Overview of Genetics - 1000 Genomes Project PDF

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This document provides an overview of genetics, with an emphasis on the Human Genome Project and related concepts. It describes the increasing pace of genetic discoveries and the role of the Human Genome Project in deciphering the human genome's massive amount of genetic information.

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1 PART I INTRODUCTION CHAPTER OUTLINE...

1 PART I INTRODUCTION CHAPTER OUTLINE 1.1 The Molecular Expression of Genes 1.2 The Relationship Between Genes and Traits 1.3 Fields of Genetics 1.4 The Science of Genetics CC (for “carbon copy” or “copy cat”), the first cloned pet. In 2002, the cat shown here was produced by cloning, a procedure described in Chapter 20. ©Texas A&M University/Getty Images OVERVIEW OF GENETICS Hardly a week goes by without a major news story involving a such as how many genes we have, how genes direct the activi- genetic breakthrough. The increasing pace of genetic discoveries ties of living cells, how species evolve, how single cells develop has become staggering. The Human Genome Project is a case in into complex tissues, and how defective genes cause disease. Fur- point. This project began in the United States in 1990, when the thermore, such understanding may lend itself to improvements in National Institutes of Health and the Department of Energy joined modern medicine by providing better diagnoses of diseases and forces with international partners to decipher the massive amount allowing the development of new treatments for them. of information contained in our genome—the deoxyribonucleic A controversial example of a genetic technology is mammalian acid (DNA) found within all of our chromosomes (Figure! 1.1). cloning. In 1997, Ian Wilmut and his colleagues produced clones Remarkably, in only a decade, the researchers working on this of sheep, using mammary cells from an adult animal (Figure 1.2). project determined the DNA sequence of 90% of the human More recently, such cloning has been achieved in several mammalian genome. The completed sequence, published in 2003, has an accu- species, including cows, mice, goats, pigs, and cats. In 2002, the first racy greater than 99.99%; fewer than one mistake was made in pet was cloned, a cat named CC (for “carbon copy” or “copy cat”; every 10,000 base pairs (bp)! see the photo at the beginning of the chapter). The cloning of mam- In 2008, a more massive undertaking, called the 1000 Genomes mals provides the potential for many practical applications. Clon- Project, was launched, with the goal of establishing a detailed under- ing of livestock would enable farmers to use cells from their best standing of human genetic variation. In this international project, individuals to create genetically homogeneous herds. This could be researchers set out to determine the DNA sequence of at least 1000 advantageous in terms of agricultural yield, although such a geneti- anonymous participants from around the globe. In 2015, the sequenc- cally homogeneous herd may be more susceptible to certain diseases. ing of over 2500 genomes was described in the journal Nature. However, people have become greatly concerned with the possibil- Studying the human genome allows us to explore fundamental ity of human cloning. As discussed in Chapter 20, this prospect has details about ourselves at the molecular level. The results of human raised serious ethical questions. Within the past few years, legislative genome projects have shed considerable light on basic questions, bills have been introduced that involve bans on human cloning. 1 2 C H A P T E R 1 : : OVERVIEW OF GENETICS Chromosomes DNA, the molecule of life Cell The adult human body is composed of trillions of cells. Most human cells contain the following: Gene 46 human chromosomes, found in 23 pairs C G T A T A 2 meters of DNA G C G T A T A Approximately 22,000 genes T A coding for proteins that perform A T C G most life functions A T Approximately 3 billion DNA DNA base pairs per set of chromosomes, mRNA containing the bases A, T, G, and C Amino acid Protein (composed of amino acids) FIGURE 1.1 The Human Genome Project.!The human genome is a complete set of human chromosomes. People have two sets of chromosomes, one set from each parent. Collectively, each set of chromosomes is composed of a DNA sequence that is approximately 3 billion nucleotide base pairs long. Estimates suggest that each set contains about 22,000 protein-encoding genes. This figure emphasizes the DNA found in the cell nucleus. Humans also have a small amount of DNA in their mitochondria, which has also been sequenced. Concept Check: How might a better understanding of our genes be used in the field of medicine? Better understanding of our genes could end the problem of birth defects Finally, genetic technologies provide the means of modify- ing the traits of animals and plants in ways that would have been unimaginable just a few decades ago. Figure 1.3a shows a bizarre example in which scientists introduced a gene from jellyfish into mice. Certain species of jellyfish emit a “green glow” produced by a gene that encodes a bioluminescent protein called green fluorescent protein (GFP). When exposed to blue or ultraviolet (UV) light, the protein emits a striking green-colored light. Scientists were able to clone the GFP gene from a sample of jellyfish cells and then intro- duce this gene into laboratory mice. The green fluorescent protein is made throughout the cells of their bodies. As a result, their skin, eyes, FIGURE 1.2 The cloning of a mammal.!The lamb on the left is and organs give off an eerie green glow when exposed to UV light. Dolly, the first mammal to be cloned. She was cloned from a cell of a The expression of green fluorescent protein allows research- Finn Dorset (a white-faced sheep). The sheep on the right is Dolly’s ers to identify particular proteins in cells or specific body parts. surrogate mother, a Blackface ewe. A description of how Dolly was pro- For example, Andrea Crisanti and colleagues have altered mos- duced is presented in Chapter 20. ©R. Scott Horner KRT/Newscom quitoes to express GFP only in the gonads of males (Figure 1.3b). Concept Check: What ethical issues may be associated with This enables the researchers to distinguish males from females and human cloning? Being born just to be a copy might mess with the persons mind. sort mosquitoes by sex. Why is this useful? The ability to rapidly 1.1 THE MOLECULAR EXPRESSION OF!GENES 3 than it has ever been. Nevertheless, new genetic knowledge and technologies will also create many ethical and societal challenges. In this chapter, we begin with an overview of genetics and then explore the various fields of genetics and their experimental approaches. 1.1 THE MOLECULAR EXPRESSION OF!GENES Learning Outcomes: 1. Describe the biochemical composition of cells. 2. Outline how DNA stores the information to make proteins. 3. Explain how proteins are largely responsible for cell struc- ture and function. (a) GFP expressed in mice Genetics is the branch of biology that deals with heredity and GFP variation. It stands as the unifying discipline in biology by allow- ing us to understand how life can exist at all levels of complex- ity, ranging from the molecular to the population level. Genetic variation is the root of the natural diversity that we observe among members of the same species and among different species. Genetics is centered on the study of genes. A gene is classi- cally defined as a unit of heredity, but such a vague definition does not do justice to the exciting characteristics of genes as intricate molecular units that manifest themselves as critical contributors to cell structure and function. (b) GFP expressed in the gonads of a male mosquito " At the molecular level, a gene is a segment of DNA that FIGURE 1.3 The introduction of a jellyfish gene into laboratory has the information to produce a functional product. The mice and mosquitoes.!(a) A gene that naturally occurs in certain jellyfish functional product of most genes is a polypeptide—a linear encodes a protein called green fluorescent protein (GFP). The GFP gene was cloned and introduced into mice. When these mice are exposed to ultra- sequence of amino acids that folds into units that constitute violet light, GFP emits a bright green color. These mice glow green, just like proteins. the jellyfish! (b) GFP was introduced next to a gene sequence that causes " Genes are commonly described according to the way they the expression of GFP only in the gonads of male mosquitoes. The resulting affect traits, which are the characteristics of an organism. green glow allows researchers to identify and sort males from females. In humans, for example, we observe traits such as eye color, (a) ©Eye of Science/Science Source; (b) Courtesy of Flaminia Catteruccia, Jason hair texture, and height. An ongoing theme of this textbook Benton and Andrea Crisanti is the relationship between genes and traits. As an organism Concept Check: Why is it useful to sort male mosquitoes from grows and develops, its collection of genes provides a female mosquitoes? blueprint that determines its characteristics. In this section, we will examine the general features of life sort mosquitoes by sex makes it possible to produce populations with an emphasis on the molecular level. Genetics is the common of sterile males and then release the sterile males without the risk thread that explains the existence of life and its continuity from of releasing additional females. The release of sterile males may generation to generation. For most students, this chapter should serve as a cohesive review of topics they learned in other introduc- be an effective means of controlling mosquito populations because tory courses such as general biology. Even so, it is usually helpful females breed only once. Mating with a sterile male prevents a to see the “big picture” of genetics before delving into the finer female from producing offspring. In 2008, Osamu Shimomura, details that are covered in Chapters 2 through 24. Martin Chalfie, and Roger Tsien received the Nobel Prize in chemistry for the discovery and the development of GFP, which Living Cells Are Composed of Biochemicals has become a widely used tool in biology. To fully understand the relationship between genes and traits, we Overall, as we move forward in the twenty-first century, the need to begin with an examination of the composition of living excitement level in the field of genetics is high, perhaps higher organisms. Every cell is constructed from intricately organized 4 C H A P T E R 1 : : OVERVIEW OF GENETICS Plant cell chemical substances. Small organic molecules such as glucose and amino acids are produced by the linkage of atoms via chemical bonds. The chemical properties of organic molecules are essential for cell vitality in two key ways. ! First, the breaking of chemical bonds during the degradation of small molecules provides energy to drive cellular processes. ! A second important function of these small organic molecules Nucleus is their role as the building blocks for the synthesis of larger molecules. Four important categories of larger cellular molecules are nucleic acids (i.e., DNA and RNA), proteins, carbohydrates, and lipids. Three of these—nucleic acids, proteins, and carbohydrates—form macromolecules that are composed of many repeating units of smaller building blocks. Proteins, RNA, and carbohydrates can be made from hundreds or even thousands of repeating building blocks. DNA is the largest macromolecule found in living cells. A single DNA molecule can be composed of a linear sequence Chromosome of hundreds of millions of building blocks called nucleotides! Proteins The formation of cellular structures relies on the interac- tions of molecules and macromolecules. Figure 1.4 illustrates this concept. ! Nucleotides are small organic molecules. ! Nucleotides are linked to each other and form the building blocks of DNA, which is a macromolecule. DNA ! DNA is a component of chromosomes, which also contain proteins that contribute to chromosome structure. ! Within a eukaryotic cell, the chromosomes are contained in a compartment called the cell nucleus. The nucleus is bounded by a double membrane composed of lipids and proteins that shields the chromosomes from the rest of the cell. The nucleus is an example of an organelle—a membrane-bound compartment with a specialized function. The cell nucleus protects the chromosomes from mechanical damage and provides a single compartment for genetic activities such as Nucleotides gene transcription. ! Finally, cellular molecules, macromolecules, and organelles are organized to make a complete living cell. NH2 Cytosine N H Guanine O Each Cell Contains Many Different O– O N H H N N Proteins That Determine Cell Structure O P O CH2 O O– H O– H2 N N N and!Function H H H H O P O CH2 O O– To a great extent, the characteristics of a cell depend on the types OH H H H H H of proteins that it makes. The entire collection of proteins that a OH H cell makes at a given time is called its proteome. As we will learn throughout this textbook, proteins are the “workhorses” of all liv- FIGURE 1.4 Molecular organization of a living cell.!Cellular structures are constructed from smaller building blocks. In this example, ing cells. The range of functions among different types of proteins DNA is formed from the linkage of nucleotides, producing a very long is truly remarkable. Some examples include the following: macromolecule. The DNA associates with proteins to form a chromo- ! Proteins help determine the shape and structure of a given some. The chromosomes are located within a membrane-bound organelle cell. For example, the protein known as tubulin can assemble called the nucleus, which, along with many different types of organelles, is found within a complete cell. into large structures known as microtubules, which provide ©Biophoto Associates/Science Source the cell with internal structure and organization. ! Proteins are inserted into cell membranes and aid in the Concept Check: Is DNA a small molecule, a macromolecule, or transport of ions and small molecules across the membrane. an organelle? 1.1 THE MOLECULAR EXPRESSION OF!GENES 5 " Proteins may also function as biological motors. An interesting case is the protein known as myosin, which is involved in the contractile properties of muscle cells. " Within multicellular organisms, certain proteins function in cell-to-cell recognition and signaling. For example, hormones such as insulin are secreted by endocrine cells and bind to the insulin receptor protein found within the plasma membrane of target cells. " Enzymes, which accelerate chemical reactions, are a particularly important category of proteins. Some enzymes play a role in the breakdown of molecules or macromolecules into smaller units. These enzymes are important in the utilization of energy. Molecular biologists have come to realize that the functions of proteins underlie the cellular characteristics of every organism. At the molecular level, proteins can be viewed as the active par- ticipants in the enterprise of life. DNA Stores the Information for Protein Synthesis FIGURE 1.5 A micrograph of the 46 chromosomes found in a cell from a human male. As mentioned, the genetic material of living organisms is com- ©Kateryna Kon/Shutterstock posed of a substance called deoxyribonucleic acid, abbreviated Concept Check: Which types of macromolecules are found in DNA. The DNA stores the information needed for the synthesis of chromosomes? all proteins. In other words, the main function of the genetic blue- print is to code for the production of proteins in the correct cell, at the proper time, and in suitable amounts. This task is extremely In living cells, DNA is found within large structures known as complicated because living cells make thousands of different pro- chromosomes. Figure 1.5 is a micrograph of the 46 chromosomes in teins. Genetic analyses have shown that a typical bacterium can a cell from a human male, which are found in pairs. The DNA of an make a few thousand different proteins, and estimates of the num- average human chromosome is an extraordinarily long, linear, double- bers of proteins produced by complex eukaryotes range in the tens stranded structure that contains well over a hundred million nucleotides. of thousands. Along the immense length of a chromosome, the genetic information DNA’s ability to store information is based on its structure. is parceled into functional units known as genes. An average-sized human chromosome is expected to carry about 1000 different genes. " DNA is composed of a linear sequence of nucleotides, each of which contains one of four nitrogen-containing bases: The Information in DNA Is Accessed During adenine (A), thymine (T), guanine (G), or cytosine (C). the!Process of Gene Expression " The linear order of these bases along a DNA molecule contains information similar to the way that groups of letters To synthesize its proteins, a cell must be able to access the infor- of the alphabet represent words. For example, the “meaning” mation that is stored within its DNA. The process of using a gene of the sequence of bases ATGGGCCTTAGC differs from sequence to affect the characteristics of cells and organisms is that of TTTAAGCTTGCC. referred to as gene expression. At the molecular level, the infor- " DNA sequences within most genes contain the information mation is accessed in a stepwise process (Figure 1.6). to direct the order of amino acids within polypeptides 1. In the first step, known as transcription, the DNA sequence according to the genetic code. In the code, a three-base within a gene is copied into a nucleotide sequence of sequence, called a codon, specifies one particular amino ribonucleic acid (RNA). Most genes encode RNAs that contain acid among the 20 possible choices. the information for the synthesis of a particular polypeptide. " The sequence of amino acids in a polypeptide causes it to This type of RNA is called messenger RNA (mRNA). fold into a particular structure; one or more polypeptides 2. During the process of translation, the sequence of nucleotides form a functional protein. in an mRNA provides the information (using the genetic code) In this way, the DNA can store the information to specify the to produce the amino acid sequence of a polypeptide. proteins made by an organism. 3. A polypeptide folds into a three-dimensional structure. As mentioned, a protein is a functional unit. Some proteins are DNA Sequence Amino Acid Sequence composed of a single polypeptide, and other proteins consist of two or more polypeptides. ATG GGC CTT AGC Methionine Glycine Leucine Serine 4. The functioning of proteins largely determines cell structure TTT AAG CTT GCC Phenylalanine Lysine Leucine Alanine and function. 6 C H A P T E R 1 : : OVERVIEW OF GENETICS DNA 3. The function of the genetic code is to a. promote transcription. Gene b. specify the amino acids within a polypeptide. c. alter the sequence of DNA. Transcription d. do none of the above. 4. The process of transcription directly results in the synthesis of RNA (messenger RNA) a. DNA. b. RNA. c. a polypeptide. Translation d. all of the above. Protein 1.2 THE RELATIONSHIP BETWEEN (sequence of amino acids) GENES AND TRAITS Learning Outcomes: 1. Outline how the expression of genes leads to an organism’s traits. Functioning of proteins within living 2. Define genetic variation. cells influences an organism’s traits. 3. Discuss the relationship between genes, traits, and the environment. FIGURE 1.6 Gene expression at the molecular 4. Describe how genes are transmitted in sexually reproducing level.!The expression of a gene is a multistep process. During species. transcription, one of the DNA strands is used as a template to 5. Describe the process of evolution. make an RNA strand. During translation, the RNA strand is used to specify the sequence of amino acids within a polypeptide. One or more polypep- tides produce a functional protein, thereby influencing an organism’s traits. A trait is any characteristic that an organism displays. In genetics, Concept Check: Where is the information to make a polypeptide we can place traits into different categories. stored? " Morphological traits affect the appearance, form, and structure of an organism. The color of a flower and the 1.1 REVIEWING THE KEY CONCEPTS height of a pea plant are morphological traits. Geneticists frequently study these types of traits because they are easy to " Living cells are composed of nucleic acids (DNA and RNA), evaluate. For example, an experimenter can simply look at a proteins, carbohydrates, and lipids. The proteome largely deter- plant and tell if it has red or white flowers. mines the structure and function of cells (see Figure 1.4). " Physiological traits affect the ability of an organism " DNA, which is found within chromosomes, stores the informa- to function. For example, the rate at which a bacterium tion to make proteins (see Figure 1.5). metabolizes a sugar such as lactose is a physiological trait. " Most genes encode polypeptides that are units within functional Like morphological traits, physiological traits are controlled, proteins. Gene expression at the molecular level involves tran- in part, by the expression of genes. scription to produce mRNA and translation to produce a poly- " Behavioral traits affect the ways an organism responds to its peptide (see Figure 1.6). environment. An example is the mating calls of bird species. In animals, the nervous system plays a key role in governing 1.1 COMPREHENSION QUESTIONS such traits. 1. Which of the following is not a constituent of a cell’s proteome? a. An enzyme In this section, we will examine the relationship between the b. A motor protein expression of genes and an organism’s traits. c. A receptor in the plasma membrane d. An mRNA 2. A gene is a segment of DNA that has the information to produce a The Molecular Expression of Genes Within functional product. The functional product of most genes is Cells!Leads to an Organism’s Traits a. DNA. A complicated, yet very exciting, aspect of genetics is that our b. mRNA. observations and theories span four levels of biological organi- c. a polypeptide. zation: molecules, cells, organisms, and populations. This broad d. none of the above. scope can make it difficult to appreciate the relationship between 1.2 THE RELATIONSHIP BETWEEN GENES AND TRAITS 7 genes and traits. To understand this connection, we need to relate the following four phenomena: 1. As we learned in Section 1.1, genes are expressed at the Pigmentation gene Pigmentation gene molecular level. In other words, gene transcription and (dark allele) (light allele) found in a dark butterfly found in a light butterfly translation lead to the production of a particular protein, which is a molecular process. Transcription and translation 2. Proteins often function at the cellular level. The function of a protein within a cell affects the structure and workings of that cell. 3. An organism’s traits are determined by the characteristics of its cells. We do not have microscopic vision, yet when we view morphological traits, we are really observing the Highly functional Poorly functional properties of an individual’s cells. For example, a red flower pigmentation enzyme pigmentation enzyme has its color because its cells make a red pigment. The trait of red flower color is an observation at the organism level, (a) Molecular level yet the trait is rooted in the molecular characteristics of the organism’s cells. 4. A species is a group of organisms that maintains a distinctive Pigment molecule set of attributes in nature. The occurrence of a trait within a species is an observation at the population level. Along with Wing cells learning how a trait occurs, we also want to understand why a trait becomes prevalent in a particular species. In many cases, researchers discover that a trait predominates within a Lots of pigment made Little pigment made population because it promotes the reproductive success of the members of the population. (b) Cellular level As a schematic example to illustrate the four levels of genetics, Figure 1.7 shows the trait of pigmentation in a spe- cies of butterflies. One member of this species is light-colored and the other is very dark. Let’s consider how we can explain this trait at the molecular, cellular, organism, and population levels. Dark butterfly Light butterfly 1. At the molecular level, we need to understand the nature of the gene or genes that govern this trait. As shown in (c) Organism level Figure 1.7a, a gene, which we will call the pigmentation gene, is responsible for the amount of pigment produced. The pigmentation gene can exist in two different forms called alleles. In this example, one allele confers a dark pigmentation and one causes a light pigmentation. Each of these alleles encodes a protein that functions as a pigment-synthesizing enzyme. However, the DNA sequences of the two alleles differ slightly from each other. This difference in the DNA sequence leads to a variation in the structure and function of the respective pigmentation enzymes. 2. At the cellular level (Figure 1.7b), the functional differences between the pigmentation enzymes affect Dark butterflies are usually Light butterflies are usually the amount of pigment produced. The allele causing in forested regions. in unforested regions. dark pigmentation, which is shown on the left, encodes an enzyme that functions very well. Therefore, when (d) Population level this gene is expressed in the cells of the wings, a large amount of pigment is made. By comparison, the allele FIGURE 1.7 The relationship between genes and traits at the (a)!molecular, (b) cellular, (c) organism, and (d) population levels. causing light pigmentation encodes an enzyme that functions poorly. Therefore, when this allele is the only Concept Check: Which butterfly has a more active pigment- pigmentation gene expressed, little pigment is made. synthesizing enzyme, the light- or dark-colored one? 8 C H A P T E R 1 : : OVERVIEW OF GENETICS 3. At the organism level (Figure 1.7c), the amount of pigment in the wing cells governs the color of the wings. If the pigment-synthesizing enzymes produce high amounts of pigment, the wings are dark-colored; if the enzymes produce little pigment, the wings are light. 4. Finally, at the population level (Figure 1.7d), geneticists want to know why a species of butterfly has some members with dark wings and others with light wings. One possible explanation is differential predation. The butterflies with dark wings might avoid being eaten by birds if they happen to live within the dim light of a forest. The dark wings would help to camouflage the butterfly if it were perched on a dark surface such as a tree trunk. In contrast, the light-colored wings would be an advantage if the butterfly inhabited a brightly lit meadow. Under these conditions, a bird might be less likely to notice a light-colored butterfly that was perched on a sunlit surface. A geneticist might study this species of FIGURE 1.8 Two dyeing poison frogs (Dendrobates tinctorius) butterfly and find that the dark-colored members usually live showing different morphs within a single species. in forested areas and the light-colored members reside in (a) ©Natalia Kuzmina/Shutterstock; (b) ©Valt Ahyppo/Shutterstock unforested regions. Concept Check: Why do these two frogs look so different? Inherited Differences in Traits " Major alterations can also occur in the structure of a chromosome. A large segment of a chromosome can be lost, Are!Due!to!Genetic Variation rearranged, or reattached to another chromosome. In Figure 1.7, we considered how gene expression can lead to " Variation may also occur in the total number of variation in a trait of an organism, specifically, dark- versus light- chromosomes. In some cases, an organism may inherit one colored wings in butterflies. Variation in traits among members of too many or one too few chromosomes. In other cases, it may the same species is very common. For example, some people have inherit an extra set of chromosomes. black hair, and others have brown hair; some petunias have white flowers, but others have purple flowers. These are examples of Variations within the sequences of genes are a common genetic variation. This term describes the differences in inherited source of genetic variation among members of the same species. traits among individuals within a population. In humans, familiar examples of sequence variation involve genes In large populations that occupy a wide geographic range, for eye color, hair texture, and skin pigmentation. Chromosome genetic variation can be quite striking. Morphological differ- variation—a change in chromosome structure or number (or ences have often led geneticists to misidentify two members of both)—is also found, but this type of change is often detrimen- the same species as belonging to separate species. As an example, tal. Many human genetic disorders are the result of chromosomal Figure!1.8 shows two dyeing poison frogs that are members of the alterations. An example is Down syndrome, which is due to the same species, Dendrobates tinctorius. They display dramatic dif- presence of an extra chromosome (Figure 1.9a). By comparison, ferences in their markings. Such contrasting forms within a single chromosome variation in plants is common and often results in species are termed morphs. You can easily imagine how someone plants with superior characteristics, such as increased resistance might mistakenly conclude that these frogs are not members of the to disease. Plant breeders have frequently exploited this observa- same species. tion. Cultivated varieties of wheat, for example, have many more Changes in the nucleotide sequence of DNA underlie the chromosomes than the wild species (Figure 1.9b). genetic variation that we see among individuals. Throughout this textbook, we will routinely examine how variation in the genetic Traits Are Governed by Genes material results in changes in the outcome of traits. At the molecu- and!by!the!Environment lar level, genetic variation can be attributed to different types of In our discussion thus far, we have considered the role that genes play modifications. in the outcome of traits. Another critical factor is the environment— " Small or large differences can occur within gene sequences. the surroundings in which an organism exists. A variety of factors When such changes initially occur, they are called gene in an organism’s environment profoundly affect its morphologi- mutations, which are heritable changes in the genetic cal and physiological features. For example, a person’s diet greatly material. Gene mutations result in genetic variation in influences many traits, such as height, weight, and even intelligence. which a gene is found in two or more alleles, as previously Likewise, the amount of sunlight a plant receives affects its growth described in Figure 1.7. In many cases, gene mutations alter rate and the color of its flowers. The term norm of reaction refers to the expression or function of a protein that a gene specifies. the effects of environmental variation on an individual’s traits. 1.2 THE RELATIONSHIP BETWEEN GENES AND TRAITS 9 (a) (b) FIGURE 1.9 Examples of chromosome variation.!(a) A person FIGURE 1.10 Environmental influence on the outcome of with Down syndrome. She has 47 chromosomes rather than the common PKU. !This girl with PKU has developed normally because she followed number of 46, because she has an extra copy of chromosome 21. a diet that is very low in phenylalanine. (b) A wheat plant. Bread wheat is derived from the contributions of three ©Noah Goodrich/Newscom related species with two sets of chromosomes each, producing an organ- Concept Check: What would have been the consequences if ism with six sets of chromosomes. this girl had followed a standard diet, which contains a higher (a) ©Stockbyte/Alamy Stock Photo; (b) ©Pixtal/age fotostock amount of phenylalanine? Concept Check: Are these examples of gene mutations, variation in chromosome structure, or variation in chromosome number? our understanding of inheritance came from the studies of pea plants by Gregor Mendel in the nineteenth century. His work revealed that External influences may dictate the way that genetic varia- genetic determinants, which we now call genes, are passed from tion is manifested in an individual. An interesting example is the parent to offspring as discrete units. We can predict the outcome of human genetic disease phenylketonuria (PKU). Humans have a many genetic crosses based on Mendel’s laws of inheritance. gene that encodes an enzyme known as phenylalanine hydroxy- The inheritance patterns identified by Mendel can be lase. Most people have two functional copies of this gene. People explained by the existence of chromosomes and their behavior with one or two functional copies of the gene can eat foods con- during cell division. taining the amino acid phenylalanine and metabolize it properly. A rare variation in the sequence of the phenylalanine hydroxy- " Like Mendel’s pea plants, sexually reproducing species are lase gene results in a nonfunctional version of this protein. Individu- commonly diploid. This means that their cells contain two als with two copies of this rare, inactive allele cannot metabolize copies of each chromosome, one from each parent. The two phenylalanine properly. Such individuals represent about 1 in 8000 copies are called homologs of each other. births in the United States. When given a standard diet containing " Because genes are located within chromosomes, diploid phenylalanine, individuals with this disorder are unable to break organisms have two copies of most genes. Humans, for down this amino acid. Phenylalanine accumulates and is converted example, have 46 chromosomes, which are found in into phenylketones, which are detected in the urine. PKU individuals homologous pairs (Figure 1.11a). With the exception of the manifest a variety of detrimental traits, including mental impairment, sex chromosomes (X and Y), each homologous pair contains underdeveloped teeth, and foul-smelling urine. In contrast, when the same kinds of genes. For example, both copies of human PKU individuals are identified at birth and raised on a restricted diet chromosome 12 carry the gene that encodes phenylalanine that is low in phenylalanine, they develop normally (Figure!1.10). hydroxylase, which was discussed previously. Therefore, an Fortunately, through routine newborn screening, most affected individual has two copies of this gene that may or may not be babies in the United States are now diagnosed and treated early. PKU identical alleles. provides a dramatic example of how the environment and an indi- " Most cells of the human body that are not directly involved vidual’s genes can interact to influence the traits of the organism. in sexual reproduction contain 46 chromosomes. These cells are called somatic cells. In contrast, the gametes—sperm and egg cells—contain half that number (23) and are termed During Reproduction, Genes Are Passed haploid (Figure 1.11b). from!Parent to Offspring " The union of gametes during fertilization restores the Now that we have considered how genes and the environment gov- diploid number of chromosomes. The primary advantage of ern the outcome of traits, we can turn to the issue of inheritance. sexual reproduction is that it enhances genetic variation. For How are traits passed from parents to offspring? The foundation for example, a tall person with blue eyes and a short person with 10 C H A P T E R 1 : : OVERVIEW OF GENETICS 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 9 10 11 12 13 14 15 16 17 18 19 20 21 22 XX 17 18 19 20 21 22 X (a) Chromosomal composition found in most female (b) Chromosomal composition found in human cells (46 chromosomes) a human gamete (23 chromosomes) FIGURE 1.11 The complement of human chromosomes in somatic cells and gametes.!(a) A schematic drawing of the 46 chromosomes of a human. With the exception of the sex chromosomes, these are always found in homologous pairs in somatic cells, such as skin or nerve cells. (b) The chromosomal composition of a gamete, which contains only 23 chromosomes, one from each pair. This gamete contains an X chromosome. Half of the gametes from human males contain a Y chromosome instead of an X chromosome. Concept Check: The leaf cells of a corn plant contain 20 chromosomes each. How many chromosomes are found in a gamete made by a corn plant? brown eyes may have short offspring with blue eyes or tall Over a long period of time, the accumulation of many offspring with brown eyes. Therefore, sexual reproduction genetic changes may lead to rather striking modifications in a can result in new combinations of two or more traits that species’ characteristics. As an example, Figure 1.12 depicts the differ from those of either parent. evolution of the modern-day horse. A variety of morphological changes occurred, including an increase in size, fewer toes, and modified jaw structure. The changes can be attributed to natu- The Genetic Composition of a Species Evolves ral selection producing adaptations to changing global climates. over the Course of Many Generations Over North America, where much of horse evolution occurred, As we have just seen, sexual reproduction has the potential to large areas of dense forests were replaced with grasslands. The enhance genetic variation. This can be an advantage for a popula- increase in size and changes in foot structure enabled horses tion of individuals as they struggle to survive and compete within to escape predators more easily and travel greater distances in their natural environment. The term biological evolution, or sim- search of food. The changes seen in horses’ teeth are consistent ply, evolution, refers to the phenomenon that the genetic makeup with a shift from eating tender leaves to eating grasses and other of a population can change from one generation to the next. types of vegetation that are more abrasive and require more As proposed by Charles Darwin, the members of a species are chewing. in competition with one another for essential resources. Random 1.2 REVIEWING THE KEY CONCEPTS genetic changes (i.e., mutations) occasionally occur within an indi- " Genetics spans the molecular, cellular, organism, and popula- vidual’s genes, and sometimes these changes lead to a modification tion levels (see Figure 1.7). of traits that promote reproductive success. For example, over the course of many generations, random gene mutations have length- " Genetic variation underlies variation in traits. In addition, the ened the snout of the anteater, enabling it to feed on ants located in environment plays a key role (see Figures 1.8–1.10). the ground. When a mutation creates a new allele that is beneficial, " During reproduction, genetic material is passed from parents to the allele may become prevalent in future generations because the offspring. In many species, somatic cells are diploid and have individuals carrying the allele are more likely to survive and repro- two sets of chromosomes, whereas gametes are haploid and duce and pass the beneficial allele to their offspring. This process have a single set (see Figure 1.11). is known as natural selection. In this way, a species becomes bet- " Evolution refers to a change in the genetic composition of a pop- ter adapted to survive and reproduce in its native environment. ulation from one generation to the next (see Figure 1.12). 1.2 THE RELATIONSHIP BETWEEN GENES AND TRAITS 11 Equus 0 Hippidium and other genera Nannippus Stylohipparion 5 Pliohippus Hipparion Neohipparion 10 Sinohippus Megahippus Calippus Archaeohippus 20 Merychippus Millions of years ago (mya) Anchitherium Hypohippus Parahippus Miohippus Mesohippus 40 Paleotherium Epihippus Propalaeotherium FIGURE 1.12 The evolutionary changes that led to the mod- Pachynolophus Orohippus ern horse genus, Equus.!Three important morphological changes that occurred were larger size, fewer toes, and a shift toward a jaw structure suited for grazing. 55 Concept Check: !According to the theory of evolution, why have Hyracotherium these changes occurred in horse populations over the course of many generations? 1.2 COMPREHENSION QUESTIONS 3. A human skin cell has 46 chromosomes. A human sperm cell has 1. Gene expression can be viewed at which of the following levels? a. 23. a. Molecular and cellular levels b. 46. b. Organism level c. 92. c. Population level d. None of the above is the number of chromosomes in a sperm cell. d. All of the above 4. Evolutionary change caused by natural selection results in species 2. Variation in the traits of organisms may be attributable to with a. gene mutations. a. greater complexity. b. alterations in chromosome structure. b. less complexity. c. variation in chromosome number. c. greater reproductive success in their native environment. d. all of the above. d. the ability to survive longer. 12 C H A P T E R 1 : : OVERVIEW OF GENETICS The study of genetics has been traditionally divided into 1.3 FIELDS OF GENETICS three areas—transmission, molecular, and population genetics— although there is some overlap of these three fields. In this section, Learning Outcome: we will examine the general questions that scientists in these areas 1. Compare and contrast the three major fields of genetics: are attempting to answer. transmission, molecular, and population genetics. Transmission Genetics Explores the Inheritance Genetics is a broad discipline encompassing molecular, cellu- Patterns of Traits as They Are Passed from lar, organism, and population biology. Many scientists who are interested in genetics have been trained in supporting disciplines Parents to Offspring such as biochemistry, biophysics, cell biology, mathematics, A scientist working in the field of transmission genetics examines microbiology, population biology, ecology, agriculture, and medi- the relationship between the transmission of genes from parent to cine. Experimentally, geneticists often focus their efforts on model offspring and the outcome of the offspring’s traits. For example, organisms—organisms studied by many different researchers so how can two brown-eyed parents produce a blue-eyed child? Or they can compare their results and determine scientific principles why do tall parents tend to produce tall children, but not always? that apply more broadly to other species. Figure 1.13 shows some Our modern understanding of transmission genetics began with the examples of model organisms, including Escherichia coli (a bac- studies of Gregor Mendel. His work provided the conceptual frame- terium), Saccharomyces cerevisiae (a yeast), Drosophila mela- work for transmission genetics. In particular, he originated the idea nogaster (fruit fly), Caenorhabditis elegans (a nematode worm), that genetic determinants, which we now call genes, are passed as Mus musculus (mouse), and Arabidopsis thaliana (a flowering discrete units from parents to offspring via sperm and egg cells. plant). Model organisms offer experimental advantages over other Since Mendel’s pioneering studies of the 1860s, our knowledge species. For example, E. coli is a very simple organism that can be of genetic transmission has greatly increased. Many patterns of easily grown in the laboratory. By limiting their work to a few such genetic transmission are more complex than the simple Mendelian model organisms, researchers can more easily unravel the genetic patterns that are described in Chapter 3. The additional complexi- mechanisms that govern the traits of a given species. Furthermore, ties of transmission genetics are examined in Chapters 4 through 10. the genes found in model organisms often function in a similar Experimentally, the fundamental technique used by a trans- way to those found in humans. mission geneticist is the genetic cross. A genetic cross involves 0.3 μm 7 μm (a) Escherichia coli (b) Saccharomyces cerevisiae (c) Drosophila melanogaster 133 μm (d) Caenorhabditis elegans (e) Mus musculus (f) Arabidopsis thaliana FIGURE 1.13 Examples of model organisms studied by geneticists.!(a) Escherichia coli (a bacterium), (b) Saccharomyces cerevisiae (a yeast), (c) Drosophila melanogaster (fruit fly), (d) Caenorhabditis elegans (a nematode worm), (e) Mus musculus (mouse), and (f) Arabidopsis thaliana (a flowering plant). (a) Source: CDC/Peggy S. Hayes & Elizabeth H. White, M.S; (b) ©Science Photo Library/Alamy Stock Photo; (c) ©janeff/Getty Images; (d) ©Sinclair Stammers/Science Source; (e) ©G.K. & Vikki Hart/Getty Images; (f) ©WILDLIFE GmbH/Alamy Stock Photo Concept Check: Can you think of another example of a model organism? 1.3 FIELDS OF GENETICS 13 breeding two selected individuals and then analyzing their offspring addition, advances within molecular genetics have shed consider- in an attempt to understand how traits are passed from parents to able light on the areas of transmission and population genetics. Our offspring. In the case of experimental organisms, the researcher quest to understand molecular genetics has spawned a variety of chooses two parents with particular traits and then categorizes the modern molecular technologies and computer-based approaches. offspring according to the traits they possess. In many cases, this Furthermore, discoveries within molecular genetics have had wide- analysis is quantitative in nature. For example, an experimenter spread applications in agriculture, medicine, and biotechnology. may cross two tall pea plants and obtain 100 offspring that fall into The following are some general questions within the field of two categories: 75 tall and 25 dwarf. As we will see in Chapter 3, molecular genetics: the ratio of tall to dwarf offspring (3:1) provides important infor- What are the molecular structures of DNA and RNA? mation concerning the inheritance pattern of the height trait. Chapter 11 Throughout Chapters 2 to 10, we will learn how researchers try to answer many fundamental questions concerning the passage What is the composition and conformation of chromosomes? of genetic material from cell to cell and the passage of traits from Chapter 12 parents to offspring. Here are some of these questions: How is the genetic material copied? Chapter 13 How are genes expressed at the molecular level? How are chromosomes transmitted during cell division and Chapters 14, 15 gamete formation? Chapter 2 How is gene expression regulated so that it occurs under the What are the common patterns of inheritance for genes? appropriate conditions, in the appropriate cell type, and at Chapters 3–5 the correct stage of development? Chapters 16, 17 Are there unusual patterns of inheritance that cannot be What are the roles of RNA molecules that do not encode explained by the simple transmission of genes located on polypeptides? Chapter 18 chromosomes in the cell nucleus? Chapter 6 What is the molecular nature of mutations? How are When two or more genes are located on the same chromosome, mutations repaired? Chapter 19 how is the pattern of inheritance affected? Chapter 7 How have genetic technologies advanced our understanding How do variations in chromosome structure or chromosome of genetics? Chapter 20 number occur, and how are they transmitted from parents to offspring? Chapter 8 What is the genetic composition and function of whole genomes? Chapter 21 How are genes transmitted by bacterial species? Chapter 9 How do viruses proliferate? Chapter 10 Population Genetics Is Concerned with Genetic Variation and Its Role in Evolution Molecular Genetics Focuses on a Biochemical The foundations of population genetics arose during the first few Understanding of the Hereditary Material decades of the twentieth century. Although many scientists of this era did not accept the findings of Mendel or Darwin, the theo- The goal of molecular genetics, as the name of the field implies, ries of population genetics provided a compelling way to connect is to understand how the genetic material works at the molecular the two viewpoints. Mendel’s work and that of many succeeding level. In other words, molecular geneticists want to understand the geneticists gave insight into the nature of genes and how they are molecular features of DNA and how these features underlie the transmitted from parents to offspring. The theory of evolution by expression of genes. The experiments of molecular geneticists are natural selection proposed by Darwin provided a biological expla- usually conducted within the confines of a laboratory. Their efforts nation for the variation in characteristics observed among the frequently progress to a detailed analysis of DNA, RNA, and pro- members of a species. To relate these two phenomena, popula- teins, using a variety of techniques that are described throughout tion geneticists have developed mathematical theories to explain Parts III, IV, and V of this textbook. the prevalence of certain alleles within populations of individuals. Molecular geneticists often study mutant genes that have abnor- The work of population geneticists helps us understand how pro- mal function. This is called a genetic approach to the study of a cesses such as natural selection have resulted in the prevalence of research question. In many cases, researchers analyze the effect of a individuals that carry particular alleles. gene mutation that eliminates the function of a gene. This is called Population geneticists are particularly interested in genetic a loss-of-function mutation, and the resulting gene is called a loss- variation and how that variation is related to an organism’s of-function allele. Studying the effect of such a mutation often environment. In this field, the frequencies of alleles within a reveals the role of the functional, nonmutant gene. For example, let’s population are of central importance. The following are some suppose that a particular plant species produces purple flowers. If a general questions in population genetics: loss-of-function mutation within a given gene causes a plant of that species to produce white flowers, one would suspect that the role of What is the underlying relationship between genes and the functional gene involves the production of purple pigmentation. genetic diseases? Chapter 22 Studies within molecular genetics interface with other dis- Why are two or more different alleles of a gene maintained in ciplines such as biochemistry, biophysics, and cell biology. In a population? Chapter 23 14 C H A P T E R 1 : : OVERVIEW OF GENETICS What factors alter the prevalence of alleles within a about which particular genes may be involved. The collection and population? Chapter 23 analysis of data without the need for a preconceived hypothesis is What are the contributions of genetics and environment in called discovery-based science or, simply, discovery science. the outcome of a trait? Chapter 24 In traditional science textbooks, the emphasis often lies on How do genetics and the environment influence quantitative the product of science. That is, many textbooks are aimed primar- traits, such as size and weight? Chapter 24 ily at teaching the student about the observations scientists have made and the hypotheses they have proposed to explain these 1.3 REVIEWING THE KEY CONCEPTS observations. Along the way, the student is provided with many " Model organisms are studied by many different researchers so bits and pieces of experimental techniques and data. Although this they can compare their results and determine scientific princi- textbook provides you with many observations and hypotheses, it ples that apply more broadly to other species (see Figure 1.13). attempts to go one step further. Many of the following chapters " Genetics is traditionally divided into transmission genetics, contain one or two figures presenting experiments that have been molecular genetics, and population genetics, though overlap “dissected” into five individual components to help you to under- occurs among these fields. stand the entire scientific process. The five steps are as follows: 1.3 COMPREHENSION QUESTIONS 1. Background information is provided so that you can appreciate observations that were known prior to conducting 1. Which of the following is not a model organism? the experiment. a. Mus musculus (laboratory mouse) 2. Most experiments involve hypothesis testing. In those cases, b. Escherichia coli (a bacterium) the figure presenting the experiment states the hypothesis the c. Saccharomyces cerevisiae (a yeast) scientists were trying to test. In other words, what scientific d. Sciurus carolinensis (gray squirrel) question was the researcher trying to answer? 2. A person studying the rate of transcription of a particular gene is 3. Next, the figure follows the experimental steps the scientists working in the field of took to test the hypothesis. The steps necessary to carry a. molecular genetics. out the experiment are listed in the order in which they b. transmission genetics. were conducted. The figure presents parallel illustrations c. population genetics. labeled “Experimental level” and “Conceptual level.” The d. None of the above is correct. experimental level helps you to understand the techniques that were used. The conceptual level helps you to understand what is actually happening at each step in the procedure. 4. The raw data from the experiment are then presented. 5. Last, an interpretation of the data is offered within the text. 1.4 THE SCIENCE OF GENETICS The rationale behind this approach is that it enables you to see Learning Outcomes: the experimental process from beginning to end. Hopefully, you will 1. Describe what makes genetics an experimental science. find this a more interesting and rewarding way to learn about genet- 2. Outline different strategies for solving problems in genetics. ics. As you read through the chapters, the experiments will help you to see the relationship between science and scientific theories. Science is a way of knowing about our natural world. The science As a student of genetics, you will be given the opportunity to of genetics allows us to understand how the expression of our genes involve your mind in the experimental process. As you are reading produces the traits that we possess. In this section, we will consider an experiment, you may find yourself thinking about alternative how scientists attempt to answer questions via experimentation. approaches and hypotheses. Different people can view the same We will also consider general approaches for solving problems. data and arrive at very different conclusions. As you progress through the experiments in this book, you will enjoy genetics far more if you try to develop your own skills at formulating hypoth- Genetics Is an Experimental Science eses, designing experiments, and interpreting data. Also, some of Regardless of what field of genetics they work in, researchers typi- the questions in the problem sets are aimed at refining these skills. cally follow two general types of scientific approaches: hypothesis testing and discovery-based science. In hypothesis testing, also Genetic TIPS Will Help You to Improve Your called the scientific method, scientists follow a series of steps to Problem-Solving Skills reach verifiable conclusions about the world. Although scientists As you progress through this textbook, your learning will involve arrive at their theories in different ways, the scientific method pro- two general goals: vides a way to validate (or invalidate) a particular hypothesis. Alter- natively, research may also involve the collection of data without " You will gather foundational knowledge. In other words, you a preconceived hypothesis. For example, researchers might analyze will be able to describe core concepts in genetics. For example, the genes found in cancer cells to identify those genes that have you will be able to explain how DNA replication occurs and become mutant. In this case, the scientists may not have a hypothesis describe the proteins that are involved in this process. 1.4 THE SCIENCE OF GENETICS 15 " You will develop problem-solving skills that allow you to of each chapter. Though there are many different problem-solving apply that foundational knowledge in different ways. For strategies, Genetic TIPS will focus on ten strategies that will help example, you will learn how to use statistics to determine if a you to solve problems. You will see these ten strategies over and genetic hypothesis is consistent with experimental data. over again as you progress through the textbook: The combination of foundational knowledge and problem-solving 1. Define key terms. In some cases, a question may be difficult skills will enable you not only to understand genetics, but also to to understand because you don’t know the meaning of one apply your knowledge in different situations. To help you develop or more key terms in the question. If so, you will need to these skills, Chapters 2 through 24 contain solved problems named begin your problem solving by defining such terms, either by Genetic TIPS, which stands for Topic, Information, and Problem- looking them up in the glossary or by using the index to find solving Strategy. These solved problems follow a consistent pattern. the location in the text where the key terms are explained. 2. Make a drawing. Genetic problems are often difficult to solve in your head. Making a drawing may make a big Genetic TIPS difference in your ability to see the solution. The Question: All of the Genetic TIPS begin with a question. 3. Predict the outcome. Geneticists may want to predict the As an example, let’s consider the following question: outcome of an experiment. For example, in Chapters 3 through 6, The coding strand of DNA in a segment of a gene is as follows: you will learn about different ways to predict the outcome of ATG GGC CTT AGC. This strand carries the information to genetic crosses. Becoming familiar with these methods will make a region of a polypeptide with the amino acid sequence help you to predict the outcomes of particular experiments. methionine-glycine-leucine-serine. What would be the con- 4. Compare and contrast. Making a direct comparison between sequences if a mutation changed the second cytosine (C) two things, such as two RNA sequences, may help you to in this sequence to an adenine (A)? understand how they are similar and how they are different. 5. Relate structure and function. A recurring theme in biology Topic: What topic in genetics does this question address? and genetics is that structure determines function. This The topic is gene expression. More specifically, the question relationship holds true at many levels of biology, including is about the relationship between a gene sequence and the the molecular, microscopic, and macroscopic levels. For genetic code. some questions, you will need to understand how certain Information: What information do you know based on the structural features are related to their biological functions. question and your understanding of the topic? 6. Describe the steps. At first, some questions may be difficult In the question, you are given the base sequence of a short seg- to understand because they may involve mechanisms that ment of a gene and told that one of the bases has been changed. occur in a series of several steps. Sometimes, if you sort out From your understanding of the topic, you may remember that the steps, you may identify the key step that you need to a polypeptide sequence is determined by reading the mRNA understand to solve the problem. (transcribed from a gene) in groups of three bases called codons. 7. Propose a hypothesis. A hypothesis is an attempt to explain an observation or data. Hypotheses may be made in many Problem-Solving Strategy: Compare and contrast. forms, including statements, models, equations, and diagrams. One strategy to solve this problem is to compare the mRNA 8. Design an experiment. Experimental design lies at the heart of sequence (transcribed from this gene) before and after the science. In many cases, an experiment begins with some type mutation: of starting material(s), such as strains of organisms or purified molecules, and then the starting materials are subjected to a Original: AUG GGC CUU AGC series of steps. The experiments featured throughout the textbook Mutant: AUG GGC AUU AGC will also help you refine the skill of designing experiments. 9. Analyze data. Because genetics is an experimental science, many problems involve the analysis of data, which are the Answer: The mutation has changed the sequence of bases in product of experiments. A variety of different statistical the mRNA so that the third codon has changed from CUU to methods are used to analyze data and make conclusions AUU (see arrow). Because codons specify amino acids, this about what the data mean. may change the third amino acid to something else. Note: If 10. Make a calculation. Genetics is a quantitative science. you look ahead to Chapter 15 (see Table 15.1), you will see Researchers have devised mathematical relationships to that CUU specifies leucine, whereas AUU specifies isoleu- understand and predict genetic phenomena. Becoming cine. Therefore, you would predict that the mutation would familiar with these mathematical relationships will help you change the third amino acid from leucine to isoleucine. to better understand genetic concepts and to make predictions. For most problems throughout this textbook, one or more of Throughout Chapters 2 through 24, each chapter will contain these strategies may help you to arrive at the correct solution. several Genetic TIPS. Some of these will be within the chapter Genetic TIPS will provide you with practice at applying these ten itself and some will precede the problem sets that are at the end problem-solving strategies. 16 C H A P T E R 1 : : OVERVIEW OF GENETICS 1.4 REVIEWING THE KEY CONCEPTS 1.4 COMPREHENSION QUESTION " Researchers in genetics carry out hypothesis testing or 1. The scientific method involves which of the following? discovery-based science. a. The collection of observations and the formulation of a hypothesis " Genetic TIPS are aimed at improving your ability to solve b. Experimentation problems. c. Data analysis and interpretation d. All of the above KEY TERMS Page 1. genome, deoxyribonucleic acid (DNA) Page 8. genetic variation, morphs, gene mutations, environment, Page 3. genetics, gene, traits norm of reaction Page 4. nucleic acids, proteins, carbohydrates, lipids, macromol- Page 9. phenylketonuria (PKU), diploid, homologs, somatic ecules, organelle, proteome cells, gametes, haploid Page 5. enzymes, nucleotides, polypeptides, genetic code, codon, Page 10. biological evolution, evolution, natural selection amino acid, chromosomes, gene expression, transcription, Page 12. model organisms, genetic cross ribonucleic acid (RNA), messenger RNA (mRNA), translation Page 13. genetic approach, loss-of-function mutation, loss-of- Page 6. morphological traits, physiological traits, behavioral traits function allele Page 7. molecular level, cellular level, organism level, species, Page 14. hypothesis testing, scientific method, discovery-based population level, alleles science CHAPTER SUMMARY " The complete genetic composition of a cell is called a genome. " During reproduction, genetic material is passed from parents to The genome encodes all of the proteins a cell can make. Many offspring. In many species, somatic cells are diploid and have key discoveries in genetics are related to the study of genes and two sets of chromosomes, whereas gametes are haploid and genomes (see Figures 1.1–1.3). have a single set (see Figure 1.11). " Evolution refers to a change in the genetic composition of a pop- 1.1 The Molecular Expression of Genes ulation from one generation to the next (see Figure 1.12). " Living cells are composed of nucleic acids (DNA and RNA), proteins, carbohydrates, and lipids. The proteome largely deter- 1.3 Fields of Genetics mines the structure and function of cells (see Figure 1.4). " Model organisms are studied by many different researchers so " DNA, which is found within chromosomes, stores the informa- they can compare their results and determine scientific princi- tion to make proteins (see Figure 1.5). ples that apply more broadly to other species (see Figure 1.13). " Most genes encode polypeptides that are units within functional " Genetics is traditionally divided into transmission genetics, proteins. Gene expression at the molecular level involves tran- molecular genetics, and population genetics, though overlap scription to produce mRNA and translation to produce a poly- occurs among these fields. peptide (see Figure 1.6). 1.2 The Relationship Between Genes and Traits 1.4 The Science of Genetics " Genetics spans the molecular, cellular, organism, and popula- " Researchers in genetics carry out hypothesis testing or tion levels (see Figure 1.7). discovery-based science. " Genetic variation underlies variation in traits. In addition, the " Genetic TIPS are aimed at improving your ability to solve environment plays a key role (see Figures 1.8–1.10). problems. PROBLEM SETS & INSIGHTS More Genetic TIPS 1. Most genes encode proteins. Explain how proteins produce an Information: What information do you know based on the organism’s traits. Provide examples. question and your understanding of the topic? Topic: What topic in genetics does this question address? In the question, you are reminded that most genes encode proteins The topic is the relationship between genes and traits. More spe- and that proteins play a role in producing an organism’s traits. cifically, the question is about how proteins, which are encoded by From your understanding of the topic, you may remember that genes, produce an organism’s traits. CONCEPTUAL QUESTIONS 17 proteins carry out a variety of functions that are critical to cell so common is that people who have one copy of the functional structure and function. CFTR gene and one copy of the mutant gene may be more Problem-Solving Strategy: Relate structure and function. resistant to diarrheal diseases such as cholera. Therefore, even though individuals with two mutant copies are very sick, peo- One strategy for you to solve this problem is to consider the ple with one mutant copy and one functional copy might have relationship between protein structure and function. Think about a survival advantage over people with two functional copies of examples in which the structure and function of proteins govern the gene. the structure and function of living cells. Also, consider how the structure and functions of cells determine an organism’s traits. Topic: What topic in genetics does this question address? The topic is how genetics can be viewed at different levels, rang- Answer: The structure and function of proteins govern the struc- ing from the molecular to the population level. ture and function of living cells. For example, specific proteins help determine the shape and structure of a given cell. The pro- Information: What information do you know based on the question and your understanding of the topic? tein known as tubulin can assemble into large structures known as microtubules, which provide the cell with internal structure The question describes the disease called cystic fibrosis. Parts and organization. The proteins that a cell makes are largely A through D give descriptions of various aspects of the disease. responsible for the cell’s structure and function. For example, From your understanding of the topic, you may remember that the proteins made by a nerve cell cause the cell to be very elon- genetics can be viewed at the molecular, cellular, organism, and gated and to be able to transmit signals from one cell to another. population level. This concept is described in Figure 1.7. The structure of a nerve cell provides animals with many traits, Problem-Solving Strategies: Make a drawing. Compare and

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