Biology 1101 Final Study Guide
Biology 1101 Final Study Guide BIOLOGY 1101 (Evan Waletzko)
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Biology 1101 Final Study Guide These notes are derived from the What is Life? Textbook used in our class. The take- home messages are important to know for the exam. Quiz 8 1. The distribution of terrestrial biomes across the globe corresponds most to which of the following? a. Climate b. Organisms c. Human influences d. Entropy 2. ____ and _____ are the major factors that determine climate. a. Wind; elevation b. Temperature; precipitation c. Elevation; precipitation d. Precipitation; wind e. Elevation; temperature 3. Most of the carbon in the earth's atmosphere exists as: a. Methane b. Carbon tetrachloride c. Chlorofluorocarbons (CFCs) d. Carbonic acid e. Carbon dioxide 4. When two species battle for resources until one is driven locally extinct, the process is called: a. Competitive exclusion b. Resource partitioning c. Mutualism d. Parasitism e. Character displacement 5. The "competitive exclusion" principle states that: a. K-selected species will always outcompete r-selected species b. Dominant species can always outcompete recessive species c. Only two species may occupy the same niche in a given community d. No two species competing for the same resources can stably coexist e. Predators will always competitively exclude their prey 6. Which of the following is an example of an abiotic factor? a. Disease b. Average life span c. Rainfall in an ecosystem d. Rate of predation on a prey population e. Average litter size 7. Which of the following organisms would occupy a higher trophic level? a. Bacteria b. Rabbit 1 c. Wolf d. Fungus e. Grass 8. Which type of organism takes the abiotic nutrients (nitrogen and phosphorus) out of the soil and brings it into the food chain/web? a. Hawks b. Moles c. Snakes d. Worms e. Plants 9. The phenomenon in which species that live together in the same environment tend to diverge in those characteristics that overlap is known as: a. Competitive exclusion b. Character displacement c. Resource partitioning d. Evolution e. Symbiosis 10. Keystone species: a. Occur only in intertidal zones b. Can be removed from a habitat without any impact on the remaining species in the habitat c. Are primary producers and, therefore, are usually plants d. Are more expendable than commensal species, from a conservation perspective e. Play an unusually important role in determining the species composition in a habitat 11. An ecosystem: a. Consists of all of the living animals in a given area b. Is composed of the plant life and climate of a given area c. Consists of all of the living and non-living things functioning together in an area d. Is composed of all of the abiotic factors that influence living organisms in a given area e. Consists of all of the living organisms in a given area 12. Roughly how much energy in a trophic level is available to the next trophic level? a. 10% b. 1% c. 75% d. 90% e. 50% 13. The largest reservoir of phosphorus in most ecosystems is: a. Dissolved in water b. In mycorrhizae c. Bound in below-ground plant tissues d. In the atmosphere e. In rocks and sediments 14. The "fundamental niche" of an organism is defined as the: a. Physical space that the organism occupies 2 b. Range of environmental conditions under which the organism can survive and reproduce c. Area that an organism occupies immediately after birth d. Area in which the organism reproduces e. Climatic conditions required for the organism to survive and reproduce 15. "Ecological succession" refers to: a. The effects on a trophic web when one species is removed b. A series of predictable changes in the composition or structure of an ecological community c. Human-induced (anthropogenic) changes in an ecological community d. The replacement of one type of vegetation with another e. The effects on an ecological community when a non-native species is introduced Quiz 9 1. Conservation biologists employ the methods and knowledge derived from which of these other fields of biology? a. Genetics b. Ecology c. Molecular biology d. Systematics and taxonomy e. All of the above choices are correct 2. What does it mean to say a species is a "threatened" species? a. That species has many natural predators. b. That species is having a portion of its habitat destroyed. c. That species was brought up to the point of extinction, then miraculously saved through conservative measures. d. That species is in immediate danger of extinction. e. That species is not in immediate danger of extinction but is likely to be so in the near future. 3. Which of the following troublesome exotic species was NOT introduced intentionally? a. Both the brown tree snake in Guam and the zebra mussel in the U.S. b. The zebra mussel in the United States. c. The brown tree snake in Guam. d. The monk parakeet in the United States. e. The cane toad in Australia. 4. Ozone depletion is a cause for concern because: a. Without ozone in the atmosphere, greenhouse gases can escape the Earth's atmosphere, leading to global warming. b. Without the protection against solar radiation provided by ozone, rates of skin cancer are likely to increase. c. Without ozone, fossil fuels are more likely to react with water vapor in the atmosphere, increasing levels of precipitation throughout the world. d. Reduced levels of ozone can increase the rate of respiratory problems in humans, particularly children. e. Without ozone, solar radiation can irradiate the Antarctic ice shelf, causing unsafe levels of radioactive isotopes. 3 5. A species can engender significant public support for conservation of other species and the ecosystem they inhabit is called a(n): a. Phylogenetic species b. Keystone species c. Indicator species d. Flagship species e. Endangered species 6. Madagascar is most important to conservation because it: a. Has the highest percentage of endemic plants and animals of any comparably sized area on earth b. Is the fourth-largest island in the world c. Is home to more endangered species than any other country d. Is the native habitat of the orsy periwinkle, a rainforest plant that helped to cure childhood lymphocytic leukemia e. Has more species per unit area than any place on earth 7. Which of the following is currently the leading cause of extinction? a. Habitat loss b. Overexploitation c. Disease d. Pollution e. Introduced species 8. In the United States, purple loosestrife is: a. A native flowering plant nearly brought to extinction by consumption by exotic insect pests. b. A native flowering plant that has been exported to Europe, where it has caused significant problems for native European species. c. An exotic flowering plant that has caused relatively few problems for native species. d. An exotic flowering plant that has caused significant problems for native species. e. A native flowering plant nearly brought to extinction by competition from exotic plants. 9. When sulfur burns, its IMMEDIATE product is: a. The gas sulfur monoxide (SO) b. The gas sulfur dioxide (SO2) c. Elemental sulfur (S) d. Sulfuric acid (H2S04) e. Sulfuric acid (H2S03) 10. In conservation biology, what does the term "corridor" mean? a. The edge of a given patch of habitat b. The path an organism takes from its home to its food source c. The path a conservative biologist takes through a habitat to minimize any negative effects on the habitat's biodiversity d. Sections of habitat that organisms use to travel between two or more isolated patches of habitat e. None of the above answers is correct. 4 11. Which of the following systems has the highest level of species richness and endemic species? a. Tropical dry forests b. Boreal forests c. Tropical moist forests d. Tropical grasslands e. Mangrove forests 12. What bird, once the most abundant in North America, was hunted to extinction by means of shooting and trapping during most of the 1800s? a. Eurasian house sparrow b. American robin c. Passenger pigeon d. Mourning dove e. European starling 13. The first species of plants to grow in after a farm field is abandoned are often: a. Orchids b. Shrubs c. Weeds d. Trees e. Epiphytes 14. Ozone is a molecule with the chemical formula: a. HO b. O4- c. O3 d. HO2- e. O2- 15. What does it mean in conservative biology to say a habitat is fragmented? a. The habitat consists of many different species living together. b. The habitat is found in more than one part of the world. c. The habitat is being studied by more than one conservative biologist. d. The habitat exists in patches that are physically isolated from each other. e. The habitat has areas that experience vastly different weather conditions. Quiz 10 1. Which of the following is currently the leading cause of extinction? a. Introduced species b. Habitat loss c. Overexploitation d. Pollution e. Disease 2. Which of the following is the most species-rich terrestrial biome? a. Tropical dry forests b. Tropical rain forests c. Mountains d. Mediterranean forests and shrublands e. Temperate forests 5 3. Which of the following is NOT a factor that limits population growth? a. Accumulation of waste products b. Density of parasites c. Insufficient food supply d. Lack of nesting space e. All of these factors may limit population growth 4. Keystone species: a. Are primary producers and, therefore, are usually plants. b. Occur only in intertidal zones. c. Are more expendable than commensal species, from a conservation perspective. d. Play an unusually important role in determining the species composition in a habitat. e. Can be removed from a habitat without any impact on the remaining species in the habitat. 5. An ecosystem: a. Consists of all of the living animals in a given area. b. Consists of all of the living and non-living things functioning together in an area. c. Is composed of all of the abiotic factors that influence living organisms in a given area. d. Is composed of the plant life and climate of a given area. e. Consists of all of the living organisms in a given area. Chapter 1: Scientific Overview 1.1 What is science? What is biology? TAKE-HOME MESSAGE 1.1: Through its emphasis on objective observation, description, and experimentation, science is a pathway by which we can discover and better understand the world around us. 1.2 Biological literacy is essential in the modern world. TAKE-HOME MESSAGE 1.2: Biological issues permeate all aspects of our lives. To make wise decisions, it is essential for individuals and societies to attain biological literacy. 1.3 Scientific thinking is a powerful approach to understanding the world. TAKE-HOME MESSAGE 1.3: There are numerous ways of gaining an understanding of the world. Because it is empirical, rational, testable, repeatable, and self-correcting, the scientific method is a particularly effective approach. 1.4 Thinking like a scientist: how do you use the scientific method? TAKE-HOME MESSAGE 1.4: The scientific method (observation, hypothesis, prediction, test, and conclusion) is a flexible, adaptable, and efficient pathway to understanding the world, because it tells us when we must change our beliefs. 1.5 Step 1: Make observations. TAKE-HOME MESSAGE 1.5: The scientific method begins by making observations about the world, noting apparent patterns or cause-and-effect relationships. 6 1.6 Step 2: Formulate a hypothesis. TAKE-HOME MESSAGE 1.6: A hypothesis is a proposed explanation for an observed phenomenon. 1.7 Step 3: Devise a testable prediction. TAKE-HOME MESSAGE 1.7: For a hypothesis to be useful, it must generate a testable hypothesis. 1.8 Step 4: Conduct a critical experiment. TAKE-HOME MESSAGE 1.8: A critical experiment is one that makes it possible to decisively determine whether a particular hypothesis is correct. 1.9 Step 5: Draw conclusions, make revisions. TAKE-HOME MESSAGE 1.9: Based on the results of experimental tests, we can revise a hypothesis and explain the observable world with increasing accuracy. A great strength of scientific thinking, therefore, is that it helps us understand when we should change our minds. 1.10 When do hypotheses become theories, and what are theories? TAKE-HOME MESSAGE 1.10: Scientific theories do not represent speculation or guesses about the natural world. Rather, they are hypotheses—proposed explanations for natural phenomena—that have been so strongly and persuasively supported by empirical observation that the scientific community views them as very unlikely to be altered by new evidence. 1.11 Controlling variables makes experiments more powerful. TAKE-HOME MESSAGE 1.11: To draw clear conclusions from experiments, it is essential to hold constant all those variables we are not interested in. Control and experimental groups should differ only with respect to the treatment of interest. Differences in outcomes between the groups can then be attributed to the treatment. 1.12 This is how we do it: Is arthroscopic surgery for arthritis of the knee beneficial? TAKE-HOME MESSAGE 1.12: In a well-controlled experiment, researchers demonstrated that arthroscopic knee surgery for osteoarthritis was no more beneficial for patients – in terms of knee pain and knew functioning – than a placebo surgery. 1.13 Repeatable experiments increase our confidence. TAKE-HOME MESSAGE 1.13: Experiments and their outcomes must be repeatable for their conclusions to be considered valid and widely accepted. 1.14 We’ve got to watch out for our biases. TAKE-HOME MESSAGE 1.14: Biases can influence our behavior, including our collection and interpretation of data. With careful controls, it is possible to minimize such biases. 1.15 Visual displays of data can help us understand and explain phenomena. TAKE-HOME MESSAGE 1.15: Visual displays of data, which condense large amounts of information, can aid in the presentation and exploration of the data. The effectiveness of such displays is influenced by the precision and clarity of the presentation, and it cbe reduced by 7 ambiguity, biases, hidden assumptions, and other issues that reduce a viewer’s confidence in the underlying truth of the presented phenomenon. 1.16 Statistics can help us in making decisions. TAKE-HOME MESSAGE 1.16: Because much variation exists in the world, statistics can help us evaluate whether any differences between a treatment group and a control group can be attributed to the treatment rather than random chance. 1.17 Pseudoscience and misleading anecdotal evidence can obscure the truth. TAKE-HOME MESSAGE 1.17: Pseudoscience and anecdotal observations often lead people to believe that links between two phenomena exist, when in fact there are no such links. 1.18 There are limits to what science can do. TAKE-HOME MESSAGE 1.18: Although the scientific method may be the most effective path toward understanding the observable world, it cannot give us insights into the generation of value judgments and other types of non-quantifiable, subjective information. 1.19 What is life? A few important themes unify and connect diverse topics in biology. TAKE-HOME MESSAGE 1.19: “Life” is not easily described with a simple definition. The characteristics shared by all living organisms include complex and ordered organization; the use and transformation of energy; responsiveness to the external environment; regulation and homeostasis; growth, development, and reproduction; and evolutionary adaptation leading to descent with modification. Chapter 2: Chemistry 2.1 Everything is made of atoms. TAKE-HOME MESSAGE 2.1: Everything around us, living or not, is made up of atoms, the smallest units into which material can be divided without losing its essential properties. All atoms have the same general structure. They are made up of protons and neutrons in the nucleus, and electrons, which circle far and fast around the nucleus. 2.2 An atom’s electrons determine whether (and how) the atom will bond with other atoms. TAKE-HOME MESSAGE 2.2: The chemical characteristics of an atom depend on the number of electrons in its outermost shell. Atoms are most stable and least likely to bond with other atoms when their outermost electron shell is filled to capacity. 2.3 Atoms can bond together to form molecules or compounds. TAKE-HOME MESSAGE 2.3: Atoms can be bound together in three different ways. Covalent bonds occur when atoms share electrons. In ionic bonds, one atom transfers its electrons to another and the two oppositely charged ions are attracted to each other, forming an ionic compound. Hydrogen bonds, which are weaker than covalent and ionic bonds, are formed from the attraction between a hydrogen atom and another atom with a slight negative charge. 2.4 Hydrogen bonds make water cohesive. TAKE-HOME MESSAGE 2.4: Water molecules easily form hydrogen bonds, giving water great cohesiveness. 8 2.5 Water has unusual properties that make it critical to life. TAKE-HOME MESSAGE 2.5: The hydrogen bonds between water molecules give water several of its most important characteristics, including cohesiveness, reduced density as a solid, the ability to resist temperature changes, and broad effectiveness as a solvent for ionic and polar substances. 2.6 Living systems are highly sensitive to acidic and basic conditions. TAKE-HOME MESSAGE 2.6: The pH of a fluid is a measure of how acidic or basic the + solution is and depends on the concentration of dissolved H ions present. Acids, such as vinegar, can donate protons to other chemicals; bases, including baking soda, bind with free protons. 2.7 This is how we do it: Do anti-acid drugs impair digestion and increase the risk of food allergies? TAKE-HOME MESSAGE 2.7: Dietary proteins are not digested by the stomach when the stomach’s pH is increased as a result of medications taken to treat ulcers, heartburn, and other digestive problems. This can put people who take anti-acid medications at risk for developing allergic responses to common foods. 2.8 Carbohydrates include macromolecules that function as fuel. TAKE-HOME MESSAGE 2.8: Carbohydrates are the primary fuel for running all cellular machinery and also form much of the structure of cells in all life forms. Carbohydrates contain carbon, hydrogen, and oxygen, and generally have the same number of carbon atoms as they do H 2 units. The simplest carbohydrates, including glucose, are monosaccharides or simple sugars. They contain from three to six carbon atoms. As the chemical bonds of carbohydrates are broken down and other more stable bonds are formed, a great deal of energy is released that can be used by organism. 2.9 Glucose provides energy for the body’s cells. TAKE-HOME MESSAGE 2.9: Glucose is the most important carbohydrate to living organisms. Glucose in the bloodstream can be used as an energy source, can be stored as glycogen in the muscles and liver for later use, or can be converted to fat. 2.10 Many complex carbohydrates are time-released packets of energy. TAKE-HOME MESSAGE 2.10: Multiple simple carbohydrates are sometimes linked together into more complex carbohydrates. Types of complex carbohydrates include starch, which is the primary form of energy storage in plants, and glycogen, which is a primary form of energy storage in animals. 2.11 Not all carbohydrates are digestible. TAKE-HOME MESSAGE 2.11: Some complex carbohydrates, including chitin and cellulose, cannot be digested by most animals. Such indigestible carbohydrates in the diet, called fiber, aid in digestion and have many health benefits. 2.12 Lipids are macromolecules with several functions, including energy storage. 9 TAKE-HOME MESSAGE 2.12: Lipids are insoluble in water and greasy to the touch. They are valuable to organisms for long-term energy storage and insulation, in membrane formation, and as hormones. 2.13 Fats are tasty molecules too plentiful in our diets. TAKE-HOME MESSAGE 2.13: Fats, including the triglycerides common in the food we eat, are one type of lipid. Characterized by long hydrocarbon tails, fats effectively store energy in the many carbon-hydrogen and carbon-carbon bonds. Their caloric density is responsible for humans’ preferring fats to other macromolecules in the diet, and is also responsible for their association with obesity and illness in the modern world. 2.14 Cholesterol and phospholipids are used to build sex hormones and membranes. TAKE-HOME MESSAGE 2.14: Cholesterol and phospholipids are lipids that are not fats. Both are important components in cell membranes. Cholesterol also serves as a precursor to steroid hormones, important regulators of growth and development. 2.15 Proteins are bodybuilding macromolecules. TAKE-HOME MESSAGE 2.15: Unique combinations of 20 amino acids give rise to proteins, the chief building blocks of the physical structures that make up all organisms. Proteins perform myriad functions, from assisting chemical reactions to causing blood clotting to building bones to fighting microorganisms. 2.16 Proteins are an essential dietary component. TAKE-HOME MESSAGE 2.16: Twenty amino acids make up all the proteins necessary for growth, repair, and replacement of tissue in living organisms. Of these amino acids, about half are essential for humans: they cannot be synthesized by the body so must be consumed in the diet. Complete proteins contain all essential amino acids, while incomplete proteins do not. 2.17 A protein’s function is influenced by its three-dimensional shape. TAKE-HOME MESSAGE 2.17: A protein’s particular amino acid sequence determines how it folds into a particular three-dimensional shape. This shape determines many of the protein’s properties, such as the molecules it will interact with. When a protein’s shape is deformed, the protein usually loses its ability to function. 2.18 Enzymes are proteins that speed up chemical reactions. TAKE-HOME MESSAGE 2.18: Enzymes are proteins that help initiate and speed up chemical reactions. They aren’t permanently altered in the process, but rather can be used again and again. 2.19 Enzymes regulate reactions in several ways (but malformed enzymes can cause problems). TAKE-HOME MESSAGE 2.19: Enzyme activity is influenced by physical factors such as temperature and pH, as well as by chemical factors, including enzyme and substrate concentrations. Inhibitors and activators are chemicals that bind to enzymes and, by blocking the active site or altering the shape or structure of the enzyme, can change the rate at which the enzyme catalyzes reactions. 2.20 Nucleic acids are macromolecules that store information. 10 TAKE-HOME MESSAGE 2.20: The nucleic acids DNA and RNA are macromolecules that store information in their unique sequences of bases contained in nucleotides, their building- block molecules. Both nucleic acids play central roles in directing protein production in organisms. 2.21 DNA holds the genetic information to build an organism. TAKE-HOME MESSAGE 2.21: DNA is shaped like a ladder in which the long vertical sides of the ladder are made from a sequence of sugar-phosphate-sugar-phosphate molecules and the rungs are pairs of nucleotide bases. The sequence of nucleotide bases contains the information about how to produce a particular protein. 2.22 RNA is a universal translator, reading DNA and directing protein production. TAKE-HOME MESSAGE 2.22: RNA acts as a middleman molecule—taking the instructions for protein production from DNA to another part of the cell where, in accordance with the RNA instructions, amino acids are linked together into proteins. Chapter 3: Cells 3.1 All organisms are made of cells. TAKE-HOME MESSAGE 3.1: The most basic unit of any organism is the cell, the smallest unit of life that can function independently and perform all of the necessary functions of life, including reproducing itself. All living organisms are made up of one or more cells, and all cells arise from other, pre-existing cells. 3.2 Prokaryotic cells are structurally simple but extremely diverse. TAKE-HOME MESSAGE 3.2: Every cell on earth is either a eukaryotic or a prokaryotic cell. Prokaryotes, which have no nucleus, were the first cells on earth. They are all single-celled organisms. Prokaryotes include the bacteria and archaea and, as a group, are characterized by tremendous metabolic diversity. 3.3 Eukaryotic cells have compartments with specialized functions. TAKE-HOME MESSAGE 3.3: Eukaryotes are single-celled or multicellular organisms consisting of cells with a nucleus that contains linear strands of genetic material. The cells also commonly have organelles throughout their cytoplasm; these organelles may have originated evolutionarily through endosymbiosis or invagination, or both. 3.4 Every cell is bordered by a plasma membrane. TAKE-HOME MESSAGE 3.4: Every cell of every living organism is enclosed by a plasma membrane, a two-layered membrane that holds the contents of a cell in place and regulates what enters and leaves the cell. 3.5 Molecules embedded in the plasma membrane help it perform its functions. TAKE-HOME MESSAGE 3.5: The plasma membrane is a fluid mosaic of proteins, lipids, and carbohydrates. Proteins found in the plasma membrane enable it to carry out most of its gatekeeping functions. The proteins act as receptors, help molecules enter and leave the cell, and catalyze reactions on the inner and outer cell surfaces. In conjunction with carbohydrates, some plasma membrane proteins identify the cell to other cells. And, in addition to the phospholipids 11 that make up most of the plasma membrane, cholesterol is an important lipid in some membranes, influencing fluidity. 3.6 Faulty membranes can cause disease. TAKE-HOME MESSAGE 3.6: Normal cell functioning can be disrupted when cell membranes—particularly the proteins embedded in them—do not function properly. Such malfunctions can cause health problems, such as cystic fibrosis. But intentional disruption of normal cell membrane function can have beneficial, therapeutic effects, such as in the treatment of high blood pressure and anxiety. 3.7 Membrane surfaces have a “fingerprint” that identifies the cell. TAKE-HOME MESSAGE 3.7: Every cell in your body has a “fingerprint” made from a variety of molecules on the outside-facing surface of the cell membrane. This molecular fingerprint is key to the function of your immune system. 3.8 Passive transport is the spontaneous diffusion of molecules across a membrane. TAKE-HOME MESSAGE 3.8: For proper functioning, cells must acquire food molecules and/or other necessary materials from outside the cell. Similarly, metabolic waste molecules and molecules produced for use elsewhere in the body must move out of the cell. In passive transport—which includes simple and facilitated diffusion and osmosis—the molecular movement occurs spontaneously, without the input of energy. This generally occurs as molecules move down their concentration gradient. 3.9 Osmosis is the passive diffusion of water across a membrane. TAKE-HOME MESSAGE 3.9: The diffusion of water across a membrane is a special type of passive transport called osmosis. Water moves from an area with a low concentration of solutes to an area with a higher concentration of solutes. Water molecules move across the membrane until the concentration of water inside and outside the cell is equalized. 3.10 In active transport, cells use energy to move small molecules into and out of the cell. TAKE-HOME MESSAGE 3.10: In active transport, movement of molecules across a membrane requires energy. Active transport is necessary if the molecules to be moved are very large or if they are being moved against their concentration gradient. Proteins embedded in the plasma membrane act like motorized revolving doors to actively transport (pump) the molecules. 3.11 Endocytosis and exocytosis are used for bulk transport of particles. TAKE-HOME MESSAGE 3.11: When materials cannot get into a cell by diffusion or through a pump (for example, when the molecules are too big), cells can engulf the molecules or particles with their plasma membrane in a process called endocytosis. Similarly, molecules can be moved out of a cell by exocytosis. In both processes, the plasma membrane moves to surround the molecules or particles and forms a little vesicle that is pinched off inside the cell (endocytosis) or fuses with the plasma membrane and dumps its contents outside the cell (exocytosis). 3.12 Connections between cells hold them in place and enable them to communicate with each other. TAKE-HOME MESSAGE 3.12: In multicellular organisms, most cells are connected to other cells. The connections can form a water-tight seal between the cells (tight junctions), can hold 12 sheets of cells together while allowing fluid to pass between the cell (desmosomes), or can function like secret passageways, allowing the movement of cytoplasm, molecules, and other signals between cells (gap junctions). The nucleus is the cell’s genetic control center. TAKE-HOME MESSAGE 3.13: The nucleus is usually the largest and most prominent organelle in the eukaryotic cell. It directs most cellular activities by controlling which molecules are produced and in what quantity. The nucleus is also the storehouse for hereditary information. 3.13Cytoplasm and the cytoskeleton form the cell’s internal environment, provide its physical support, and can generate movement. TAKE-HOME MESSAGE 3.14: The inner scaffolding of the cell, which is made from proteins, is the cytoskeleton. Consisting of three types of protein fibers—microtubules, intermediate filaments, and microfilaments—the cytoskeleton gives animal cells their shape and support, gives cells some ability to control their movement, and serves as a series of tracks on which organelles and molecules are guided across and around the inside of the cell. 3.13 Mitochondria are the cell’s energy converters. TAKE-HOME MESSAGE 3.15: In mitochondria, which are found in nearly all eukaryotic cells, the energy contained in the chemical bonds of carbohydrate, fat, and protein molecules is converted into carbon dioxide, water, and ATP, the energy source for all cellular functions and activities. Mitochondria may have their evolutionary origins as symbiotic bacteria living inside other cells. 3.14 This is how we do it: Can cells change their composition to adapt to their environment? TAKE-HOME MESSAGE 3.16: Form follows function in an organism’s cells and reflects their environment. When cells must perform intensive heat production, for example, they significantly increase the number and size of their mitochondria. They also increase the blood supply to the tissue and make use of existing stores of energy. 3.15 Lysosomes are the cell’s garbage disposals. TAKE-HOME MESSAGE 3.17: Lysosomes are round, membrane-enclosed, acid-filled organelles that function as a cell’s garbage disposals. They are filled with about 50 different digestive enzymes and enable a cell to dismantle macromolecules, including disease-causing bacteria. 3.16 In the endoplasmic reticulum, cells build proteins and lipids and disarm toxins. TAKE-HOME MESSAGE 3.18: The production and modification of biological molecules in eukaryotic cells occurs in a system of organelles called the endomembrane system, which includes, among other organelles, the rough and smooth endoplasmic reticulum. In rough ER, proteins that will be shipped elsewhere in the body are folded and packaged. In the smooth ER, lipids are synthesized and alcohol, antibiotics, and other drugs are detoxified. 3.17 The Golgi apparatus is the site where the cell processes products for delivery throughout the body. TAKE-HOME MESSAGE 3.19: The Golgi apparatus—another organelle within the 13 endomembrane system—processes molecules synthesized in a cell and packages those that are destined for use elsewhere in the body. 3.18 The cell wall provides additional protection and support for the plant cells. TAKE-HOME MESSAGE 3.20: The cell wall is an organelle found in plants (and in some other non-animal organisms). It is made primarily from the carbohydrate cellulose, and it surrounds the plasma membrane of the cell. The cell wall confers tremendous structural strength on plant cells, gives plants increased resistance to water loss, and provides some protection from insects and other animals that might eat the plant. In plants, plasmodesmata connect cells and enable communication and transport between them. 3.19 Vacuoles are multipurpose storage sacs for cells. TAKE-HOME MESSAGE 3.21: In plants, vacuoles can occupy most of the interior space of the cell. Vacuoles are also present in some other eukaryotic species. In planets, they function as storage spaces and play a role in nutrition, waste management, predator deterrence, reproduction, and physical support. 3.20 Chloroplasts are the plant cell’s power plant. TAKE-HOME MESSAGE 3.22: The chloroplast is the organelle in plants and algae that is the site of photosynthesis—the conversion of light energy into chemical energy, with oxygen as a by-product. Chloroplasts may have originally been bacteria that were engulfed by a predatory cell by endosymbiosis. Chapter 4: Energy 4.1 Cars that run on french fry oil? Organisms and machines need energy to work. TAKE- HOME MESSAGE 4.1: The sun is the source of the energy that powers most living organisms and other “machines.” The energy from sunlight is stored in the chemical bonds of molecules. When these bonds are broken, energy is released, regardless of whether the bond is in a molecule of food, of a fossil fuel, or of a biofuel such as the oil in which French fries are cooked. 4.2 Energy has two forms: kinetic and potential. TAKE-HOME MESSAGE 4.2: Energy, the capacity to do work, comes in two forms. Kinetic energy is the energy of moving objects, while potential energy, such as chemical energy, is stored energy that results from the position or location of an object. 4.3 As energy is captured and converted, the amount of energy available to do work decreases. TAKE-HOME MESSAGE 4.3: Energy is neither created nor destroyed but can change form. Each conversion of energy is inefficient, and some of the usable energy is converted to less useful heat energy. 4.4 ATP molecules are like free-floating rechargeable batteries in all living cells. TAKE-HOME MESSAGE 4.4: Cells temporarily store energy in the bonds of ATP molecules. This potential energy can be converted to kinetic energy and used to fuel life-sustaining chemical reactions. At other times, inputs of kinetic energy are converted to the potential energy of the energy-rich but unstable bonds in the ATP molecule. 14 4.5 Where does plant matter come from? Photosynthesis: the big picture. TAKE-HOME MESSAGE 4.5: Through photosynthesis, plants use water, the energy of sunlight, and carbon dioxide gas from the air to produce sugars and other organic materials. In the process, photosynthesizing organisms also produce oxygen, which makes all animal life possible. 4.6 Photosynthesis takes place in the chloroplasts. TAKE-HOME MESSAGE 4.6: In plants, photosynthesis occurs in chloroplasts, green organelles packed in cells near the plants’ surfaces, especially in the leaves. 4.7 Light energy travels in waves: Plant pigments absorb specific wavelengths. TAKE-HOME MESSAGE 4.7: Photosynthesis is powered by light energy, a type of kinetic energy made of energy packets called photons. Photons hit chlorophyll and other light-absorbing molecules in the chloroplasts of cells near the green surface of plants. These molecules capture some of the light energy and harness it to build sugar from carbon dioxide and water. 4.8 Photons cause electrons in chlorophyll to enter an excited state. TAKE-HOME MESSAGE 4.8: When chlorophyll is hit by photons, the light energy excites an electron in the chlorophyll molecule, increasing the chlorophyll’s potential energy. The excited electrons can be passed to other molecules, moving the potential energy through the cell. 4.9 Photosynthesis in detail: The energy of sunlight is captured as chemical energy. TAKE-HOME MESSAGE 4.9: There are two parts to photosynthesis. The first is the “photo” part, in which light energy is transformed into chemical energy, while splitting water molecules and producing oxygen. Sunlight’s energy is first captured when an electron in chlorophyll is excited. As this electron is passed from one molecule to another, energy is released at each transfer, some of which is used to build the energy-storage molecules ATP and NADPH. 4.10 Photosynthesis in detail: The captured energy of sunlight is used to make food. TAKE-HOME MESSAGE 4.10: The second part, or “synthesis” part, of photosynthesis is the Calvin cycle, which occurs in the stroma of chloroplasts. During this phase, carbon from CO 2n the atmosphere is attached (fixed) to molecules in chloroplasts, sugars are built, and molecules are regenerated to be used again in the Calvin cycle. The fixation, building, and regeneration processes consume energy from ATP and NADPH (the products of the “photo” part of photosynthesis). 4.11 The battle against world hunger can use plants adapted to water scarcity. TAKE-HOME MESSAGE 4.11: C4 and CAM photosynthesis are evolutionary adaptations at the biochemical level that, although being more energetically expensive than regular (C3) photosynthesis, allow plants in hot, dry climates to close their stomata and conserve water without shutting down photosynthesis. 4.12 How do living organisms fuel their actions? Cellular respiration: the big picture. TAKE-HOME MESSAGE 4.12: Living organisms extract energy through a process called cellular respiration, in which the high-energy bonds of sugar and other energy-rich molecules are 15 broken, releasing the energy that went into creating them. The cell captures the food molecules’ stored energy in the bonds of ATP molecules. This process requires fuel molecules and oxygen and it yields ATP molecules, water, and carbon dioxide. 4.13 The first step of cellular respiration: glycolysis is the universal energy-releasing pathway. TAKE-HOME MESSAGE 4.13: Glycolysis is the initial phase in the process by which all living organisms harness energy from food molecules. Glycolysis occurs in a cell’s cytoplasm and uses the energy released from breaking chemical bonds in food molecules to produce high- energy molecules, ATP and NADPH. 4.14 The second step of cellular respiration: the Krebs cycle extracts energy from sugar. TAKE-HOME MESSAGE 4.14: A huge amount of additional energy can be harvested by cells after glycolysis. First, the end product of glycolysis, pyruvate, is chemically modified. Then, in the Krebs cycle, the modified pyruvate is broken down step by step. This breakdown releases carbon into the atmosphere (as CO ) 2s bonds are broken, and captures some of the released energy in two ATP molecules and numerous high-energy electron carriers for every glucose molecule. 4.15 The third step in cellular respiration: ATP is built in the electron transport chain. TAKE-HOME MESSAGE 4.15: The largest energy payoff of cellular respiration comes as electrons from NADPH and FADH produ2ed during glycolysis and the Krebs cycle move along the electron transport chain. The electrons are passed from one carrier to another and energy is released, pumping protons into the mitochondrial intermembrane space. As the protons rush back into the mitochondrial matrix, the force of their flow fuels the production of large amounts of ATP. 4.16 This is how we do it: Can we combat the fatigue and reduced cognitive functioning of jet lag with NADH pills? TAKE-HOME MESSAGE 4.16: The symptoms of jet lag – including fatigue, memory loss, and reductions in cognitive performance – can impair the performance of people in many professions today. The results of a randomized, controlled, double-blind study support the hypothesis that an NADH supplement may be a suitable short-term countermeasure for these effects. 4.17 Beer, wine, and spirits are by-products of cellular metabolism in the absence of oxygen. TAKE-HOME MESSAGE 4.17: Oxygen deficiency limits the breakdown of fuel because the electron transport chain requires oxygen as the final acceptor of electrons during the chemical reactions of glycolysis and the Krebs cycle. When oxygen is unavailable, yeast resort to fermentation, in which they use a different electron acceptor, acetaldehyde, and in the process generate ethanol, the alcohol in beer, wine, and spirits. 4.18 Eating a complete diet: cells can run on protein and fat as well as on glucose. TAKE-HOME MESSAGE 4.18: Humans and other organisms have metabolic machinery that allows them to extract energy and other valuable chemicals from proteins, fats, and carbohydrates in addition to the simple sugar glucose. 16 Chapter 5: DNA, Gene Expression, and Biotechnology 5.1 Knowledge about DNA is increasing justice in the world. TAKE-HOME MESSAGE 5.1: DNA is a molecule that all living organisms carry in almost every cell in their body. Because every person’s DNA is unique and because we leave a trail of DNA behind us as we go about our lives, DNA can serve as an individual identifier. 5.2 The DNA molecule contains instructions for the development and functioning of all living organisms. TAKE-HOME MESSAGE 5.2: DNA is a nucleic acid, a macromolecule that stores information. It consists of individual units called nucleotides, which consist of a sugar, a phosphate group, and a nitrogen-containing base. DNA’s structure resembles a twisted ladder, with the sugar and phosphate groups serving as the backbones of the molecule and base pairs serving as the rungs. The sequence of bases on one side of the ladder-like DNA molecule complements that of the bases on the other side. 5.3 Genes are sections of DNA that contain instructions for making proteins. TAKE-HOME MESSAGE 5.3: DNA is a universal language that provides the instructions for building all the structures in all living organisms. The full set of DNA that an organism carries is called its genome. In prokaryotes, the DNA occurs in circular pieces. In eukaryotes, the genome is divided among smaller, linear strands of DNA. An organism’s DNA pieces are generally called chromosomes. A gene is a sequence of bases in a DNA molecule that carries the information necessary for producing a functional product, usually a polypeptide or RNA molecule. 5.4 Not all DNA contains instructions for making proteins. TAKE-HOME MESSAGE 5.4: Only a small fraction of DNA in the eukaryotic species is in genes that code for proteins; the function of much of the rest is still poorly understood, although at least some of it plays important roles in the cell, such as gene regulation. 5.5 How do genes work? An overview. TAKE-HOME MESSAGE 5.5: The genes in strands of DNA are a storehouse of information, an instruction book. The process by which this information is used to build an organism occurs in two main steps: transcription, in which a copy of a gene’s base sequence is made, and translation, in which that copy is used to direct the production of a polypeptide. 5.6 In transcription, the information coded in DNA is copied into mRNA. TAKE-HOME MESSAGE 5.6: Transcription is the first step in the two-step process of producing proteins base on instructions contained in DNA. In transcription (which occurs in the nucleus in eukaryotic cells), a single copy of one specific gene in the DNA is made, in the form of a molecule of mRNA. When the mRNA copy of a gene is completed, it moves to the cytoplasm, where it can be translated into a polypeptide. 5.7 In translation, the mRNA copy of the information from DNA is used to build functional molecules. 17 TAKE-HOME MESSAGE 5.7: Translation is the second step in the two-step process by which information carried in DNA directs the synthesis of proteins. In translation, the information from a gene that has been encoded in the nucleotide sequence of an mRNA is read, and ingredients present in the cell’s cytoplasm are used to produce a protein. 5.8 Genes are regulated in several ways. TAKE-HOME MESSAGE 5.8: Environmental signals influence the turning on and turning off of genes. By binding to DNA, regulatory proteins can block or facilitate the binding of RNA polymerase and subsequent transcription of genes. Regulation of gene expression also can occur in a variety of other ways that enhance or impede transcription, or alter mRNA’s longevity and rate of degradation, or influence translation or protein processing. 5.9 What causes a mutation, and what are its effects? TAKE-HOME MESSAGE 5.9: Mutations are alterations in a single base or changes in large segments of DNA that include several genes or more. They are rare, but when they do occur, they may disrupt normal functioning of the body (although many mutations are neutral). Extremely rarely, mutations may have a beneficial effect. Mutations play an important role in evolution. 5.10 This is how we do it: Does sunscreen use reduce skin cancer risk? TAKE-HOME MESSAGE 5.10: The relationship between sunscreen use and skin cancer is important but murky. Numerous case-controlled studies suggested that sunscreen use increase the incidence of melanoma, the most deadly type of skin cancer. But a more powerful, randomized controlled approached demonstrated that regular sunscreen use significantly reduces the risk of melanoma. 5.11 Faulty genes, coding for faulty enzymes, can lead to sickness. TAKE-HOME MESSAGE 5.11: Many genetic diseases result from mutations that cause a gene to produce a non-functioning enzyme, which in turn blocks the functioning of a metabolic pathway. 5.12 What is biotechnology? TAKE-HOME MESSAGE 5.12: Biotechnology is the use of technology to modify organisms, cells, and their molecules to achieve practical benefits. Modern molecular methods make it possible to cut and copy DNA from one organism and deliver it into another. The methods include the use of naturally occurring restriction enzymes for cutting DNA, the polymerase chain reaction for amplifying small amounts of DNA, insertion of the DNA into bacterial or viral vectors, and the cloning and identification of cells with the transferred DNA of interest. 5.13 Biotechnology can improve food nutrition and make farming more efficient and eco- friendly. TAKE-HOME MESSAGE 5.13: Biotechnology has led to important improvements in agriculture by using transgenic plants and animals to produce more nutritious food. Even more significant is the extent to which biotechnology has reduced the environmental and financial costs of producing food, through the creation of herbicide-resistant and insect-resistant crops. 18 The ecological and health risks of such widespread use of transgenic species are not full understood and are potentially great. 5.14 Fears and risks: are genetically modified foods safe? TAKE-HOME MESSAGE 5.14: More and more genetically modified foods are being created using modern methods of recombinant DNA technology. Amount the public, however, numerous legitimate fears remain about the potentially catastrophic risks of these foods, given that their development relies on such new technology, and about the long-term financial advantages they offer. 5.15 The treatment of diseases and production of medicines are improved with biotechnology. TAKE-HOME MESSAGE 5.15: Biotechnology has led to some notable successes in treating diseases, usually by producing medicines more efficiently and effectively than they can be produced with traditional methods. 5.16 Gene therapy: biotechnology can help diagnose and prevent diseases, but has had limited success in curing them. TAKE-HOME MESSAGE 5.16: Biotechnology tools have been developed to reduce suffering and the incidence of diseases, but come with significant potential costs. Gene therapy has had a poor record of success in curing human diseases, primarily because of success in curing human diseases, primarily because of technical difficulties in transferring normal-functioning genes into the cells of a person with a genetic disease. 5.17 Cloning—ranging from genes to organs to individuals—offers both promise and perils. TAKE-HOME MESSAGE 5.17: Cloning of individuals has potential benefits in agriculture and medicine, but ethical questions linger 5.18 DNA is an individual identifier: the uses and abuses of DNA fingerprinting. TAKE-HOME MESSAGE 5.18: Comparisons of highly variable DNA regions can be used to identify tissue specimens and determine the individual from whom they came. Chapter 6: Chromosomes and Cell Division 6.1 Immortal cells can spell trouble: cell division in sickness and in health. TAKE-HOME MESSAGE 6.1: Cell division is an ongoing process in most organisms and their tissues; disruptions to normal cell division can have serious consequences. In eukaryotic cells, a protective section of DNA called the telomere, at each end of every chromosome, plays a role in keeping track of cell division, getting shorter every time the cell divides. If telomeres become too short, additional cell divisions cause the loss of essential DNA and cell death. Cells that rebuild their telomeres with each division can become cancerous. 6.2 Some chromosomes are circular, others are linear. TAKE-HOME MESSAGE 6.2: In most bacteria and archaea, the genetic information is carried in a single, circular chromosome, a strand of DNA that is attached at one site to the cell membrane. Eukaryotes have much more DNA that do bacteria and organize it into linear chromosomes within the nucleus. Bacteria divide by a type of asexual reproduction called binary 19 fission: first, the circular chromosome duplicates itself, then the parent cell splits into two new, genetically identical daughter cells. 6.3 There is a time for everything in the eukaryotic cell cycle. TAKE-HOME MESSAGE 6.3: Eukaryotic somatic cells alternate in a cycle between cell division and other cell activities. The cell division portion of the cycle is called the mitotic phase. The remainder of the cell cycle, called interphase, consists of two gap phases (during which cell growth and other metabolic activities occur) separated by a DNA synthesis phase during which the genetic material is replicated. A cell-cycle control system functions through a series of checkpoints, critical points in the cell cycle at which progress is blocked – and cells are prevented from dividing – until specific signals trigger continuation of the process. 6.4 Cell division is preceded by chromosome replication. TAKE-HOME MESSAGE 6.4: Every time a cell divides, that cell’s DNA must first duplicate itself so that each of the two new daughter cells has all the genetic material of the original parent cell. The process of DNA duplication, called replication, is catalyzed by several important enzymes and occurs in two steps: unwinding and separation of the two strands, and reconstruction and elongation of the new complementary strands. The end result is two double- stranded DNA molecules that carry virtually the same genetic information as the parent DNA. Although enzymes proofread and repair DNA during and after replication, some errors may remain. 6.5 Most cells are not immortal: mitosis generates replacements. TAKE-HOME MESSAGE 6.5: Mitosis enables existing cells to generate new, genetically identical cells. This makes it possible for organisms to grow and to replace cells that die. 6.6 Overview: mitosis leads to duplicate cells. TAKE-HOME MESSAGE 6.6: Mitosis is the process by which cells duplicate themselves. Mitosis follows chromosome replication and leads to the production of two daughter cells from one parent cell. 6.7 The details: mitosis is a four-step process. TAKE-HOME MESSAGE 6.7: The ultimate result of mitosis and cytokinesis is the production of two genetically identical cells. 6.8 Cell division out of control may result in cancer TAKE-HOME MESSAGE 6.8: Cancer is unrestrained cell growth and cell division, which lead to large masses of cells that may cause serious health problems. It often results from mutations to genes important in controlling the cell cycle, thus reducing the effectiveness of the checkpoints. Treatment focuses on killing or slowing down the fast-growing and dividing cells, usually using chemotherapy and/or radiation. 6.9 Overview: Sexual reproduction requires special cells made by meiosis. TAKE-HOME MESSAGE 6.9: Meiosis is the process by which reproductive cells are produced in sexually reproducing organisms. It results in gametes that have only half as much 20 genetic material as the parent cell and that differ from one another with respect to the combinations of alleles they carry. 6.10 Sperm and eggs are produced by meiosis: the details, step by step. TAKE-HOME MESSAGE 6.10: Meiosis in animals occurs only in gamete-producing cells. It is preceded by DNA replication and consists of two rounds of cellular division, one in which homologous pairs of sister chromatids separate and a second in which the sister chromatids separate. The final product of meiosis in a diploid organism is four haploid gametes. 6.11 Male and female gametes are produced in slightly different ways. TAKE-HOME MESSAGE 6.11: In species with two sexes—including nearly every sexually reproducing plant and animal species—females are the sex that produces the larger gamete, and males produce the smaller gamete. Whether it is male or female gametes that are being produced, each gamete ends up with just one copy of each chromosome. 6.12 Crossing over and meiosis are important sources of variation. TAKE-HOME MESSAGE 6.12: Although it doesn’t create any new traits, crossing over during the first prophase of meiosis creates gametes with collections of traits that may never have existed before; this variation is important for evolution. 6.13 What are the costs and benefits of sexual reproduction? TAKE-HOME MESSAGE 6.13: There are two fundamentally different ways that cells and organisms can reproduce: (1) mitosis and asexual reproduction, and (2) meiosis and sexual reproduction. Asexual reproduction can be fast and efficient, but it leads to genetically identical offspring that carry all of the genes that their parent carried, which could be disadvantageous in a changing environment. Sexual reproduction leads to offspring that are genetically different from one another and from either parent, but it takes more time and energy and can be risky. 6.14 How is sex determined in humans? TAKE-HOME MESSAGE 6.14: In humans, the sex chromosomes carry information that directs a growing fetus to develop as either a male (if a Y chromosome is present) or a female (if no Y chromosome is present). Sex determination depends on the sex chromosomes inherited from the father. 6.15 The sex of offspring is determined in a variety of ways in non-human species. TAKE-HOME MESSAGE 6.156: A variety of methods are used for sex determination across the world of plant and animal species. These include the presence or absence of sex chromosomes, the number of chromosome sets, and environmental factors such as incubation temperature. 6.16 This is how we do it: Can the environment determine the sex of a turtle’s offspring? TAKE-HOME MESSAGE 6.16: Observations of some lizard and turtle species reveal that sex determination is influenced by the temperature during incubation of the eggs. Both in the lab and in natural habitats, at cooler temperatures more males develop, and at warmer temperatures more females develop. Climate change is predicted to bring about an increase of about 4° C in North America in the next 100 years. This increase is likely to have adverse effects on turtles with 21 temperature-dependent sex determination and may signal far greater impacts of climate change on biological systems. 6.17 Down syndrome can be detected before birth: karyotypes reveal an individual’s entire chromosome set. TAKE-HOME MESSAGE 6.17: A karyotype is a visual display of a complete set of chromosomes. It is a useful diagnostic tool because it can be prepared early in fetal development to assess whether there is an abnormality in the number of chromosomes or in their structure, such as in Down syndrome. Down syndrome is caused by having an extra copy of chromosome 21. 6.18 Life is possible with too many or too few sex chromosomes. TAKE-HOME MESSAGE 6.18: Although it is usually fatal to have one too many or one too few of the non-sex chromosomes, individuals born with only a single sex chromosome that is an X, or with an additional X or Y chromosome, usually survive—though often with physical and/or mental problems. Chapter 7: Genes and Inheritance 7.1 Family resemblance: Your mother and father each contribute to your genetic makeup. TAKE-HOME MESSAGE 7.1: Offspring resemble their parents because they inherit genes— instruction sets for biochemical, physical, and behavioral traits, some of which are responsible for diseases—from their parents. 7.2 Some traits are controlled by a single gene. TAKE-HOME MESSAGE 7.2: Some traits are determined by instructions an individual carries on a single gene, and these traits exhibit straightforward patterns of inheritance. 7.3 Mendel learned about heredity by conducting experiments. TAKE-HOME MESSAGE 7.3: In the mid-1800s, Gregor Mendel conducted studies that helped us understand heredity. He focused on easily observed and categorized traits in garden peas and applied methodical experimentation and rigorous hypothesis testing to determine how traits are inherited. 7.4 Segregation: You’ve got two copies of each gene but put only one copy in each sperm or egg. TAKE-HOME MESSAGE 7.4: Each parent puts a single set of instructions for building a particular trait into every sperm or egg it makes. This instruction set is called a gene. The trait observed in an individual depends on the two copies (alleles) of the gene it inherits from its parents. 7.5 Observing an individual’s phenotype is not sufficient for determining its genotype. TAKE-HOME MESSAGE 7.5: It is not always possible to determine an individual’s genetic makeup, known as its genotype, by observation of the organism’s outward appearance, known as its phenotype. For a particular trait, an individual may carry a recessive allele whose phenotypic effect is masked by the presence of a dominant allele. Much genetic analysis makes use of clever 22 experiments and careful record-keeping, often using Punnett squares, to determine organisms’ genotypes. 7.6 Chance is important in genetics. TAKE-HOME MESSAGE 7.6: Probability plays a central role in genetics. In segregation, each gamete that an individual produces receives only one of the two copies of each gene the individual carries in its other cells, but it is impossible to know which allele goes into the gamete. Chance plays an important role in fertilization, too: all of the sperm or eggs produced by an individual are different from one another, and any one of those gametes may be the gamete involved in fertilization. 7.7 A test-cross enables us to figure out which alleles an individual carries. TAKE-HOME MESSAGE 7.7: In a test-cross, an individual that exhibits a dominant trait but has an unknown genotype is mated with an individual that is homozygous recessive. The phenotypes of the offspring reveal whether the unknown-genotype individual is homozygous dominant (all of the offspring exhibit the dominant trait) or heterozygous (half of the offspring show the dominant trait and half show the recessive trait). 7.8 We use pedigrees to decipher and predict the inheritance patterns of genes. TAKE-HOME MESSAGE 7.8: Pedigrees help scientists, doctors, animal and plant breeders, and prospective parents determine the genes that individuals carry and the likelihood that the offspring of two individuals will exhibit a given trait. 7.9 Incomplete dominance and codominance: the effects of both alleles in a genotype can show up in the phenotype. TAKE-HOME MESSAGE 7.9: Sometimes the effects of both alleles in a heterozygous genotype are evident in the phenotype. With incomplete dominance, the phenotype of a heterozygote appears to be an intermediate blend of the phenotypes of the two homozygotes. With codominance, a heterozygote has a phenotype that exhibits characteristics of both homozygotes. 7.10 What’s your blood type? Some genes have more than two alleles. TAKE-HOME MESSAGE 7.10: In multiple allelism, a single gene has more than two alleles. Each individual still carries only two alleles, but in the population, more than just two alleles exist. This is the case for the ABO blood groups in humans. 7.11 Multigene traits: how are continuously varying traits such as height influenced by genes? TAKE-HOME MESSAGE 7.11: Many traits, including continuously varying traits such as height, eye color, and skin color, are influenced by multiple genes. 7.12 Sometimes one gene influences multiple traits. TAKE-HOME MESSAGE 7.12: In pleiotropy, one gene influences multiple, unrelated traits. Most, if not all, genes may be pleiotropic. 7.13 Why are more men than women colorblind? Sex-linked traits differ in their patterns of expression in males and females. 23 TAKE-HOME MESSAGE 7.13: The patterns of inheritance of most traits do not differ between males and females. However, when a trait is coded for by a gene on a sex chromosome, such as color vision on the X chromosome, the pattern of expression differs for males and females. 7.14 This is how we do it: What is the cause of male-pattern baldness? TAKE-HOME MESSAGE 7.14: Observations of male-pattern baldness within families and comparisons with unrelated individuals suggest that the baldness is caused by a sex-linked gene that codes for an androgen receptor. Males inheriting an allele – always from their mother – for higher activity of the androgen receptor gene are more likely to have male-pattern baldness than males inheriting an alternative allele. 7.15 Environmental effects: Identical twins are not identical. TAKE-HOME MESSAGE 7.15: Genotypes are not like blueprints that specify phenotypes. Phenotypes are generally a product of the genotype in combination with the environment. 7.16 Most traits are passed on as independent features: Mendel’s law of independent assortment. TAKE-HOME MESSAGE 7.16: Genes tend to behave independently, such that the inheritance pattern of one trait doesn’t usually influence the inheritance of any other trait. 7.17 Red hair and freckles: genes on the same chromosome are sometimes inherited together. TAKE-HOME MESSAGE 7.17: Sometimes, having one trait influences the
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