LS 15 Practice Questions

What’s the most important outcome of scientific thinking?

Scientific thinking tells us when to change our minds about the natural world. Let’s consider a question that might seem silly—even trivial—and watch our minds open.

In nationwide advertisements, the Dannon Company claimed that its Activia yogurt relieved irregularity and helped with “slow intestinal transit time.” Dannon also claimed that its DanActive dairy drink helped prevent colds and flu. These claims were based on no evidence. How would you design an experiment to try to test these statements?

Two claims were made, requiring two double-blind experiments, because only one variable can be changed or tested at a time. Experiment 1: Measure intestinal transit time (the time for consumed foods and other substances to travel through the digestive system) for two groups. Group 1 (control group) does not receive Activia yogurt, but instead receives an inert yogurt substitute. Group 2 (experimental group) receives Activia yogurt. Intestinal transit times are measured for both groups. Neither the participants nor the administrators know which participants are consuming Activia and which are consuming the substitute. Data from group 2, the experimental group, are compared with data from group 1, the control group, to determine the effectiveness of Activia. If the data show that intestinal transit time is shorter in the experimental group, the statement by the Dannon Company is supported. Experiment 2: Determine whether DanActive dairy drink helps prevent colds and flu, again using two groups. Group 1 (control group) does not receive DanActive dairy drink, but instead receives an inert dairy drink substitute. Group 2 (experimental group) receives DanActive dairy drink. The incidence of colds and flu is measured over a finite period of time, such as one year, for the two groups—the same period of time for both groups. Neither the participants nor the administrators know which participants are consuming DanActive dairy drink and which are consuming the substitute. Data collected from the experimental group are compared with data from the control group. If the incidence of colds and/or flu is lower in the experimental group, the statement made by the Dannon Company is supported.

Scientific thinking can be distinguished from alternative ways of acquiring knowledge in that it is empirical. What does empirical mean, and how does it relate specifically to the study of biology?

Empirical refers to knowledge gathered through observations and experiments that are rational, testable, and repeatable. Because the study of biology is concerned with phenomena in the natural world, it is appropriate that it is an empirical science. In contrast to this empirical approach, knowledge can also be gained through reason, reflection, and observation in the absence of experimentation.

An especially important feature of scientific thinking is that it is a self-correcting process. What does self-correcting mean in this context?

Predictions are made based on a formulated hypothesis and are tested with experiments. If a hypothesis is not supported by the experimental results, it can be revised, new predictions made, and these predictions tested with new experiments. Experiments can be repeated many times by many different scientists, and the hypothesis can again be revised and tested if conflicting results arise. In this way, the scientific method, done properly, is self-correcting.

Describe something that you have observed in the world around you that you could study using scientific thinking. Describe something that you could not study scientifically.

When a phenomenon can be measured, it can be tested through experimentation. To be testable by the scientific method, all examples would need to comply with this requirement. If a phenomenon cannot be measured, it cannot be tested using the scientific method; examples include determining the existence of God or the beauty of a Shakespearean sonnet.

A researcher hypothesizes that the more a person exercises, the less acne he or she will have. Write the null hypothesis for this experiment.

The null hypothesis would state that exercise does not affect the amount of acne a person develops.

What would be a reasonable prediction for the hypothesis “Eating fresh fruit reduces the likelihood that you will get sick”?

The prediction would be that a group of people who eat fresh fruit will have a lower rate of illness than another group of people who do not consume fresh fruit.

Describe the key features of a critical experiment

Key features of a critical experiment: Treatment: The condition that is changed for members of the experimental group. It is the variable (e.g., temperature, pressure, light) that is changed to test a hypothesis. Only the experimental group experiences this treatment; otherwise, comparison with the control group will not be possible and the hypothesis cannot be properly evaluated. Experimental group: The group that receives the treatment, which is the variable being manipulated to test the hypothesis. Control group: The group for which all variables, with the exception of the treatment, are the same as for the experimental group. This makes it possible to determine differences between the experimental group and the control group that result from the variable being tested. Variables: Characteristics that can be changed, such as temperature, light, pressure, or quantity and type of food, and can potentially elicit a change. Blind experimental design: For studies involving human beings, a study design in which the placebo effect and bias can be minimized by preventing members of the experimental group from knowing whether they are part of the experimental (treatment) or control group. However, the experimenters know who is a member of the experimental group or the control group. Double-blind experimental design: For studies involving human beings, a study design in which the placebo effect and bias can be minimized by preventing members of the experimental and control groups and the experimenters from knowing who is part of the experimental (treatment) and control groups. Randomized: The random selection of participants to be in either the experimental or control group.

Many claims have been made concerning the health benefits of green tea. Suppose you read a claim that alleges drinking green tea causes weight loss. You are provided with the following information about the studies that led to this claim: People were weighed at the beginning of the study. People were asked to drink two cups of green tea every day for 6 weeks. People were weighed at the end of the study. People who drank green tea for 6 weeks lost some weight by the end of the study. It was concluded that green tea is helpful for weight loss. This study obviously had some holes in its design. Assuming no information other than that provided here, list at least four things that could be done to improve the experimental design.

a) Include a control group that does not drink green tea and instead drinks water or some other inert drink. (b) Conduct this as a double-blind experiment. (c) Include a large number of people who would be representative of the population advised to drink green tea as a weight-loss method. (d) Stipulate a certain amount of green tea of the same brand to be consumed by study participants. (e) Ensure that only one variable, green tea consumption, differs between the experimental and control groups. In neither group should participants alter their consumption of food and beverages or change their total calories burned or consumed. (f) Perform a statistical analysis of collected data to determine whether weight loss is significant.

Following an experimental conclusion, what is a likely next step? Why?

If the experimental conclusion does not support the hypothesis, the hypothesis can be revised and retested. If the hypothesis is supported by the experimental conclusion, the experiment can be repeated by other scientists. This will ensure that the process is self-correcting. (Also important is the reporting of findings in professional science publications.)

Compare and contrast the terms theory and hypothesis

Both theories and hypotheses are attempts to understand a natural phenomenon. A hypothesis is an idea that has been proposed but not yet widely tested, whereas a theory has been widely tested by many different scientists, with repeated results to support it. Both hypotheses and theories can be revised as new information arises from research and experimentation.

A recent headline in The Washington Post read: “Your gym is teeming with germs.” The article was based on a 2019 research paper which documented that 50% of the treadmill handles and 56% of the weight plates (and many other surfaces) across dozens of gyms had been contaminated with Staphylococcus aureus, a bacterium sometimes associated with respiratory infections. But the bacterium is also found in the noses and throats of some healthy individuals. Propose an experiment and analyses to test whether using public gym facilities increases your likelihood of infection with S. aureus and your risk of illness. In your answer, be sure to state your hypotheses and their associated prediction(s).

Hypotheses: (a) Using public gym facilities increases the likelihood that your nose and throat will be contaminated with Staphylococcus aureus. (b) Using public gym facilities increases your risk of illness relative to not using such facilities.

Prediction: (a) The proportion of individuals having S. aureus present in their nose/throat will be greater among those using public gym facilities than among those not using such facilities. (b) Individuals using public gym facilities will experience a greater rate of illness than individuals not using such facilities. Experiment: Recruit 500 research participants representing a broad demographic range (including males and females of a wide range of ages). Randomly assign them to one of two groups: (a) public gym usage or (b) no public gym usage. Have those in group 1 use the gym for 45 minutes, four times a week, for ten weeks. Have those in group 2 not use a public gym facility (but work out for 45 minutes, four times a week, for ten weeks, in their own gym, not used by anyone else or completely sanitized after each usage). Have the individuals in each group record their daily food consumption and sleep patterns. Using a swab, take a bacterial sample from the nose/throat of each person at the beginning and the end of the study. Have each person keep a health diary for the ten weeks. In it, have them record their health status, including whether they are sick. Analyses: (a) For each of the two groups, evaluate the proportion with S. aureus present before and after the study. Compare these data for the two groups. You can consider your hypothesis supported if there is no difference in the proportion with S. aureus at the beginning of the study (meaning the groups are equivalent), but the proportion is higher among the public gym users at the end of the ten weeks. (b) For each of the two groups, evaluate the incidence and duration of illness. You can consider your hypothesis supported if the proportion of individuals experiencing illness is greater among those in the public-gym use group.

Describe what is meant by a randomized, controlled, double-blind study.

Randomized: Participants are randomly selected to be in either the experimental or control group. Controlled: There is a control group in which all variables, with the exception of the treatment, are the same as for the treatment group. This makes it possible to determine whether differences between outcomes for the experimental group and the control group result from the variable being tested. Double-blind: For studies involving human beings, the placebo effect and bias can be minimized by preventing members of the experimental and control groups and the experimenters from knowing who is part of the experimental (treatment) group and who is in the control group.

The American Dental Association (ADA) recommends cleaning between teeth with dental floss once a day to reduce the likelihood of suffering from a condition called gingivitis, the early stage of gum disease. In a recently published review of 12 published studies on the effects of flossing, however, researchers have questioned whether flossing has any value. What is the ADA’s hypothesis? What are the associated predictions? Propose an experiment and analysis you would conduct and describe how it would test this assertion.

Hypothesis: Flossing each day will reduce the frequency and severity of gingivitis. Prediction: Individuals flossing daily will experience a lower frequency of gingivitis than individuals not flossing. Experiment: Select a large group (more than 100,000) of individuals representative, demographically and socioeconomically, of the overall U.S. population. Randomly divide them into an experimental group, who are instructed how to use dental floss and required to floss once each day, and a control group, who are instructed to not floss. Then, at regular intervals throughout their lives—perhaps twice a year—evaluate all subjects. Determine the frequency and severity of gingivitis in the individuals in each group. Analyses: Compare whether the research participants who flossed daily have reduced frequency and severity of gingivitis. Because the only consistent and systematic difference between the two groups is whether they flossed, if flossing is beneficial, the flossing group will have reduced incidence and severity of gingivitis. (In the recent published study, the researchers concluded that there was no significant benefit to flossing!)

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How are statistical analyses used to analyze data?

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Statistics can help scientists determine whether differences between the control and experimental groups are a result of chance or are due to the experimental variable (treatment).

You notice that all of the male students who signed up for French tutoring have blue eyes. What conclusions can you draw from this observation using scientific thinking?

No conclusion can be drawn. The fact that the male students who signed up for French tutoring have blue eyes could be due to chance or to some biological differences. Without a randomized, controlled, double-blind experiment, there is no way to determine whether these observations result from chance.

You have heard that people from Scandinavia usually have blond hair and fair skin. Your roommate is from Sweden and he has darker hair and more olive skin. Does this disprove the idea that Scandinavians are fair and blond? Why or why not?

This does not disprove the hypothesis that Scandinavians are usually fair-skinned and blonde. There is a greater likelihood of people from this region being fair-skinned and blonde, but there will always be some people with genetic variation that includes darker skin and/or hair. This could result from variation within the population or migration of individuals into the area.

Describe the five major unifying themes in biology

The study of life is unified by five themes. (a) Evolution: Populations have the capacity to change over time. As a consequence of organisms’ ability to reproduce and of evolutionary change—the change in genetic characteristics of a population over time—populations may become better adapted to their environments. This accounts for the diversity of organisms and the unity among them. (b) Structure and function: Across all levels of organization, the physical features of organisms are closely related to function. (c) Information flow, exchange, and storage: Genetic information—instructions for biochemical, physical, and behavioral traits—is carried in all organisms and passed to their offspring. Its expression influences the organisms’ growth and functioning. (d) Pathways and transformations of energy and matter: Within organisms, pathways of chemical reactions use energy to transform matter and enable growth, movement, reproduction, and other processes. (e) Systems: Life is organized on many interconnected levels within individual organisms, including atoms, cells, tissues, and organs. And in the larger world, organisms themselves are organized into many levels: populations, communities, and ecosystems within the biosphere.

superstitions are:

A. evidence that not all phenomena can be understood purely through scientific thinking.

B. just one of many possible forms of scientific thinking. C. true beliefs that have yet to be fully understood.

D. irrational beliefs that actions not logically related to a course of events influence its outcome.

E. proof that the scientific method is not perfect.

D

Empirical results:

A. rely on intuition.

B. are generated by theories.

C. are based on observation.

D. cannot be replicated.

E. must support a tested hypothesis.

C

Which of the following statements is correct?

A. A hypothesis that does not generate a testable prediction is not useful.

B. Common sense is usually a good substitute for scientific thinking when trying to understand the world.

C. Scientific thinking can be used only to understand scientific phenomena.

D. It is not necessary to make observations as part of scientific thinking.

E. All of the above are correct.

A

The placebo effect:

A. is the frequently observed phenomenon that people tend to respond favorably to any treatment.

B. reveals that sugar pills are more effective than actual medications.

C. reveals that experimental treatments cannot be proven effective.

D. demonstrates that most scientific studies cannot be replicated.

E. is an urban legend.

A

In controlled experiments:

A. one variable is manipulated while others are held constant.

B. all variables are dependent on each another.

C. all variables are held constant.

D. all variables are independent of each another.

E. All critical variables are manipulated

A

If a researcher uses the same experimental setup as in another study, but with different research participants, the process is considered:

A. an uncontrolled experiment.

B. intuitive reasoning.

C. extrapolation.

D. replication.

E. exploration

D

An independent variable:

A. can cause a change in a dependent variable.

B. generally varies less than a dependent variable.

C. is plotted on the y-axis in a line graph.

D. can be controlled less well than a dependent variable. E. is typically more important than a dependent variable.

A

Statistical methods make it possible to:

A. prove that any hypothesis is true.

B. determine how likely it is that certain results have occurred by chance.

C. unambiguously learn the truth.

D. reject any hypothesis.

E. test non-falsifiable hypotheses.

B

Anecdotal evidence:

A. is the basis of scientific thinking.

B. tends to be more reliable than data based on observations of large numbers of diverse individuals.

C. is a necessary part of the scientific method.

D. is often the only way to prove important causal links between two phenomena.

E. can seem to reveal links between two phenomena, but the links may not actually exist.

E

Which of the following issues would be least helped by application of scientific thinking?

A. developing more effective high school curricula

B. evaluating the relationship between violence in video games and criminal behavior in teens

C. determining the most effective safety products for automobiles

D. formulating public policy on euthanasia

E. comparing the effectiveness of two potential antibiotics

D

Misconception: Evolution is a theory about the origin of life.

Reality: Although scientists are interested in (and are investigating) how life began, evolutionary biology focuses on the processes that occurred (and still occur) after life began, such as the diversification and branching of species and the adaptation of populations to their environments.

Misconception: Individuals evolve (and are “trying” to adapt).

Reality: Natural selection can lead to populations of organisms adapted to their environment (see Sections 10.10 and 10.13). But the process does not involve “trying” to adapt—it is a consequence of some individuals passing on their genes at a higher rate than individuals carrying alternative versions of those genes. The population, rather than any one individual, changes over time (see Section 10.10).

Misconception: Evolution is goal-oriented and leads to progress and “optimal” solutions.

Reality: Alleles that make it impossible for the individuals carrying them to live in a particular environment are weeded out of the population. But this process is not the same as creating perfect or optimal organisms (see Section 10.11). Rather, the traits and features of organisms that increase in frequency within a population are those that cause the organisms carrying them to leave more descendants than organisms with alternative traits (see Sections 10.8 and 10.11). As the environment changes, so do the traits that are most fit.

Misconception: There are no transitional fossils; the gaps in the fossil record disprove evolution

Reality: Because fossils are formed only under a narrow range of environmental conditions, biologists do not expect to find all transitional forms, so the gaps do not disprove evolution. Numerous transitional fossils have been found, including those between dinosaurs and modern birds, and those between whales and their terrestrial mammalian ancestors

Misconception: The theory of evolution says that life originated, and evolution proceeds, randomly, by chance.

Reality: Although chance is one element of evolution—there is a random element to the generation of mutations, for example—many nonrandom factors are central to how evolution proceeds. For example, the differential reproductive success of some variants—such as bacteria with antibiotic-resistance genes—that leads to natural selection is not random

Describe one benefit of conducting research on evolution in a population of short-lived species such as fruit flies.

Fruit flies reach maturity in about two weeks, so a scientist can study many generations during an experiment on starvation resistance. With an organism having a late age of maturity, it would require many years to complete a study such as the starvation resistance experiments.

Describe three commonly held Western beliefs about the natural world that were overturned in the 18th and 19th centuries.

It was commonly thought that (a) as in the biblical account, the earth was only about 6,000 years old; (b) the earth had not changed very much over time, with the exception of the occasional earthquake, flood, or volcanic eruption; and (c) all species, including humans, were created at the same time, and species never change or die out.

How did Darwin’s time traveling around the world help him develop his ideas on evolution?

Darwin found many fossils that were very similar to species still living in the same area. He also noticed that finches on the Galápagos Islands, which he assumed were all of the same species, had different physical characteristics on different islands. Biologists at the Zoological Society in London later found that there were 13 different finch species. All of these species closely resembled a single species of finch on the nearest mainland, in Ecuador.

What is evolution?

Evolution is a genetic change in a population over time. A change in allele frequency in the population occurs as a result of natural selection, mutations, genetic drift, and/or migration.

What does it mean when fixation for an allele occurs in a population?

Fixation for an allele occurs as a result of genetic drift, when an allele frequency becomes 100% in a population. As a result, there is no genetic variation for that gene. This can be detrimental because the population may be less able to adapt to environmental changes.

Do recessive alleles tend to decrease in frequency in a population? Why or why not?

The frequency of a recessive allele does not change in a population as long as the allele does not affect reproductive success. If a recessive allele does affect reproductive success, its frequency will change. Recessive alleles that adversely affect reproduction will decrease in frequency, and those that improve reproductive success will increase in frequency.

Describe three important components to an organism’s evolutionary fitness.

Fitness is a measure of the reproductive success of individuals. (a) An organism’s fitness depends on the environment in which it lives. The organism may be more fit in one environment, but less fit in another environment. (b) Fitness is relative to that of other organisms in the same environment. The fitness of one individual is measured against that of other individuals with specific genotypes and phenotypes. (c) The relative fitness of an individual is measured against that of other individuals of the same species in the same population. Those individuals that reproduce more frequently than others are more fit, irrespective of the age at which they die.

How does the increasing frequency of antibiotic-resistant strains of bacteria represent an example of evolution in progress?

The use of an antibiotic creates environmental pressure, and bacteria with one or more alleles imparting resistance to that antibiotic are more likely to reproduce than those that do not possess such alleles. Over time, bacteria with antibiotic-resistance alleles become more frequent in the population. Should another antibiotic be introduced, other alleles present in the population that impart resistance to that antibiotic will become more frequent over time.

In The Origin of Species, Charles Darwin wrote: “We may look with some confidence to a secure future of great length. And as natural selection works solely by and for the good of each being, all corporeal and mental endowments will tend to progress towards perfection.” Give three reasons why he was wrong.

(a) Natural selection requires time—sometimes more time than is allowed by the rate at which changes are occurring in the environment. The world’s environments are constantly changing and require continual evolutionary change if organisms are to maintain fitness. No single perfect organism for all environments will ever evolve. (b) Because mutations occur regardless of the needs of the organism, new mutations (i.e., new alleles) may not be those required for the organism to adapt to changes in its environment. (c) More than one allele may be suitable for selection in an environment, so there is no single optimal adaptation.

How does modern medicine alter the selective pressures on birth weight in humans?

Most babies weigh between 7 and 8 pounds at birth, as a result of stabilizing selection. Modern medicine has enabled very-low-weight, premature babies to survive, and very large babies to be delivered through cesarean births. This has prevented selection against extremes in the size of babies born, and greater numbers of babies of extreme size (small or large) may become more prevalent in the future.

How is biogeography useful when studying the evolutionary history of a population?

Scientists in the field of biogeography have observed that species migrate to nearby locations and adapt to their new habitats. Different species living less than 100 miles apart and occupying drastically different habitats resemble each other more than they resemble species many thousands of miles away that occupy very similar habitats. This suggests shared common ancestors among species that are close together geographically. An example is found on the continent of Australia. Many niches in Australia that are occupied by marsupials would elsewhere be occupied by placental mammals. For example, the Tasmanian wolf, now thought to be extinct, occupied the same type of habitat as gray wolves. However, the Tasmanian wolf is much more closely related to other marsupials living in Australia than to gray wolves found thousands of miles away.

Giving an example of each, compare and contrast homologous and analogous structures.

Analogous structures have similar functions and developed as a result of convergent evolution, forming from different starting materials in unrelated species. For example, the wings of insects, birds, and bats are all used for flying, but developed independently in each group of animals. Homologous structures develop in related species in which the same structures are modified to perform different functions. An example is the modification of the bones making up the forelimbs of mammals. All mammals have a common ancestor, but the bones of the forelimbs have been modified for different functions, such as flying, swimming, and walking.

When evolutionary biologists speak of a “molecular clock,” what do they mean?

A molecular clock is a way for biologists to determine how long two different species have been evolving separately. The longer the two species have been evolving separately, the greater are the differences in their genetic sequences. For example, humans and chimpanzees share many genes, whereas fruit flies and humans share much fewer genes.

Selecting for increased starvation resistance in fruit flies: a. has no effect, because starvation resistance is not a trait that influences fruit flies’ fitness.

B. has little effect, because ongoing mutation counteracts any benefits from selection.

C. cannot increase their survival time, because there is no genetic variation for this trait.

D. has no effect, because starvation resistance is dependent on the effects of too many genes.

E. can produce populations in which the average time to death from starvation is 160 hours.

E

Which of the following statements about mutations is incorrect?

A. Mutations are almost always random with respect to the needs of the organism.

B. A mutation is any change in an organism’s DNA.

C. Most mutations are harmful or neutral for the organism in which they occur.

D. The origin of genetic variation is mutation.

E. All of these statements are correct.

E

In a fish population in a shallow stream, the genotypic frequencies of yellowish-brown fish and greenish-brown fish changed significantly after a flash flood randomly swept away fish from the stream. This change in genotypic frequency was most likely attributable to:

A. gene flow.

B. disruptive selection.

C. directional selection.

D. stabilizing selection.

E. genetic drift.

A

When a small group of individuals colonizes a new habitat, an evolutionary event is likely to occur, because: a. members of a small population have reduced rates of mating.

B. gene flow increases.

C. mutations are more common in novel environments.

D. new environments tend to be inhospitable, reducing survival there.

E. small founding populations are rarely genetically representative of the initial population.

E

“Survival of the fittest” may be a misleading phrase to describe the process of evolution by natural selection because:

A. it is impossible to determine the fittest individuals in nature.

B. survival matters less to natural selection than does reproductive success.

C. natural variation in a population is generally too great to be influenced by differential survival.

D. during population bottlenecks, it is the least fit individuals that have the greatest survival.

E. reproductive success, on its own, does not necessarily guarantee evolution.

B

Adaptations shaped by natural selection:

A. are magnified and enhanced through genetic drift.

B. are unlikely to be present in humans living in industrial societies.

C. may be out of date, having been shaped in the past under conditions that differed from those in the present. D. represent perfect solutions to the problems posed by nature.

E. are continuously modified so that they are always matched to the environment in which a population lives.

C

In a population in which a trait is exposed to stabilizing selection:

A. neither the average value nor the variation for the trait changes.

B. both the average value and the variation for the trait increase.

C. the average value for the trait increases or decreases, and the variation for the trait decreases.

D. the average value for the trait stays approximately the same, and the variation for the trait decreases.

E. the average value for the trait stays approximately the same, and the variation for the trait increases.

D

Maze-running behavior in rats:

A. is too complex a trait to be influenced by natural selection.

B. is a heritable trait.

C. is not influenced by natural selection, because it does not occur in rats’ natural environment.

D. shows no variation.

E. is influenced primarily by mutation.

B

What can be concluded from comparing differences in molecular biology among different species?

A. Extremely different species are fundamentally unrelated in any way.

B. Only DNA sequences can be used to compare species’ relatedness.

C. Birds are more closely related to humans than dogs are.

D. Genetic similarities and differences demonstrate species relatedness.

E. The longer two species have been evolving on their separate paths, the fewer the genetic differences between them.

D

Evolution:

A. occurs too slowly to be observed.

B. can occur in the wild but not in the laboratory.

C. is responsible for the increased occurrence of antibiotic-resistant bacteria.

D. does not occur in human-occupied habitats.

E. None of these statements is correct.

C

Some fertility clinics now promise that, for as little as $200, they can help a couple choose the sex of their baby. Q: Given our knowledge of the behavior of the X and the Y chromosomes, is it more likely that these sex-selection techniques involve manipulations of sperm or of eggs? Why?

Because all eggs have X chromosomes, the X chromosome doesn’t play a role in determining the sex of a baby. Instead, fertility clinics must somehow sort and separate sperm cells based on whether they carry an X or a Y chromosome (men produce approximately equal numbers of each).

Q: What feature of sperm must be used in separating them?

This process must be based on a way of distinguishing between sperm carrying an X chromosome and sperm carrying a Y chromosome. After the two types of sperm are distinguished, they must then be separated. Sperm with the desired sex chromosome are then used to fertilize the woman’s egg (which may be done within her body, using artificial insemination, or in a Petri dish, after which a fertilized embryo is transferred into the woman’s reproductive tract).

Q: What techniques might make it possible to separate sperm with an X chromosome from those with a Y chromosome?

One method involves determining, by weighing sperm, which cells have more DNA. The heavier sperm must be carrying the X rather than the Y chromosome because the X is so much larger. Another method is to add a fluorescent dye to sperm that temporarily attaches to their DNA. Because sperm with an X chromosome have more DNA, more of the dye attaches to them and they are more fluorescent. A machine then sorts the sperm one by one, and at ovulation, insemination is performed using the sperm with the desired sex chromosome.

Q: Does it work?

The technique is still fairly new, but initial results suggest that the procedures have a success rate of 70% to 90%.

Each time a cell divides, telomeres get a little shorter. If a cell’s telomeres get too short, this can interfere with genes and lead to cell death. Why would a therapy that helped cells rebuild their telomeres not be the answer to the quest for “eternal youth”?

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Rebuilding telomeres could result in cells becoming cancerous. Individuals undertaking therapy of this kind would have a greater chance of developing and dying from cancer.

Compare and contrast the genetic information found in prokaryotes and in eukaryotes.

Prokaryotes typically have circular chromosomes attached to the cell membrane, whereas eukaryotes have linear, free-floating chromosomes in the nucleus. Eukaryotic cells have more genetic material than do prokaryotic cells. Eukaryotic cells typically have two (or sometimes more) copies of their chromosomes, contributed by maternal and paternal gametes. Because prokaryotic cells do not reproduce sexually, they do not typically have multiple copies of chromosomes.

Describe the two main phases of the eukaryotic cell cycle and what occurs during each

The main phases of the eukaryotic cell cycle are interphase, during which the cell grows and prepares for division, and mitosis (or M phase), during which the nucleus and genetic material divide, followed by division of the rest of the cellular contents. Interphase consists of several subphases. During Gap 1 ( G 1 ), the cell grows and performs normal functions such as protein synthesis and waste removal; cells spend most of their time in Gap 1. Some cells may enter a resting phase (sometimes for days or even years), called G 0 , when no cell division occurs. Cells destined for mitosis enter S phase (DNA synthesis), in which chromosomes are replicated such that each has an exact copy. In Gap 2 ( G 2 ), cells undergo a high rate of growth and protein synthesis in preparation for division. In M phase, or mitosis, the cell nucleus (including its chromosomes) divides. This generally is followed by cytokinesis, leading to division of the cytoplasm between the two daughter cells.

Describe the phase of the cell cycle during which the events make it possible to end up with sufficient genetic material for two identical daughter cells.

During the S phase (DNA synthesis) of interphase, the cell replicates its DNA, creating a duplicate copy of each chromosome (sister chromatids). Once this process is complete, the cell is ready to divide into two identical daughter cells.

Describe three ways in which cancer cells differ from “typical” cells. When someone dies of cancer, why aren’t the properties of the cancer cells a direct cause of death?

Cancer cells are unlike normal cells in that (a) they lack contact inhibition, (b) they can divide indefinitely, and (c) they have reduced “stickiness.” These cancer cell properties are indirect causes of death. Often, death occurs when cancer cells crowd out normal, healthy cells, causing organs and organ systems to fail

Why do the gametes produced by meiosis have only half the genetic material of the parent cell?

During meiosis, two cell divisions occur. The first division separates homologous chromosomes equally between two daughter cells. The second division separates sister chromatids equally into four daughter cells. The four daughter cells, called gametes, are haploid, meaning that they contain a single copy of each chromosome. When haploid gametes fuse during fertilization (the union of two gametes), the fertilized egg returns to the diploid state—in other words, it has two copies of each chromosome. If gametes were not haploid, fertilization would result in increasing numbers of chromosomes in each generation of offspring.

One important type of genetic variation does not require the creation of a new allele. How can that be?

Sexual reproduction produces genetic variation without the creation of new alleles, for the following reasons: (a) Alleles from two different parents are contributed during fertilization, producing offspring with a genetic makeup different from that of either parent. (b) Crossing over during prophase I of meiosis results in a mixture of maternal and paternal genetic information on sister chromatids. (c) Homologous chromosomes are randomly distributed to daughter cells during meiosis I, resulting in a mixing of maternal and paternal chromosomes.

Why is crossing over considered such an important event with respect to evolution?

During crossing over, sister chromatids swap segments of equal size between the homologous pairs of maternal and paternal chromosomes, producing sister chromatids that are no longer genetically identical. This results in unique genetic combinations and offspring that are genetically different from their parents—creating genetic variation on which natural selection can act in the process of evolution.

what are the differences between sex chromosomes and non-sex chromosomes

Non-sex chromosomes contain genetic information for traits other than gender-specific characteristics. The sex chromosomes contain genetic information that instructs the body to develop as male or female.

Healthy individuals may have just one sex chromosome, as long as it is an X chromosome. Why can’t a person survive with a Y chromosome and no X chromosome?

Unlike the X chromosome, the Y chromosome lacks essential genetic information needed for normal development. Information contained on the much smaller Y chromosome is limited to directing the development of male gonads.

Which of the following statements about telomeres is incorrect?

A. They function like a counter, keeping track of how many times a cell has divided.

B. At birth, they are long enough to permit approximately 80–90 cell divisions in most cells.

C. They are slightly shorter in prokaryotic cells than in eukaryotic cells.

D. They function like a protective cap on chromosomes. E. They contain no critical genes.

1. C

Prokaryotic cells can divide by:

A. mitosis.

B. binary fission.

C. meiosis.

D. both mitosis and binary fission.

E. none of the above.

2. B

Mitosis results in:

A. mitosis.

B. binary fission.

C. meiosis.

D. both mitosis and binary fission.

E. none of the above.

3. C

Using a light microscope, it is easiest to see chromosomes:

A. during mitosis and meiosis, because the condensed chromosomes are thicker and therefore more prominent. B. during interphase, when they are concentrated in the nucleus.

C. in the mitochondria, because the chromosomes are circular.

D. during asexual reproduction.

E. during interphase, because they are uncoiled and more linear.

4. A

Which of the following statements about tumors is incorrect? 

A. Benign tumors pose less of a health risk than malignant tumors.

B. Malignant tumors shed cancer cells that can spread in the body.

C. Tumors are caused by excessive cell growth and division.

D. Malignant tumor cells can travel to other parts of the body in a process called metastasis.

E. Tumors contain cells with abnormally high contact inhibition.

5. E

During meiosis but not during mitosis:

A. haploid gametes are produced that are identical in their allelic composition.

B. the cytoplasm divides.

C. chromosomes line up in the center of the cell during metaphase.

D. genetic variation among the daughter cells is increased.

E. two identical daughter cells are produced.

6. D

Sister chromatids are:

A. the result of crossing over.

B. identical molecules of DNA resulting from replication. C. homologous chromosomes.

D. produced in meiosis but not in mitosis.

E. single-stranded.

7. B

A potential disadvantage of asexual reproduction is that:

A. it increases the time required to find a mate.

B. it allows perpetuation of a population even when the members are isolated.

C. it allows population size to increase rapidly.

D. it produces genetically uniform populations.

E. it requires additional rounds of meiosis, which is energetically much more costly than mitosis.

8. D

A karyotype of one of your skin cells would reveal a total of 46 chromosomes. How many of these are maternally inherited non-sex chromosomes?

A. 20

B. 22

C. 23

D. 24

E. 46

9. B

Which of the following is not a method of chromosomal sex determination that occurs in nature?

A. In birds, the mother’s sex chromosomes determine the sex of the offspring.

B. In sea turtles, eggs laid in hot sand become females and eggs laid in cooler sand become males.

C. In humans, the presence of a Y chromosome makes an individual male, even if he also possesses two X chromosomes.

D. In bees, the eggs that the queen allows to be fertilized become females and the eggs not fertilized become males.

E. All of the above are naturally occurring examples of sex determination.

10. E

Why do offspring resemble their parents?

Offspring usually resemble their biological parents because they receive genes from their parents. Genes code for traits that determine physical characteristics that parents and offspring may share.

What was Mendel hoping to understand from his research on pea plants? How did he design his experiment to accomplish his goals?

Gregor Mendel wanted to understand how traits are passed from one generation to the next. For his research, he chose easily observable traits that he could categorize and count, in organisms (pea plants) that he could manipulate experimentally to understand their patterns of inheritance.

Why does a sperm or egg have only one copy of a gene?

The cells of diploid organisms, with the exception of gametes, have two copies (alleles) of each gene, one allele on each of a pair of homologous chromosomes. For offspring to inherit the same (diploid) number of chromosomes and genes, they must receive only one copy in each parent’s gamete. The two gametes combine to form a diploid zygote.

Compare and contrast the terms phenotype and genotype.

Phenotype describes the expression of alleles as physical, physiological, or behavioral characteristics. Genotype describes the alleles an organism has inherited, which determine the phenotypes expressed.

In what situation is a test-cross helpful in determining an individual’s genotype?

If an individual exhibits a dominant trait but has an unknown genotype, a test-cross can reveal the genotype (whether homozygous or heterozygous). The individual is crossed with an individual that is homozygous recessive for the trait. If all the offspring express the dominant trait, the unknown genotype is homozygous dominant. If half the offspring exhibit the recessive trait and half exhibit the dominant trait, the individual with the unknown genotype must be heterozygous for that trait.

What does it mean to be the carrier of a trait?

A “carrier” is an individual who carries one copy of a recessive allele, which is not expressed in the phenotype. There is no outward evidence that the individual is carrying the recessive allele.

Describe the phenotype of an individual heterozygous for an allele with incomplete dominance.

In incomplete dominance, an individual that is heterozygous for a trait expresses a phenotype intermediate between the phenotypes associated with the two alleles. An example is the snapdragon flower. If a true-breeding red-flowered plant is crossed with a true-breeding white-flowered plant, all their offspring have pink flowers—intermediate between red and white.

Describe why being heterozygous for the sickle-cell allele can confer resistance to malaria.

An individual who is heterozygous for the sickle-cell allele has resistance to malaria because the parasite that causes malaria does not survive well in sickled cells, the red blood cells with the defective hemoglobin.

Why are sex-linked recessive traits more commonly observed in males than in females?

Both the X and Y chromosomes are sex chromosomes. There are many more genes on the much larger X chromosome, with no corresponding alleles on the Y chromosome. If a recessive allele on the X chromosome is inherited by a male (XY), that allele will always be expressed.

How does the case of PKU support the notion that our phenotype is a product of our genotype in combination with the effect of our environment?

An individual with PKU inherits a defective allele for an enzyme needed to convert phenylalanine to tyrosine; in such an individual, phenylalanine builds up in cells, eventually increasing to toxic levels. If the person modifies his or her diet—that is, modifies the environment—to largely exclude phenylalanine, this amino acid will not increase to toxic levels in the body.

What is Mendel’s second law? Why doesn’t it apply for all traits?

Mendel’s second law states that the inheritance of one trait does not influence the inheritance of another trait. However, this is not true for traits determined by alleles located on the same chromosome, especially if they are close together. When a chromosome is inherited, groups of alleles, especially those found close together, are inherited together.

The more closely two genes are located on a chromosome, the less likely it is that alleles for those genes will be inherited independently. Why?

During meiosis, only one of the two copies of each chromosome ends up in a gamete. That copy is from either your mother or your father. All of the alleles on the chromosome from one parent get passed on as a group to the child at fertilization. The alleles for those genes remain together unless, during meiosis, recombination occurs, exchanging one or more alleles with the other chromosome in the pair so that they now become linked with the alleles on that chromosome. The closer two genes are on a chromosome, the less likely it is that a recombination event will occur between them, splitting up the maternal and paternal alleles.

Most genes come in alternative forms called:

A. alleles.

B. heterozygotes.

C. gametes.

D. chromosomes.

E. homozygotes.

1. A

All of the following are accurate descriptions of why pea plants were well suited for Mendel’s breeding experiments except:

A. pea plants exhibit variations for numerous observable characteristics, such as flower color and seed shape. 

B. Mendel could control the pollination between different pea plants.

C. it is easy to obtain large numbers of offspring from any given cross.

D. many of the characteristics that vary in pea plants are not linked closely on the same chromosome.

E. pea plants have a particularly long generation time.

2. E

The law of segregation states that:

A. the transmission of genetic diseases within families is always recessive.

B. an allele on one chromosome will always segregate from an allele on a different chromosome.

C. gametes cannot be separate and equal.

D. the number of chromosomes in a cell is always divisible by 2.

E. the two alleles for a given trait segregate into different gametes.

3. E

In pea plants, purple flower color is dominant to white flower color. If two pea plants that are true-breeding for purple flowers are crossed, in the offspring:

A. all of the flowers will be purple.

B. three-quarters of the flowers will be purple and one-quarter will be white.

C. half of the flowers will be purple and one-quarter will be white.

D. one-quarter of the flowers will be purple and three-quarters will be white.

E. all of the flowers will be white.

4. A

A test-cross:

A. makes it possible to determine the genotype of an individual of unknown genotype that exhibits the dominant version of a trait.

B. is a cross between an individual whose genotype for a trait is not known and an individual homozygous recessive for the trait.

C. sometimes requires the production of multiple offspring to reveal the genotype of an individual whose genotype is unknown (but who exhibits the dominant phenotype).

D. Only an and b are correct. 

E. Choices a, b, and c are correct.

5. E

All of the offspring of a black hen and a white rooster are gray. The simplest explanation for this pattern of inheritance is: 

\
A. multiple alleles.

B. Co-dominance.

C. incomplete dominance.

D. incomplete heterozygosity.

E. sex linkage.

6. C

A woman with type B blood and a man with type A blood could have children with which of the following phenotypes?

A. AB only

B. AB or O only

C. A, B, or O only

D. A or B only

E. A, B, AB, or O

7. E

A rare X-linked dominant condition in humans, congenital generalized hypertrichosis (CGH), is marked by excessive hair growth all over a person’s body. Which of the following statements about this condition is incorrect?

A. The son of a woman with CGH has just slightly more than a 50% chance of having this disease.

B. All daughters of a man with CGH will have this disease. C. The daughter of a woman with CGH has just slightly more than a 50% chance of having this disease.

D. Every son of a woman with CGH will have this disorder. E. The son of a man with CGH is no more likely to have this condition than the son of a man who doesn’t have CGH.

8. D