of hemoglobin. During transcription (step 1), one strand of the HBB DNA (here the bottom strand shown highlighted) serves as a template for the synthesis of a complementary strand of RNA. After undergoing modifications, the result￾ing mRNA (messenger RNA) is used as a template to synthesize the -globin polypeptide. This process is called translation (step 2). During translation each triplet codon in the mRNA specifies the incorporation of an amino acid in the polypeptide chain. Translation is initiated by a start codon, which specifies the incorporation of the amino acid methionine (met), and it is terminated by a stop codon, which does not specify the incorpo￾ration of any amino acid. After translation is completed, the initial methionine is removed (step 3) to produce the mature -globin polypeptide.

DNA as the Genetic Material 9 an intermediate,which is composed of RNA(Figure 1.8).Thus, Gene Transcript Polypeptide in the broad sense,the flow of information is DNA>RNA> (DNA) (RNA) (amino acids) polypeptide,a progression often spoken of as the central dogma of mo- ranscription Translation (AA lecular biology.In several chapters we will see circumstances in which the first part of this progression is reversed-that is,RNA is used as a template for the synthesis of DNA.This process,called reverse tran- Replication Reverse transcription scription,plays an important role in the activities of certain types of viruses,including the virus that causes acquired immune deficiency FIGURE 1.8 The central dogma of molecular syndrome,or AIDS;it also profoundly affects the content and structure of the genomes biology showing how genetic information is of many organisms,including the human genome.We will examine the impact of reverse propagated (through DNA replication]and transcription on genomes in Chapter 17. expressed (through transcription and transla- It was once thought that all or nearly all genes encode polypeptides.However, tion).In reverse transcription,RNA is used as a recent research has shown this idea to be incorrect.Many genes do not encode poly- template for the synthesis of DNA. peptides;instead,their end products are RNA molecules that play important roles with- in cells.We will explore these RNAs and the genes that produce them in Chapters 11 and 19. MUTATION:CHANGING GENETIC INFORMATION DNA replication is an extraordinarily accurate process,but it is not perfect. Normal Mutant sickle-cell At a low but measurable frequency,nucleotides are incorporated incorrectly B-globin gene B-globin gene into growing DNA chains.Such changes have the potential to alter or dis- HBBA Mutation HBBS rupt the information encoded in genes.DNA molecules are also sometimes damaged by electromagnetic radiation or by chemicals.Although the damage induced by these agents may be repaired,the repair processes often leave scars.Stretches of nucleotides may be deleted or duplicated. or they may be rearranged within the overall structure of the DNA DNA molecule.We call all these types of changes mutations.Genes that are altered by the occurrence of mutations are called mutant genes. Often mutant genes cause different traits in organisms 1 Transcrption Figure 1.9).For example,one of the genes in the human genome encodes the polypeptide known as B-globin.This polypeptide,146 mRNA c uc amino acids long,is a constituent of hemoglobin,the protein that TEA transports oxygen in the blood.The 146 amino acids in B-globin ②Translation correspond to 146 codons in the B-globin gene.The sixth of these Polypeptide Glutamic acid- -Vafne- codons specifies the incorporation of glutamic acid into the poly- peptide.Countless generations ago,in the germ line of some name- less individual,the middle nucleotide pair in this codon was changed from A:T to T:A,and the resulting mutation was passed on to the individual's descendants.This mutation,now widespread in some human populations,altered the sixth codon so that it specifies the Normal, Mutant, incorporation of valine into the B-globin polypeptide.This seem- disc-shaped sickle-shaped ingly insignificant change has a deleterious effect on the structure red blood cells red blood cells of the cells that make and store hemoglobin-the red blood cells. People who carry two copies of the mutant version of the B-globin 24m 2 um gene have sickle-shaped red blood cells,whereas people who carry Normal transport Sicklecell two copies of the nonmutant version of this gene have disc-shaped of oxygen disease red blood cells.The sickle-shaped cells do not transport oxygen ef- ficiently through the body.Consequently,people with sickle-shaped FIGURE 1.9 The nature and consequence of a mutation in the red blood cells develop a serious disease,so serious in fact that they gene for human B-globin.The mutant gene(HBBs top right)respon- may eventually die from it.This sickle-cell disease is therefore sible for sickle-cell disease resulted from a single base-pair substi- traceable to a mutation in the B-globin gene.We will investigate tution in the B-globin gene (HBB4 top left).Transcription and trans- the nature and causes of mutations like this one in Chapter 13. lation of the mutant gene produce a B-globin polypeptide containing the amino acid valine (center right)at the position where normal The process of mutation has another aspect-it introduces B-globin contains glutamic acid (center left].This single amino acid variability into the genetic material of organisms.Over time,the change results in the formation of sickle-shaped red blood cells mutant genes created by mutation may spread through a popula- [bottom right)rather than the normal disc-shaped cells (bottom tion.For example,you might wonder why the mutant B-globin left).The sickle-shaped cells cause a severe form of anemia

DNA as the Genetic Material 9 an intermediate, which is composed of RNA ( Figure 1.8). Thus, in the broad sense, the fl ow of information is DNA → RNA → polypeptide, a progression often spoken of as the central dogma of mo￾lecular biology. In several chapters we will see circumstances in which the fi rst part of this progression is reversed—that is, RNA is used as a template for the synthesis of DNA. This process, called reverse tran￾scription, plays an important role in the activities of certain types of viruses, including the virus that causes acquired immune defi ciency syndrome, or AIDS; it also profoundly affects the content and structure of the genomes of many organisms, including the human genome. We will examine the impact of reverse transcription on genomes in Chapter 17. It was once thought that all or nearly all genes encode polypeptides. However, recent research has shown this idea to be incorrect. Many genes do not encode poly￾peptides; instead, their end products are RNA molecules that play important roles with￾in cells. We will explore these RNAs and the genes that produce them in Chapters 11 and 19. MUTATION: CHANGING GENETIC INFORMATION DNA replication is an extraordinarily accurate process, but it is not perfect. At a low but measurable frequency, nucleotides are incorporated incorrectly into growing DNA chains. Such changes have the potential to alter or dis￾rupt the information encoded in genes. DNA molecules are also sometimes damaged by electromagnetic radiation or by chemicals. Although the damage induced by these agents may be repaired, the repair processes often leave scars. Stretches of nucleotides may be deleted or duplicated, or they may be rearranged within the overall structure of the DNA molecule. We call all these types of changes mutations. Genes that are altered by the occurrence of mutations are called mutant genes. Often mutant genes cause different traits in organisms ( Figure 1.9). For example, one of the genes in the human genome encodes the polypeptide known as -globin. This polypeptide, 146 amino acids long, is a constituent of hemoglobin, the protein that transports oxygen in the blood. The 146 amino acids in -globin correspond to 146 codons in the -globin gene. The sixth of these codons specifi es the incorporation of glutamic acid into the poly￾peptide. Countless generations ago, in the germ line of some name￾less individual, the middle nucleotide pair in this codon was changed from A:T to T:A, and the resulting mutation was passed on to the individual’s descendants. This mutation, now widespread in some human populations, altered the sixth codon so that it specifi es the incorporation of valine into the -globin polypeptide. This seem￾ingly insignifi cant change has a deleterious effect on the structure of the cells that make and store hemoglobin—the red blood cells. People who carry two copies of the mutant version of the -globin gene have sickle-shaped red blood cells, whereas people who carry two copies of the nonmutant version of this gene have disc-shaped red blood cells. The sickle-shaped cells do not transport oxygen ef- fi ciently through the body. Consequently, people with sickle-shaped red blood cells develop a serious disease, so serious in fact that they may eventually die from it. This sickle-cell disease is therefore traceable to a mutation in the -globin gene. We will investigate the nature and causes of mutations like this one in Chapter 13. The process of mutation has another aspect—it introduces variability into the genetic material of organisms. Over time, the mutant genes created by mutation may spread through a popula￾tion. For example, you might wonder why the mutant -globin FIGURE 1.8 The central dogma of molecular biology showing how genetic information is propagated (through DNA replication) and expressed (through transcription and transla￾tion). In reverse transcription, RNA is used as a template for the synthesis of DNA. FIGURE 1.9 The nature and consequence of a mutation in the gene for human -globin. The mutant gene (HBBS top right) respon￾sible for sickle-cell disease resulted from a single base-pair substi￾tution in the -globin gene (HBBA top left). Transcription and trans￾lation of the mutant gene produce a -globin polypeptide containing the amino acid valine (center right) at the position where normal -globin contains glutamic acid (center left). This single amino acid change results in the formation of sickle-shaped red blood cells (bottom right) rather than the normal disc-shaped cells (bottom left). The sickle-shaped cells cause a severe form of anemia. Normal β –globin gene Mutant sickle-cell –globin gene Mutation Transcription DNA mRNA Polypeptide — Glutamic acid — — Valine — Normal transport of oxygen Sickle-cell disease Mutant, sickle-shaped red blood cells Normal, disc-shaped red blood cells β Translation HBBA HBBS C G C A G T C G C T G G G A G G A U 1 STEP 2 STEP 2 μm 2 μm Gene (DNA) Transcript (RNA) Transcription Reverse transcription Translation Polypeptide (amino acids) AA1 AA2 AA3 Replication

10 Chapter 1 The Science of Genetics gene is relatively common in some human populations.It turns out that people who carry both a mutant and a nonmutant allele of this gene are less susceptible to infection by the blood parasite that causes malaria.These people therefore have a better chance of surviving in environments where malaria is a threat.Because of this enhanced sur- vival,they produce more children than other people,and the mutant allele that they carry can spread.This example shows how the genetic makeup of a population-in this case,the human population-can evolve over time. KEY POINTSWben DNA replicates,each strand of a duplex molecule serves as the template for the syntbesis of a complementary strand. Wben genetic information is expressed,one strand of a gene's DNA duplex is used as a template for the syntbesis of a complementary strand of RNA. For most genes,RNA syntbesis (transcription)generates a molecule (the RNA transcript)that becomes a messenger RNA (mRNA). Coded information in an mRNA is translated into a sequence of amino acids in a polypeptide. Mutations can alter the DNA sequence of a gene. The genetic variability created by mutation is the basis for biological evolution. Genetics and Evolution Genetics has much to contribute to the scientific As mutations accumulate in the DNA over many generations,we study of evolution. see their effects as differences among organisms.Mendel's strains of peas carried different mutant genes,and so do people from dif- ferent ancestral groups.In almost any species,at least some of the observable variation has an underlying genetic basis.In the middle of the nineteenth century,Charles Darwin and Alfred Wallace,both contemporaries of Mendel,proposed that this variation makes it Finback whale possible for species to change-that is,to evolve-over time. The ideas of Darwin and Wallace revolutionized scientific Blue whale thought.They introduced an historical perspective into biology Cow and gave credence to the concept that all living things are related Rat by virtue of descent from a common ancestor.However,when Mouse these ideas were proposed,Mendel's work on heredity was still in progress and the science of genetics had not yet been born.Research Opossum on biological evolution was stimulated when Mendel's discoveries Chicken came to light at the beginning of the twentieth century,and it took a Toad new turn when DNA sequencing techniques emerged at the century's end.With DNA sequencing we can see similarities and differences Trout in the genetic material of diverse organisms.On the assumption that Loach sequences of nucleotides in the DNA are the result of historical pro- Carp cesses,it is possible to interpret these similarities and differences in a temporal framework.Organisms with very similar DNA sequences FIGURE 1.10 Phylogenetic tree showing the evolutionary rela- are descended from a recent common ancestor,whereas organisms tionships among 11 different vertebrates.This tree was constructed with less similar DNA sequences are descended from a more remote by comparing the sequences of the gene for cytochrome b.a common ancestor.Using this logic,researchers can establish the his- protein involved in energy metabolism.The 11 different animals torical relationships among organisms Figure 1.10).We call these have been positioned in the tree according to the similarity of their cytochrome b gene sequences.This tree is consistent with relationships a phylogenetic tree,or more simply,a phylogeny,from other information (e.g.,data obtained from the study of fossils). Greek words meaning"the origin of tribes." except for the positions of the three fish species.The loach is ac- Today the construction of phylogenetic trees is an important tually more closely related to the carp than it is to the trout.This part of the study of evolution.Biologists use the burgeoning DNA discrepancy points out the need to interpret the results of DNA sequence data from the genome projects and other research ven- sequence comparisons carefully. tures,such as the United States National Science Foundation's "Tree

10 Chapter 1 The Science of Genetics gene is relatively common in some human populations. It turns out that people who carry both a mutant and a nonmutant allele of this gene are less susceptible to infection by the blood parasite that causes malaria. These people therefore have a better chance of surviving in environments where malaria is a threat. Because of this enhanced sur￾vival, they produce more children than other people, and the mutant allele that they carry can spread. This example shows how the genetic makeup of a population—in this case, the human population—can evolve over time. KEY POINTS  When DNA replicates, each strand of a duplex molecule serves as the template for the synthesis of a complementary strand.  When genetic information is expressed, one strand of a gene’s DNA duplex is used as a template for the synthesis of a complementary strand of RNA.  For most genes, RNA synthesis (transcription) generates a molecule (the RNA transcript) that becomes a messenger RNA (mRNA).  Coded information in an mRNA is translated into a sequence of amino acids in a polypeptide.  Mutations can alter the DNA sequence of a gene.  The genetic variability created by mutation is the basis for biological evolution. Genetics has much to contribute to the scientific study of evolution. Genetics and Evolution As mutations accumulate in the DNA over many generations, we see their effects as differences among organisms. Mendel’s strains of peas carried different mutant genes, and so do people from dif￾ferent ancestral groups. In almost any species, at least some of the observable variation has an underlying genetic basis. In the middle of the nineteenth century, Charles Darwin and Alfred Wallace, both contemporaries of Mendel, proposed that this variation makes it possible for species to change—that is, to evolve—over time. The ideas of Darwin and Wallace revolutionized scientifi c thought. They introduced an historical perspective into biology and gave credence to the concept that all living things are related by virtue of descent from a common ancestor. However, when these ideas were proposed, Mendel’s work on heredity was still in progress and the science of genetics had not yet been born. Research on biological evolution was stimulated when Mendel’s discoveries came to light at the beginning of the twentieth century, and it took a new turn when DNA sequencing techniques emerged at the century’s end. With DNA sequencing we can see similarities and differences in the genetic material of diverse organisms. On the assumption that sequences of nucleotides in the DNA are the result of historical pro￾cesses, it is possible to interpret these similarities and differences in a temporal framework. Organisms with very similar DNA sequences are descended from a recent common ancestor, whereas organisms with less similar DNA sequences are descended from a more remote common ancestor. Using this logic, researchers can establish the his￾torical relationships among organisms ( Figure 1.10). We call these relationships a phylogenetic tree, or more simply, a phylogeny, from Greek words meaning “the origin of tribes.” Today the construction of phylogenetic trees is an important part of the study of evolution. Biologists use the burgeoning DNA sequence data from the genome projects and other research ven￾tures, such as the United States National Science Foundation’s “Tree Finback whale Cow Rat Mouse Opossum Chicken Toad Trout Loach Carp Blue whale FIGURE 1.10 Phylogenetic tree showing the evolutionary rela￾tionships among 11 different vertebrates. This tree was constructed by comparing the sequences of the gene for cytochrome b, a protein involved in energy metabolism. The 11 different animals have been positioned in the tree according to the similarity of their cytochrome b gene sequences. This tree is consistent with other information (e.g., data obtained from the study of fossils), except for the positions of the three fish species. The loach is ac￾tually more closely related to the carp than it is to the trout. This discrepancy points out the need to interpret the results of DNA sequence comparisons carefully.

Levels of Genetic Analysis 11 of Life"program,in combination with anatomical data collected from living and fos- silized organisms to discern the evolutionary relationships among species.We will explore the genetic basis of evolution in Chapters 23 and 24. Evolution depends on the occurrence,transmission,and spread of mutant genes in groups KEY POINTS of organisms. DNA sequence data provide a way of studying the bistorical process ofevolution. Levels of Genetic Analysis Genetic analysis is practiced at different levels.The Geneticists approach their science from different points oldest type of genetic analysis follows in Mendel's of view-from that of a gene,a DNA molecule,or a footsteps by focusing on how traits are inherited when different strains of organisms are hybridized.Another population of organisms. type of genetic analysis follows in the footsteps of Wat- son and Crick and the army of people who have worked on the various genome projects by focusing on the molecular makeup of the genetic material.Still another type of genetic analysis imitates Darwin and Wallace by focusing on entire populations of organisms.All these levels of genetic analysis are routinely used in research today. Although we will encounter them in many different places in this book,we provide brief descriptions of them here. CLASSICAL GENETICS The period prior to the discovery of the structure of DNA is often spoken of as the era of classical genetics.During this time,geneticists pursued their science by analyzing the outcomes of crosses between different strains of organisms,much as Mendel had done in his work with peas.In this type of analysis,genes are identified by studying the in- heritance of trait differences-tall pea plants versus short pea plants,for example-in the offspring of crosses.The trait differences are due to the alternate forms of genes. Sometimes more than one gene influences a trait,and sometimes environmental conditions-for example,temperature and nutrition-exert an effect.These compli- cations can make the analysis of inheritance difficult. The classical approach to the study of genes can also be coordinated with studies on the structure and behavior of chromosomes,which are the cellular entities that contain the genes.By analyzing patterns of inheritance,geneticists can localize genes to specific chromosomes.More detailed analyses allow them to localize genes to spe- cific positions within chromosomes-a practice called chromosome mapping.Because these studies emphasize the transmission of genes and chromosomes from one genera- tion to the next,they are often referred to as exercises in transmission genetics.However, classical genetics is not limited to the analysis of gene and chromosome transmission. It also studies the nature of the genetic material-how it controls traits and how it mutates.We present the essential features of classical genetics in Chapters 3-8. MOLECULAR GENETICS With the discovery of the structure of DNA,genetics entered a new phase.The repli- cation,expression,and mutation of genes could now be studied at the molecular level This approach to genetic analysis was raised to a new level when it became possible to sequence DNA molecules easily.Molecular genetic analysis is rooted in the study of DNA sequences.Knowledge of a DNA sequence and comparisons to other DNA se- quences allow a geneticist to define a gene chemically.The gene's internal components- coding sequences,regulatory sequences,and noncoding sequences-can be identified, and the nature of the polypeptide encoded by the gene can be predicted

Levels of Genetic Analysis 11 of Life” program, in combination with anatomical data collected from living and fos￾silized organisms to discern the evolutionary relationships among species. We will explore the genetic basis of evolution in Chapters 23 and 24.  Evolution depends on the occurrence, transmission, and spread of mutant genes in groups of organisms.  DNA sequence data provide a way of studying the historical process of evolution. KEY POINTS Geneticists approach their science from different points of view—from that of a gene, a DNA molecule, or a population of organisms. Levels of Genetic Analysis Genetic analysis is practiced at different levels. The oldest type of genetic analysis follows in Mendel’s footsteps by focusing on how traits are inherited when different strains of organisms are hybridized. Another type of genetic analysis follows in the footsteps of Wat￾son and Crick and the army of people who have worked on the various genome projects by focusing on the molecular makeup of the genetic material. Still another type of genetic analysis imitates Darwin and Wallace by focusing on entire populations of organisms. All these levels of genetic analysis are routinely used in research today. Although we will encounter them in many different places in this book, we provide brief descriptions of them here. CLASSICAL GENETICS The period prior to the discovery of the structure of DNA is often spoken of as the era of classical genetics. During this time, geneticists pursued their science by analyzing the outcomes of crosses between different strains of organisms, much as Mendel had done in his work with peas. In this type of analysis, genes are identifi ed by studying the in￾heritance of trait differences—tall pea plants versus short pea plants, for example—in the offspring of crosses. The trait differences are due to the alternate forms of genes. Sometimes more than one gene infl uences a trait, and sometimes environmental conditions—for example, temperature and nutrition—exert an effect. These compli￾cations can make the analysis of inheritance diffi cult. The classical approach to the study of genes can also be coordinated with studies on the structure and behavior of chromosomes, which are the cellular entities that contain the genes. By analyzing patterns of inheritance, geneticists can localize genes to specifi c chromosomes. More detailed analyses allow them to localize genes to spe￾cifi c positions within chromosomes—a practice called chromosome mapping. Because these studies emphasize the transmission of genes and chromosomes from one genera￾tion to the next, they are often referred to as exercises in transmission genetics. However, classical genetics is not limited to the analysis of gene and chromosome transmission. It also studies the nature of the genetic material—how it controls traits and how it mutates. We present the essential features of classical genetics in Chapters 3–8. MOLECULAR GENETICS With the discovery of the structure of DNA, genetics entered a new phase. The repli￾cation, expression, and mutation of genes could now be studied at the molecular level. This approach to genetic analysis was raised to a new level when it became possible to sequence DNA molecules easily. Molecular genetic analysis is rooted in the study of DNA sequences. Knowledge of a DNA sequence and comparisons to other DNA se￾quences allow a geneticist to defi ne a gene chemically. The gene’s internal components— coding sequences, regulatory sequences, and noncoding sequences—can be identifi ed, and the nature of the polypeptide encoded by the gene can be predicted

12 Chapter 1 The Science of Genetics But the molecular approach to genetic analysis is much more than the study of DNA sequences.Geneticists have learned to cut DNA molecules at specific sites. Whole genes,or pieces of genes,can be excised from one DNA molecule and inserted into another DNA molecule.These "recombinant"DNA molecules can be replicated in bacterial cells or even in test tubes that have been supplied with appropriate enzymes. Milligram quantities of a particular gene can be generated in the laboratory in an after- noon.In short,geneticists have learned how to manipulate genes more or less at will. This artful manipulation has allowed researchers to study genetic phenomena in great detail.They have even learned how to transfer genes from one organism to another. We present examples of molecular genetic analysis in many chapters in this book. POPULATION GENETICS Genetics can also be studied at the level of an entire population of organisms.Indi- viduals within a population may carry different alleles of a gene;perhaps they carry different alleles of many genes.These differences make individuals genetically dis- tinct,possibly even unique.In other words,the members of a population vary in their genetic makeup.Geneticists seek to document this variability and to understand its significance.Their most basic approach is to determine the frequencies of specific alleles in a population and then to ascertain if these frequencies change over time.If they do,the population is evolving.The assessment of genetic variability in a popula- tion is therefore a foundation for the study of biological evolution.It is also useful in the effort to understand the inheritance of complex traits,such as body size or disease susceptibility.Often complex traits are of considerable interest because they have an agricultural or a medical significance.We discuss genetic analysis at the population level in Chapters 22,23,and 24. KEY PONTSIn classical genetic analysis,genes are studied by following the inberitance of traits in crosses between different strains of an organism. In molecular genetic analysis,genes are studied by isolating,sequencing,and manipulating DNA and by examining the products of gene expression. In population genetic analysis,genes are studied by assessing the variability among individuals in a group of organisms. Genetics in the World:Applications of Genetics to Human Endeavors Genetics is relevant in many venues outside the Modern genetic analysis began in a European monastic enclo- research laboratory. sure;today,it is a worldwide enterprise.The significance and international scope of genetics are evident in today's scientific journals,which showcase the work of geneticists from many dif- ferent countries.They are also evident in the myriad ways in which genetics is applied in agriculture,medicine,and many other human endeavors all over the world.We will consider some of these applications in Chapters 14,15,16,23,and 24.Some of the highlights are introduced in this section. GENETICS IN AGRICULTURE By the time the first civilizations appeared,humans had already learned to cultivate crop plants and to rear livestock.They had also learned to improve their crops and livestock by selective breeding.This pre-Mendelian application of genetic principles had telling effects.Over thousands of generations,domesticated plant and animal species

12 Chapter 1 The Science of Genetics But the molecular approach to genetic analysis is much more than the study of DNA sequences. Geneticists have learned to cut DNA molecules at specifi c sites. Whole genes, or pieces of genes, can be excised from one DNA molecule and inserted into another DNA molecule. These “recombinant” DNA molecules can be replicated in bacterial cells or even in test tubes that have been supplied with appropriate enzymes. Milligram quantities of a particular gene can be generated in the laboratory in an after￾noon. In short, geneticists have learned how to manipulate genes more or less at will. This artful manipulation has allowed researchers to study genetic phenomena in great detail. They have even learned how to transfer genes from one organism to another. We present examples of molecular genetic analysis in many chapters in this book. POPULATION GENETICS Genetics can also be studied at the level of an entire population of organisms. Indi￾viduals within a population may carry different alleles of a gene; perhaps they carry different alleles of many genes. These differences make individuals genetically dis￾tinct, possibly even unique. In other words, the members of a population vary in their genetic makeup. Geneticists seek to document this variability and to understand its signifi cance. Their most basic approach is to determine the frequencies of specifi c alleles in a population and then to ascertain if these frequencies change over time. If they do, the population is evolving. The assessment of genetic variability in a popula￾tion is therefore a foundation for the study of biological evolution. It is also useful in the effort to understand the inheritance of complex traits, such as body size or disease susceptibility. Often complex traits are of considerable interest because they have an agricultural or a medical signifi cance. We discuss genetic analysis at the population level in Chapters 22, 23, and 24. KEY POINTS  In classical genetic analysis, genes are studied by following the inheritance of traits in crosses between different strains of an organism.  In molecular genetic analysis, genes are studied by isolating, sequencing, and manipulating DNA and by examining the products of gene expression.  In population genetic analysis, genes are studied by assessing the variability among individuals in a group of organisms. Genetics is relevant in many venues outside the research laboratory. Genetics in the World: Applications of Genetics to Human Endeavors Modern genetic analysis began in a European monastic enclo￾sure; today, it is a worldwide enterprise. The signifi cance and international scope of genetics are evident in today’s scientifi c journals, which showcase the work of geneticists from many dif￾ferent countries. They are also evident in the myriad ways in which genetics is applied in agriculture, medicine, and many other human endeavors all over the world. We will consider some of these applications in Chapters 14, 15, 16, 23, and 24. Some of the highlights are introduced in this section. GENETICS IN AGRICULTURE By the time the fi rst civilizations appeared, humans had already learned to cultivate crop plants and to rear livestock. They had also learned to improve their crops and livestock by selective breeding. This pre-Mendelian application of genetic principles had telling effects. Over thousands of generations, domesticated plant and animal species