The Question:
How might mutations occur at the genetic level, prokaryotic vs. eukaryotic genome
Background information:
This activity challenges students to explore the methods of science during a review of fundamental concepts in genetics. A brief discussion of the methods of science is included here for your convenience. How do we know what we know? This activity deals with two aspects of science that help to answer that question:(1) scientific explanations require credible evidence, and(2) new work builds on old work.
The relationship between evidence and explanation is built on several assumptions. The evidence must be credible according to scientific criteria such as accuracy and repeatability. Authority or fame alone does not establish validity for a particular hypothesis or for opinions offered as evidence. Recognition of the authority or fame of the proponent of a scientific idea may attract attention, but this notoriety in itself is not sufficient for the scientific community to accept the idea unless sufficient credible evidence supports the claim. Evidence also must relate to the issue in question; even credible evidence is not useful when it does not address the question under consideration.
Another step to determine what we know scientifically is to examine an explanation for the soundness of its reasoning. A good explanation starts with accurate assumptions, is guided by credible evidence, and is built through logical connections. If this process is done well, the explanation will have predictive power. Because many scientific explanations aim to discover causal relationships, explanations can be tested according to their ability to predict specific outcomes of the proposed causal event. Causality is a related, but not central, concept in Activity 1, and you may want to expand your discussions to include it. Question 1 of the Analysis is a good place to help students understand what is important about causality and how to establish it.
Finally, science is embedded in cultural history. Science proceeds as a chronological sequence of discoveries, but the paths it follows largely reflect the culture that supports it. Topics of concern to scientists at a particular time reflect many aspects of societal values. For example, if society views cancer, heart disease, and AIDS as important problems, agencies in the public and private sector will make funds available for research in those areas. Research that attracts attention also depends on the current level of technology for acquiring, storing, and analyzing data. Sometimes, a creative idea or question cannot be formulated as a testable hypothesis because of the limitations in technology, resources, or existing information. In these cases, the great idea must wait until history creates the right setting for it.
Strategies for teaching this activity.
The foregoing aspects of science are incorporated into the tasks of this activity. Although the Nature and Methods of Science concepts may not be entirely new to students, these concepts often are not presented in a way that makes them useful. A student may have a concrete definition of how science works, but may not be able to apply the concept either in thinking or action. This activity, is designed to take the students' understanding of the methods of science that extra step into experience.In Part I of this activity, students see how new work builds on old work and how milestone explanations attempt to establish a causal relationship. Students arrange the milestones into a sequence that makes connections between ideas, focusing on the concepts rather than on the dates of discovery (knowing the dates of the discoveries are not the objective.). Ask students to describe the basis for the sequence of milestones they construct - for example, simple to complex or general to specific - and to speculate why this sequence might differ from the historical sequence. Emphasize that the historical sequence is not necessarily "the right answer," but do not hesitate to challenge what appear to be conceptual flaws in a student's sequence. For example, molecular details about mutation and the genetic code would not precede knowledge that DNA is the genetic material.
In Part II, students focus on the role of credible evidence to support a valid scientific explanation. In each team, students study four pieces of evidence to judge their credibility and their usefulness relative to a particular milestone explanation in genetics. Each team has a different milestone. You may want to have the class define "credible" in terms of evidence. In the Annotated Student Material, we suggest questions at the start of Part II to stimulate thinking about this topic before the teams begin their work.
The questions in the Analysis help to make the aspects of the nature of science explicit for your students, but do not assume that students will recognize these characteristics without reinforcement. At this time, students may begin to brainstorm ideas for a poster about the Nature and Methods of Science (NMS). If you use this technique, students will add ideas to this NMS poster throughout the module. You will need a way to display the collection of student ideas about science, perhaps using a large sheet of butcher paper. The task of adding ideas to this poster should be open-ended, and student responses will vary widely.
Procedure.
Part I: Milestones in Understanding GeneticsIn a jigsaw fashion, teams build a meaningful sequence of the milestone explanations. Students compare the actual history of the milestone explanations with their own sequence and think about the significance of any differences.Part II: How Good Is the Evidence?
Students review the concept of credible scientific evidence. Each team evaluates four evidence cards for credibility and judges whether the credible evidence supports the team's one milestone explanation in genetics, thus exploring the requirements for an explanation to be accepted as scientifically valid.Analysis:
Students reflect on many aspects of the nature of science. They may begin to summarize these ideas on a poster to be used throughout the module.Please use the annotated version of the student materials to help you conduct this activity.
(Separate student pages contain the material shown in bold typeface. Here, is provided the objectives and an annotated version of student materials to help you conduct the activity.)OBJECTIVES
As students complete this activity, they should:
1. review some of the basic concepts of genetics;2. recognize and use criteria for determining the credibility of scientific evidence,
(i.e. precision, replicability, and controlled observation);
3. recognize the association between credible evidence and a particular scientific explanation (milestone concept);
4. recognize that new work builds on old work;
5. build an understanding that genetics has a history based on accumulated knowledge and a strong record of evidence.
If you make a scientific discovery, will people still rely on it one hundred years later?
Scientists continue to use the theories of inheritance described by Gregor Mendel -- they are remarkably durable after more than a century. Since the rediscovery of Mendel's work in about 1900, biologists have made great strides in determining the mechanisms of heredity. Knowledge about genetics has expanded in the last two decades with technical advances in molecular biology and, most recently, with the work of the Human Genome Project (HGP). This huge project will identify genetic relationships (maps) and chromosomal locations of all human genes and will attempt to determine the DNA sequence for the entire genome of Homo sapiens. Mapping and sequencing will be done for other species, too, including selected bacteria, yeast, a plant, and several animal species.
Discovery in the HGP or any field of science occurs in stages. Similarly, the history of genetics is much more than a simple record of dates, names, and discoveries; it is an account of how our understanding of inheritance and the gene has grown and changed. Modern geneticists are "standing on the shoulders of giants" who came before them.
PROCEDURE
Optional Introduction
You may want to introduce this activity with a brief preliminary discussion about how scientific knowledge is built. An interesting way to start the discussion is to ask, "Can you recall a scientific explanation that was once held to be valid and later found to be inaccurate? Describe that explanation and how it changed."
If students do not respond, suggest an explanation such as the geocentric conception of the cosmos. Point out that it appeared sensible to assume that the sun, moon, stars, and planets revolved around the Earth because they appeared to move across the sky while the Earth felt stationary to the observer. The astronomer Copernicus gathered evidence through careful observation and mathematical calculations to provide a different explanation: the Earth and the other planets orbit the sun (a heliocentric view). Although the center was perceived differently, the idea of orbits remained. Another example of an idea that changed is that of a flat Earth. Students should be able to cite evidence (such as Magellan's voyage around the world' and photographs from space) that refutes this idea. Magellan's voyage also resulted in a reconceptualization of time. Although Magellan was killed in the Philippines before the end of the voyage, his crew kept careful records of dates during the year journey. When they returned to their homeport in Spain, they found that they had lost a day. This led to understanding that the Earth's rotation results in different time zones-and even a different day-on some parts of the globe. We now acknowledge and correct for this phenomenon with the International Date Line.
Another explanation that changed in light of new evidence is that for the cause of disease. For years, people thought factors such as bad air, getting caught in the rain, or having evil spirits caused disease. These views gradually changed and, in the mid-nineteenth century, scientists provided convincing proof of the germ theory as the basis for communicable disease. The central assumption of germ theory is that microorganisms can invade other organisms and cause illness. The French scientist Louis Pasteur took one large step toward acceptance of that explanation when he dispelled the notion of spontaneous generation. Pasteur, in a series of experiments using thin necked flasks and sterile techniques, showed that organisms did not grow spontaneously from nonliving matter. He also found direct evidence to support the germ theory of disease when he identified three types of microorganisms that were pathogenic for silkworms, showing the causal connection between a pathogen and a disease. A German scientist, Robert Koch, also supplied evidence for this theory with his discovery of the bacterial pathogen that causes anthrax in cattle. The article by PA. Small and N.S. Small, Mankind's Magnificent Milestone: Smallpox Eradication, The American Biology Teacher, 58(5):264-271, provides a nice overview of the application of scientific process to the eradication of an infectious disease.
Earlier notions about causes of disease, such as getting cold and tired, are not without some merit. These conditions are not primary agents of infectious disease, but our improved understanding of immunology shows us that these factors do contribute to disease by impairing immune function. Most changes in scientific knowledge reflect the expansion of inadequate explanations rather than the expulsion of entirely inaccurate ones.
If your students have a fairly good understanding of molecular genetics, they may appreciate the additional example of discovering introns in eukaryotic genes. Because early molecular biology focused mainly on bacteria, most of which lack introns, the discovery of introns came as a surprise and required a modification in our description of genes.
Indicate that, as they work through this module, students will come to understand more about how scientific knowledge changes and develops, and about how scientists go about their work. Students also will learn about some new findings in genetics.
Part 1: Milestones in Understanding Genetics
Much of the information about genetics in your biology textbook would have amazed biologists a hundred years ago. Those scientists, driven by curiosity to answer complex questions of heredity, slowly pieced together layer after layer of the milestone explanations that we now accept as valid. The most significant explanations stand as milestone events, each of which marks a great shift in our understanding. Think about what scientists needed to know before they could add each new milestone to the body of genetics knowledge. You are going to build a sequence of milestone explanations. When you do, your sequence may reflect the actual progress of genetics during the last hundred or so years, or it may reflect other ways that history could have played itself out during these early years of discovery.
Assign students to teams and distribute a set of milestone explanations to each team.
1.Your team will receive a set of eight milestone explanations of inheritance. Decide how these milestone explanations could form a meaningful sequence, and be prepared to report your sequence and the reasons you chose it.
After the teams have had a chance to build a sequence, poll the class for examples of the choices each team has made. You may want to have each team turn in a written sequence, or you may want to call on two or three teams to report. Ask the students to explain the criteria they used to sequence the milestones. A handy way to display results of different teams is to prepare a poster for each milestone and have a student place the posters in sequence, or prepare a set of milestones strips on overhead transparency film for display. To start the discussion, you could display a milestone from the middle of the actual sequence, choose another milestone, and ask whether it should come before or after the first one.
Important: Keep in mind (and emphasize for the students) that their job is not to guess the dates and chronology of these events. Instead, their task is to reason how the milestone explanations might build in a logical way. Keep the discussion brief, and do not press for consensus.
2.Your teacher will show you the actual sequence of milestone events that occurred in the history of genetics. Compare it to the sequence you helped build with the class. Might the events have occurred just as easily in the order you built?
Display an overhead of the Historic Sequence of Milestones (below) that shows the historical sequence of milestone concepts and/or distribute a copy to each student. Emphasize to the class that the connections between discoveries are far more important than the dates on which the discoveries occurred.
The historical sequence for these events, as seen here, is somewhat arbitrary in that some ideas gained strength from multiple experiments that occurred over a number of years. For instance, an early experiment that indicated that DNA is the genetic material took place in 1944, but additional evidence provided in 1952 convinced the scientific community. The work of Mendel, which might have influenced research in genetics in the latter part of the nineteenth century, failed to do so because it was not recognized widely and understood until around 1900. The actual sequence of events in scientific history does not necessarily represent the only, or even the best, sequence in terms of logical connections because scientists are at times limited by technology or make imperfect choices about the next question to be answered.
3. What technologies or cultural issues might have influenced the timing of the milestones and other discoveries in genetics?
Sample student responses include: the use of a microscope, staining techniques, computers, and the laboratory techniques of molecular biology.
Less obvious answers include: better communication among scientists, money available to support research, political interests at the time, rediscovery and openness to Mendel's paper, the development of more sophisticated statistics, and the use of computers to store and compare data.
Part II: How Good Is the Explanation?
The milestone explanations you have been using have lasted for many years. Why? Use this part of the activity to explore how we know whether a scientific explanation is on the right track and, thus, whether it will survive the test of time.If your students have a basic grasp of what is meant by credible evidence, continue to Steps 4 and 5. If not, conduct a brief discussion to establish this idea. You may want to use Question 3 from the Analysis here as an introduction. Otherwise, this question will serve as a review of this part of the activity.
See the student version of the Evidence Cards and their corresponding Milestone Explanation. An annotated version appears in these teacher pages.
4. Your teacher will give your team a set of Evidence Cards and one Milestone Card. Your first task is to evaluate the Evidence Cards and keep only those that are credible. To determine whether the information on any given Evidence Card is credible, discuss with your teammates the criteria you can use to evaluate the evidence. Write your reasons for accepting or rejecting the stated evidence.
Allow a few minutes for teams to make their choices. This step is brief because the "bogus" evidence cards are fairly obvious. The value of this step is that students must articulate their criteria.
Ask a few teams to give examples of why they accepted or rejected evidence.
5. Now decide whether the evidence you retained is helpful in supporting or refuting the milestone explanation. Explain your decision. (Hint. Some evidence will be helpful; other evidence may not be related to the milestone explanation.)
Allow enough time for students to compare ideas about the relationship between evidence and the genetics concept on the Milestone Card, but keep their discussion brief. Ask several teams to justify the helpfulness of the evidence relative to the milestone explanations. (To keep the activity moving, you may not want to have all the teams do this.)
You may need to remind students that, for evidence to support a scientific explanation, the evidence must be related to the main ideas in that explanation. Related evidence should help to distinguish between one explanation and competing views. If the explanation discusses causality, supporting evidence needs to show a constant and regular association between the stated cause and effect. For example, in 1927, after H. J. Muller exposed fruit flies to heavy doses of X rays, offspring of the exposed flies exhibited mutations. This experiment can be repeated with the same results, supporting the notion that radiation is a causal agent of mutations. Later experiments provided a more detailed explanation of the cause by demonstrating the molecular damage in DNA that results from exposure to X rays.
Here, we number each milestone in sequence, followed by its corresponding evidence.
Actual version omits numbers to avoid influencing students.
Question: Why do offspring resemble their parents?
Milestone Explanation: Parents contribute genetic material to their offspring.Evidence 1A:
A scientist looked through a microscope at dividing cells in the tail fins of a salamander. As mitosis proceeded, she saw that chromosomes moved apart in equal numbers into the newly forming daughter cells. Other scientists observed this phenomenon in cells undergoing mitosis.
This evidence is credible; the statement is based on careful observation and is repeatable. This evidence is helpful, but only partly supportive because, although it shows a possible role for chromosomes in passing information to daughter cells, mitosis and cell division in tail fins have nothing to do with inheritance from parents to offspring.
(This is one of the observations that led to an explanation of DNA replication and cell division [W. Flemming, 1879].)Evidence IB:
A scientist crossed pea plants and carefully recorded the appearance of certain traits in the offspring. When he crossed a strain that has only purple flowers with one that has only white flowers, the offspring always had purple flowers. When he crossed these offspring to produce the next generation, however, he saw both colors of flower in the new offspring in regular proportions.
This work was repeated later by other scientists who saw the same results.
This evidence is credible; this experiment is repeatable with the same results. This evidence is helpful: this information provides evidence that flower color is a trait inherited from parents, and that both parents may contribute to the phenotype of future offspring. The white color trait is not lost, but hidden; there must be at least two factors involved one from each parent.
(This is one of Mendel's classic experiments, reported in 1865 and repeated by others in 1900. The implication is that each parent contributes one of the alleles for flower color, but that purple color is the dominant trait.)Evidence 1C:
Jorge noticed that a classmate, Susan, has curly hair. When he met her mother, he noticed that she also has curly hair.
This evidence is scientifically bogus; although it suggests that genetic information passes from parents to the offspring (Susan), the evidence is so limited in its scope that it is not of much value. The curly hair may result from other than genetic influences; for example, maybe Susan and her mother have used perms to curl their hair.Evidence 1D:
Mendel said that characteristics of offspring likely come from something the offspring get from their parents.
This evidence is scientifically bogus; stated this way, the information is unclear and unsubstantiated. lf it said "Tom" instead of "Mendel," people would be even less likely to credit the statement. Authority without a basis in scientific evidence is not meaningful in supporting a scientific explanation.
Question: How are traits distributed in offspring?
Milestone Explanation: Alleles of one gene segregate in the formation of gametes. (Reproductive cells [gametes] form during meiosis. Each gamete contains one allele from the pair of alleles present in the parent.)Evidence 2A:
People say that sons express the traits of the father, while daughters have all the mother's characteristics.
This evidence is scientifically bogus; it is based on hearsay and lacks a scientific basis.Evidence 2B:
Offspring in each generation are identical.
This evidence is scientifically bogus; the statement is not correct, and there is no information about how the observation was made. Even if it were credible, the evidence would not be helpful because the evidence does not address the distribution of traits.Evidence 2C:
Mendel speculated that traits are inherited based on discrete units of inheritance. He tested the law of segregation by observing height in several generations of pea plants. He saw a distribution of tall to dwarf in the F2 generation of 3: 1. Then the F2 plants were fertilized with their own pollen (selfed). Mendel found that the dwarf F2 plants produced dwarf F3 plants, but two thirds of the tall F2 plants produced mixed offspring, dwarf and tall. This F3 test of segregation has been repeated many times with the same results.
This evidence is credible; the evidence is drawn from careful observation and collections of quantitative data; it is repeatable. This evidence is helpful; the ratios in F2 and F3 generations fit the predictions based on the segregation of alleles in the production of gametes.Evidence 2D:
A scientist named W.S. Sutton observed chromosomes in cells undergoing meiosis. He noticed that the chromosomes behaved in a way that is consistent with Mendel's observations about inheritance patterns. Many other observations of meiosis by other scientists confirmed this behavior of chromosomes in the nucleus.
This evidence is credible; other scientists have replicated Sutton's findings. This evidence is helpful; this is evidence that chromosomes behave in a way that is consistent with Mendel's laws of inheritance and will fit with their mathematical predictions .
(This observation by Walter S. Sutton in 1903 helped establish the chromosome theory of inheritance. This same evidence was used to support another milestone explanation; this overlap is a common occurrence in science.)
Question: Where are genes located?
Milestone Explanation: Genes are located on chromosomes.
(This idea is the chromosome theory of inheritance. In eukaryotic cells, genetic material is located in the nucleus in structures called chromosomes, for their dark-staining characteristic. The name comes from the Greek words chromo [color] and soma [body].)Evidence 3A:
Using staining techniques and a microscope, C. Nageli discovered a set of structures in the nuclei of cells. Other scientists observed that these structures change and become visible with a microscope at certain times in the cell cycle. Years after Nageli's observation, a developmental biologist, W. Roux, observed these structures in the cell nucleus, and another scientist, W. Waldeyer, saw the structures and named them chromosomes.
This evidence is credible; other scientists have replicated Nageli's findings. This evidence is somewhat helpful in that it establishes that chromosomes exist. It is not conclusive, however, because there is no evidence about their genetic role.
(C. Nageli discovered chromosomes in 1842; Wilhelm Roux saw them in 1883, but did not have evidence of their role in inheritance. W. Waldeyer gave chromosomes their name in 1888.)Evidence 3B:
A scientist named W.S. Sutton observed chromosomes in cells undergoing meiosis. He noticed that the chromosomes behaved in a way that is consistent with Mendel's observations about inheritance patterns. Many other observations of meiosis by other scientists confirmed this behavior of chromosomes in the nucleus.
This evidence is credible; other scientists have replicated Sutton's findings. This evidence is helpful; this is evidence that chromosomes behave in a way that is consistent with Mendel's laws of inheritance and will fit with their mathematical predictions.
(This observation by Walter S. Sutton in 1903 helped establish the chromosome theory of inheritance.)Evidence 3C:
A scientist, T. Boveri, showed that sea urchin embryos develop normally only when they have a full set of chromosomes. Embryos with more or fewer chromosomes than the normally observed number did not develop properly. Many other scientists have made the same observations in other organisms.
This evidence is credible; it is precise, testable, and repeatable. This evidence is helpful; it connects cause and effect to genotype and phenotype.
(This observation by T. Boveri in 1903 was one line of evidence that led to the chromosome theory of inheritance.)Evidence 3D:
A professor at Harvard thinks that chromosomes contain genes.
This evidence is scientifically bogus because it is imprecise. The professor has provided no details, and the evidence is based on the fame of the university alone. There is no substantiating scientific evidence. Perhaps the professor is a specialist in something unrelated to biology and knows little about genetics.
Question: What determines the sex of an organism?
Milestone Explanation: In most sexually reproducing organisms, special chromosomes determine sex.
(Humans have sex chromosomes, designated X and Y The combination of sex chromosomes in an organism determines its sex.)Evidence 4A:
Many studies have shown that human females have two X chromosomes, and human males have one X chromosome and one Y chromosome. Patricia Jacobs and other scientists also showed that, although the normal human sex chromosome complement is XX or XY, other patterns do occur rarely. For instance, in humans, XXY individuals are male, and XO individuals are female.
This evidence is credible; it is precise, and several lines of investigation support it. This evidence is helpful, especially when added to our knowledge that chromosomes are the mediators of inheritance. This is evidence that sex determination results from the lack of the Y chromosome in females, or the presence of the Y chromosome in human males.Evidence 4B:
During their lifetime, human males produce many sperm, but females produce only a few eggs. Many years of studies showed that females are born with all the egg cells they will ever have (although the eggs must mature individually during successive menstrual cycles). Males, however, produce millions of new sperm cells every few days until a very advanced age.
This evidence is credible; the conclusions are based on "many years of studies." This evidence is not helpful; the information is correct, but it has nothing to do with sex determination.Evidence 4C:
A scientist named C.E. McClung found that grasshoppers produce equal quantities of two different types of sperm, one of which contains an extra chromosome. Three years later, two other scientists, N. Stevens and E.B. Wilson, determined that female grasshoppers have two copies of one particular chromosome, whereas males have only one.
This evidence is credible; it is precise, and other scientists can replicate the data. This evidence is helpful; this is evidence that the extra chromosome in grasshopper females determines sex.
(These are the studies of C.E. McClung in 1902, and N. Stevens and E.B. Wilson in 1905.)Evidence 4D:
There is a saying that a pregnant woman can determine the sex of her child by eating spicy foods to produce a male or cool foods to produce a female.
This evidence is scientifically bogus; this evidence is based on hearsay and folklore. In addition, the question and milestone explanation address the genetic determination of sex in offspring; by the time a woman knows she is pregnant, the genotype of the potential offspring already is determined. What she eats or drinks can affect the developing phenotype of her fetus, sometimes adversely (alcohol, for example, can cause a variety of problems in the developing fetus, including low birth weight and retardation), but it cannot affect the sex of the fetus.
Question: Why do some traits occur together in offspring?
Milestone Explanation: Some genes are located on the same chromosome (linkage).Evidence 5A:
The host of a popular talk-show about sports says that the best baseball pitchers have brown eyes.
This evidence is scientifically bogus; this comment is based on hearsay rather than scientific evidence, so it is not credible. At first glance, the comment may appear relevant to students because it suggests that two traits, pitching well and eye color, are related. Even if there were data to support the statement, many other factors might be involved, for example, light sensitivity in blue-eyed players. This situation does not indicate a genetic connection between two heritable traits.Evidence 5B:
Many microscopic studies show that chromosome pairs can exchange material during meiosis, resulting in a new combination of alleles in that pair of chromosomes. This phenomenon is genetic recombination, which results from crossing over.
This evidence is credible; it is based on "many microscopic studies." This evidence is somewhat helpful, but not strongly supportive. This observation suggests that some genes on one chromosome may not behave as though they are linked when they are separated by a substantial distance, such that recombination can occur.Evidence 5C:
Three scientists demonstrated that purple flowers and long pollen were inherited together in the sweet pea more often (more than 75% of the time) than predicted by Mendel's law of independent assortment (50%).
This evidence is credible; the scientists conducted experiments and kept track of their data. This evidence is helpful; this is evidence that these genes are associated in some way
(The experiment is the 1906 observation of W Bateson, E.R. Saunders, and R.C. Punnett; it demonstrates linkage - a noted exception to Mendel's law of independent assortment. The two genes are on the same chromosome and are not always separated by crossing over during meiosis.)Evidence 5D:
A study of human pedigrees shows that certain traits such as hemophilia and color blindness occur at a much higher frequency in males than in females. These traits appear to depend on the inheritance of mutations located on the X chromosome. When a man has both of these traits, studies show that there is a greater than 50% chance that any brothers will have both disorders, or neither one.
This evidence is credible; it relies on the observation of multiple cases and on careful mathematical analysis.
This evidence is helpful; in this case, the two traits are on the same chromosome (X) and are not inherited independently, supporting the idea that they are linked. The distance between genes on one chromosome determines how tightly they are linked. If the genes are very tightly linked, the probability is low that they will be separated during crossing over.
(This work was done by Julia Bell and J. B. S. Haldane in the 1930s and was the first to show linkage in humans. It provided early groundwork for the impetus of the Human Genome Project to map all human genes and identify those that are disease-related.)
Question: What molecular component in chromosomes carries genetic information?
Milestone Explanation: DNA carries genetic information.Evidence 6A:
Three scientists purified DNA from bacteria that grew in smooth colonies. They put this DNA into bacteria that normally grew in rough colonies. The bacteria that had been given the DNA produced many generations of offspring that formed smooth colonies. This experiment produced the same results when repeated. Other scientists (many years later) transferred a specific fragment of DNA from bacteria to a plant, and a bacterial trait appeared in the plant.
This evidence is credible; it is based on experimentation, and others have replicated the results. This evidence is helpful; this is evidence that DNA contains information that determines phenotype, such as traits that affect the appearance of a bacterial colony or particular genes in a plant. The accumulation of evidence that supports an explanation many years after it was first proposed is common and shows the durability of scientific knowledge. You may want to point out this phenomenon after the students have assembled their conceptual sequence of milestone explanations.
(The first observation is the classic experiment by O.T. Avery, C.M. MacLeod, and M. McCarty, performed in 1944. They transformed pneumococcus by transferring DNA from one strain to another. Modern genetic engineering provides many examples of transformation by DNA, even across species boundaries. For instance, the gene for a toxin from Bacillus thurengensis that is effective against insects has been inserted into several species of plants including tobacco, tomato, and cotton.)
Evidence 6B:
Two scientists studied viruses to determine how they infect bacteria. They labeled the DNA and the protein components of the viruses using radioactive chemicals. This allowed them to trace the movement of the DNA and protein. Only labeled DNA entered the bacterial cells during the infection process. Other investigations repeated these studies.
This evidence is credible; the evidence results from experiments that others have replicated helpful; this is evidence that DNA, not protein, is responsible for infection by bacteriophage viruses.
(This is the 1952 experiment of A.D. Hershey and M. Chase.)Evidence 6C:
Scientists have extracted DNA from many different cell types in one organism and from the cells of many different species.
This evidence is credible; it is based on work with numerous samples. This evidence is helpful, but certainly not conclusive. The fact that all cells appear to have DNA supports the hypothesis that DNA is the genetic material. All cells, however, also have proteins, carbohydrates, and lipids, so this evidence is in no way conclusive.Evidence 6D:
Scientist Francis Crick and a student named James Watson agreed that DNA might be the genetic material.
This evidence is scientifically bogus. Without scientific evidence, the opinion of these men proves nothing. Students might be naive about this bogus evidence because these scientists did go on to discover the structure of DNA (although even that evidence alone does not prove its role as genetic material.)
Question: How is the genetic information used to make proteins encoded?
Milestone Explanation: In DNA, a triplet of nucleotide bases encodes each amino acid in the resultant protein.Evidence 7A:
To determine how the genetic code might work, scientists noted that DNA has four different bases that can be arranged in various sequences. The code must be able to specify the 20 different amino acids found in proteins. Mathematical principles predict the following about the genetic code:one-base code specifies 4 amino acids at most
two-base code specifies 16 amino acids at most
three-base code specifies 64 amino acids at most
four-base code specifies 256 amino acids at mostThis evidence is credible; it is based on mathematical principles that are clearly demonstrable and testable helpful; the three-base code has the minimal complexity required to encode twenty amino acids. This evidence does not prove the triplet nature of the genetic code, but it is supportive and it suggests a testable hypothesis.
Evidence 7B:
Investigators found that removal of three nucleotides from a gene causes the resulting protein to lose one amino acid. However, removal of one or two nucleotides from a gene causes much more disruption in the resulting protein structure. Other scientists quickly repeated these experiments and got the same results.
This evidence is credible; it is based on experiments that others have replicated. This evidence is helpful; this is evidence that the protein-coding information occurs in groups of three nucleotides.
(This is the 1961 experiment where Francis Crick, L. Barnett, S. Brenner, and S. J. Watts-Tobin, used a mutagenic chemical [proflavin] that adds or removes nucleotides from DNA. A one- or two-nucleotide change results in a frameshift of the protein-coding information; removal of three nucleotides simply removes one amino acid from the protein and usually is less disruptive to protein function. You may want to take time to discuss this idea of a frameshift mutation and the significance of a triplet code.)Evidence 7C:
Many types of chemical analysis have shown that DNA contains about equal amounts of four different components (the nucleotides, which contain bases abbreviated A, G, C, and T). In contrast, proteins are made of 20 different components (amino acids), and they vary in amount in different proteins.
This evidence is credible; it is based on "many types of chemical analysis." This evidence is not directly supportive of the milestone explanation. In fact, this early observation led many scientists to conclude that protein is the genetic material because its structure appeared to be capable of more variation. DNA appeared to be too regular. The difficulty arose because scientists did not know the actual structure of DNA that permits the sequence of four bases to produce an enormous number of variations (hence, different genes). The variation evident in protein (phenotype) is actually the result of genetic information rather than its source.Evidence 7D:
A shampoo is advertised as containing DNA and able to enrich hair.
This evidence is scientifically bogus (but typical); the claims included in advertisements are not always substantiated, although they are supposed to be. Sometimes, advertising combines two unrelated phenomena in the hope that the consumer will infer a connection. For example, this advertisement combines "contains DNA' (which could be true) and "enriches hair" (which implies but does not state that DNA does this; it is a questionable claim in any event). In addition, there is no scientific evidence to support this association. Furthermore, even if DNA did enrich hair, that fact does not address DNA's role as genetic material (hair is protein, not a living cell).
Question: How does a new, heritable trait appear in a population?
Milestone Explanation: Mutations change the structure of DNA in reproductive cells (gametes).Evidence 8A:
People who build large muscles through exercise will have children who also have large muscles.
This evidence is scientifically bogus; it is not substantiated by any data from careful observation and measurement. In addition, even if it were credible, this evidence would not be helpful as an explanation of heritable change.Strong parents may pass along a genetic makeup for heavy build to their children, but this transfer does not depend on the acquired trait of muscle building from exercise by the parent.Evidence 8B:
A scientist named Hermann J. Muller exposed fruit flies to increasing doses of radiation in the form of X rays. He kept careful records of the number of mutant traits that appeared in their offspring. Muller found that there was a direct correlation between the number of mutations and the amount of radiation: more X rays produced more mutant offspring. (Later research showed that X rays damage chromosomes.) Other scientists have repeated this experiment, and similar experiments have been repeated many times.
This evidence is credible; it is based on careful observation and a precise record of results. Others have replicated Muller's results. This evidence is somewhat helpful; it suggests a causal relationship, although it does not directly address DNA structure as the basis for physical change.
(Muller received the 1946 Noble Prize for this [and related] work, which he reported in 1927.)Evidence 8C:
Investigators found that removal of three nucleotides from a gene causes the resulting protein to lose one amino acid. However, removal of one or two nucleotides from a gene causes much more disruption in the resulting protein structure. Other scientists quickly repeated these experiments and got the same results.
This evidence is credible; it is based on evidence that others have replicated. This evidence is helpful; this is evidence that the protein-coding information occurs in groups of three nucleotides.
(This is the 1961 experiment where Francis Crick, L. Barnett, S. Brenner, and S.J. Watts-Tobin, used a mutagenic chemical [proflavin] that adds or removes nucleotides from DNA. A one- or two-nucleotide change results in a frameshift of the protein-coding information; removal of three nucleotides simply removes one amino acid from the protein and usually is less disruptive to protein function. This same evidence was used to support another milestone explanation; this overlap is a common occurrence in science.)Evidence 8D:
Investigators at the Toronto Hospital for Sick Children studied the DNA of children who have cystic fibrosis (CF). The investigators also studied the parents of these children. They found that these children and their parents have changes in their DNA that are not present in unaffected children or in persons who do not carry the CF gene. Further investigation has revealed more than 600 different DNA mutations in the CF gene.
This evidence is credible. The investigators collected data on CF patients and their parents and compared those data to other data from unaffected persons and noncarriers. Other investigators found additional mutations in the CF gene. This evidence is helpful; the evidence shows a relationship between changes in genotype and changes in phenotype. In addition, the work builds on our basic understanding of Mendelian genetics, in this case, autosomal recessive inheritance.
(This is the 1989 work of Lap Chee Tsui and his colleagues in Toronto. This group isolated the CF gene.)
If you make a scientific discovery, will people still rely on it one hundred years later?Scientists continue to use the theories of inheritance described by Gregor Mendel -- they are remarkably durable after more than a century. Since the rediscovery of Mendel's work in about 1900, biologists have made great strides in determining the mechanisms of heredity. Knowledge about genetics has expanded in the last two decades with technical advances in molecular biology and, most recently, with the work of the Human Genome Project (HGP). This huge project will identify genetic relationships (maps) and chromosomal locations of all human genes and will attempt to determine the DNA sequence for the entire genome of Homo sapiens. Mapping and sequencing will be done for other species, too, including selected bacteria, yeast, a plant, and several animal species.
Discovery in the HGP or any field of science occurs in stages. Similarly, the history of genetics is much more than a simple record of dates, names, and discoveries; it is an account of how our understanding of inheritance and the gene has grown and changed. Modern geneticists (Figure 1-2) are "standing on the shoulders of giants" who came before them.
Part 1: Milestones in Understanding Genetics
Much of the information about genetics in your biology textbook would have amazed biologists a hundred years ago. Those scientists, driven by curiosity to answer complex questions of heredity, slowly pieced together layer after layer of the milestone explanations that we now accept as valid. The most significant explanations stand as milestone events, each of which marks a great shift in our understanding. Think about what scientists needed to know before they could add each new milestone to the body of genetics knowledge. You are going to build a sequence of milestone explanations. When you do, your sequence may reflect the actual progress of genetics during the last hundred or so years, or it may reflect other ways that history could have played itself out during these early years of discovery.1.Your team will receive a set of eight milestone explanations of inheritance.
Decide how these milestone explanations could form a meaningful sequence, then be prepared to report your sequence and the reasons why.2.Your teacher will show you the actual sequence of milestone events that occurred in the history of genetics.
Compare it to the sequence you helped build with the class.
Might the events have occurred just as easily in the order you built?3. What technologies or cultural issues might have influenced the timing of these milestones and other discoveries?
Part II: How Good Is the Explanation?
The milestone explanations you have been using have lasted for many years. Why? Use this part of the activity to explore how we know whether a scientific explanation is on the right track and, thus, whether it will survive the test of time.4. Your teacher will give your team a set of Evidence Cards and one Milestone Card. Your first task is to evaluate the Evidence Cards and keep only those that are credible. To determine whether the information on any given Evidence Card is credible, discuss with your teammates the criteria you can use to evaluate the evidence. Write your reasons for accepting or rejecting the stated evidence.
5. Now decide whether the evidence you retained is helpful in supporting or refuting the milestone explanation. Explain your decision. (Hint. Some evidence will be helpful; other evidence may not be related to the milestone explanation.)
Assessment 1.
Not all scientific discoveries are great milestones that change our understanding of the natural world. Scientists put an enormous amount of work into even relatively simple discoveries, as you would expect when you consider the rigors of investigation. These small pieces provide a valuable part of a larger puzzle. Gradually, we build our understanding of inheritance. The rarity of great leaps in understanding can be a frustrating aspect of scientific work. People carry out science who must earn a living and who want to fulfill personal goals, factors that might influence their work. Think about the social and cultural setting in which research takes place as you respond to these questions.1. What does science try to do?
2. How do we know an explanation is on the right track?
3.What counts as credible evidence in science?
4. Mendel developed a simple, yet elegant, system to explain inheritance. What has happened to his system?
Assessment 2.
Begin to make a poster that records your ideas about the characteristics of science.
MILESTONES IN UNDERSTANDING GENETICS
Question: Why do offspring resemble their parents?
Milestone Explanation: Parents contribute genetic material to their offspring.
MILESTONES IN UNDERSTANDING GENETICS
Question: Where are genes located?
Milestone Explanation: Genes are located on chromosomes.
(This idea is the chromosome theory of inheritance. In eukaryotic cells, genetic material is located in the nucleus in structures called chromosomes, for their dark staining characteristic. The name comes from the Greek words chroma [color) and soma [body].)
MILESTONES IN UNDERSTANDING GENETICS
Question: What determines the sex of an organism?
Milestone Explanation: In most sexually reproducing organisms, special chromosomes determine sex.
(Humans have sex chromosomes, designated X and Y The combination of sex chromosomes in an organism determines its sex.)
MILESTONES IN UNDERSTANDING GENETICS
Question: How are traits distributed in offspring?
Milestone Explanation: Alleles of one gene segregate in the formation of gametes.
(Reproductive cells [gametes] form during meiosis. Each gamete contains one allele from the pair of alleles present in the parent.)
MILESTONES IN UNDERSTANDING GENETICS
Question: Why do some traits occur together in offspring?
Milestone Explanation: Some genes are located on the same chromosome (linkage).
MILESTONES IN UNDERSTANDING GENETICS
Question: How is the genetic information used to make proteins encoded?
Milestone Explanation: In DNA, a triplet of nucleotide bases encodes each amino acid in the resultant protein.
MILESTONES IN UNDERSTANDING GENETICS
Question: What molecular component in chromosomes carries genetic information?
Milestone Explanation: DNA carries genetic information.
MILESTONES IN UNDERSTANDING GENETICS
Question: How does a new, heritable trait appear in a population?
Milestone Explanation: Mutations change the structure of DNA in reproductive cells (gametes).
1. Parents contribute genetic material to their offspring.
2. Alleles of one gene segregate in the formation of gametes.
3. Genes are located on chromosomes.
4. In most sexually reproducing organisms, special chromosomes determine sex.
5. Some genes are located on the same chromosome (linkage).
6. DNA carries genetic information.
7. In DNA, a triplet of nucleotide bases encodes each amino acid in the resultant protein.
8. Mutations change the structure of DNA in reproductive cells (gametes).
MILESTONES IN UNDERSTANDING GENETICS
Question: Why do offspring resemble their parents?
Milestone Explanation: Parents contribute genetic material to their offspring.parents and offspring
Evidence A:
A scientist looked through a microscope at dividing cells in the tail fins of a salamander. As mitosis proceeded, she saw that chromosomes moved apart in equal numbers into the newly forming daughter cells. Other scientists observed this phenomenon in cells undergoing mitosis.parents and offspring
Evidence B:
A scientist crossed pea plants and carefully recorded the appearance of certain traits in the offspring. When he crossed a strain that has only purple flowers with one that has only white flowers, the offspring always had purple flowers. When he crossed these offspring to produce the next generation, however, he saw both colors of flower in the new offspring in regular proportions.
This work was repeated later by other scientists who saw the same results.parents and offspring
Evidence C:
Jorge noticed that a classmate, Susan, has curly hair. When he met her mother, he noticed that she also has curly hair.parents and offspring
Evidence D:
Mendel said that characteristics of offspring likely come from something the offspring get from their parents.
MILESTONES IN UNDERSTANDING GENETICS
Question: How are traits distributed in offspring?
Milestone Explanation: Alleles of one gene segregate in the formation of gametes.
(Reproductive cells [gametes] form during meiosis. Each gamete contains one allele from the pair of alleles present in the parent.)segregation
Evidence A:
People say that sons express the traits of the father, while daughters have all the mother's characteristics.segregation
Evidence B:
Offspring in each generation are identical.segregation
Evidence C:
Mendel speculated that traits are inherited based on discrete units of inheritance. He tested the law of segregation by observing height in several generations of pea plants. He saw a distribution of tall to dwarf in the F2 generation of 3:1. Then the F2 plants were fertilized with their own pollen (selfed). Mendel found that the dwarf F2 plants produced dwarf F3 plants, but two-thirds of the tall F2 plants produced mixed offspring, dwarf and tall. This F3 test of segregation has been repeated many times with the same results.segregation
Evidence D:
A scientist named WS. Sutton observed chromosomes in cells undergoing meiosis. He noticed that the chromosomes behaved in a way that is consistent with Mendel's observations about inheritance patterns. Many other observations of meiosis by other scientists confirmed this behavior of chromosomes in the nucleus.
MILESTONES IN UNDERSTANDING GENETICS
Question: Where are genes located?
Milestone Explanation: Genes are located on chromosomes.
(This idea is the chromosome theory of inheritance. In eukaryotic cells, genetic material is located in the nucleus in structures called chromosomes, for their dark-staining characteristic. The name comes from the Greek words chroma [color] and soma [body].)genes on chromosomes
Evidence A:
Using staining techniques and a microscope, C. Nageli discovered a set of structures in the nuclei of cells. Other scientists observed that these structures change and become visible with a microscope at certain times in the cell cycle. Years after Nageli's observation, a developmental biologist, W Roux, observed these structures in the cell nucleus, and another scientist, W. Waldeyer,, saw the structures and named them chromosomes.genes on chromosomes
Evidence B:
A scientist named WS. Sutton observed chromosomes in cells undergoing meiosis. He noticed that the chromosomes behaved in a way that is consistent with Mendel's observations about inheritance patterns. Many other observations of meiosis by other scientists confirmed this behavior of chromosomes in the nucleus.genes on chromosomes
Evidence C:
A scientist, T Boveri, showed that sea urchin embryos develop normally only when they have a full set of chromosomes. Embryos with more or fewer chromosomes than the normally observed number did not develop properly. Many other scientists have made the same observations in other organisms.genes on chromosomes
Evidence D:
A professor at Harvard thinks that chromosomes contain genes.
MILESTONES IN UNDERSTANDING GENETICS
Question: What determines the sex of an organism?
Milestone Explanation: In most sexually reproducing organisms, special chromosomes determine sex.
(Humans have sex chromosomes, designated X and Y. The combination of sex chromosomes in an organism determines its sex.)sex determination
Evidence A:
Many studies have shown that human females have two X chromosomes, and human males have one X chromosome and one Y chromosome. Patricia Jacobs and other scientists also showed that, although the normal human sex chromosome complement is XX or XY, other patterns do occur rarely. For instance, in humans, XXY individuals are male, and XO individuals are female.sex determination
Evidence B:
During their lifetime, human males produce many sperm, but females produce only a few eggs. Many years of studies showed that females are born with all the egg cells they will ever have (although the eggs must mature individually during successive menstrual cycles). Males, however, produce mil lions of new sperm cells every few days until a very advanced age.sex determination
Evidence C:
A scientist named C.E. McClung found that grasshoppers produce equal quantities of two different types of sperm, one of which contains an extra chromosome. Three years later, two other scientists, N. Stevens and E.B. Wilson, determined that female grasshoppers have two copies of one particular chromosome, whereas males have only one.sex determination
Evidence D:
There is a saying that a pregnant woman can determine the sex of her child by eating spicy foods to produce a male or cool foods to produce a female.
MILESTONES IN UNDERSTANDING GENETICS
Question: Why do some traits occur together in offspring?
Milestone Explanation: Some genes are located on the same chromosome (linkage).linkage
Evidence A:
The host of a popular talk-show about sports says that the best baseball pitchers have brown eyes.linkage
Evidence B:
Many microscopic studies show that chromosome pairs can exchange material during meiosis, resulting in a new combination of alleles in that pair of chromosomes. This phenomenon is genetic recombination, which results from crossing over.linkage
Evidence C:
Three scientists demonstrated that purple flowers and long pollen were inherited together in the sweet pea more often (more than 75% of the time) than predicted by Mendel's law of independent assortment (50%).linkage
Evidence D:
A study of human pedigrees shows that certain traits such as hemophilia and color blindness occur at a much higher frequency in males than in females. These traits appear to depend on the inheritance of mutations located on the X chromosome. When a man has both of these traits, studies show that there is a greater than 50% chance that any brothers will have both disorders, or neither one.
MILESTONES IN UNDERSTANDING GENETICS
Question: What molecular component in chromosomes carries genetic information?
Milestone Explanation: DNA carries genetic information.DNA as genetic material
Evidence A:
Three scientists purified DNA from bacteria that grew in smooth colonies. They put this DNA into bacteria that normally grew in rough colonies. The bacteria that had been given the DNA produced many generations of offspring that formed smooth colonies. This experiment produced the same results when repeated. Other scientists (many years later) transferred a specific fragment of DNA from bacteria to a plant, and a bacterial trait appeared in the plant.DNA as genetic material
Evidence B:
Two scientists studied viruses to determine how they infect bacteria. They labeled the DNA and the protein components of the viruses using radioactive chemicals. This allowed them to trace the movement of the DNA and protein. Only labeled DNA entered the bacterial cells during the infection process. Other investigations repeated these studies.DNA as genetic material
Evidence C:
Scientists have extracted DNA from many different cell types in one organism and from the cells of many different species.DNA as genetic material
Evidence D:
Scientist Francis Crick and a student named James Watson agreed that DNA might be the genetic material.
MILESTONES IN UNDERSTANDING GENETICS
Question: How is the genetic information used to make proteins encoded?
Milestone Explanation: In DNA, a triplet of nucleotide bases encodes each amino acid in the resultant protein.genetic code
Evidence A:
To determine how the genetic code might work, scientists noted that DNA has four different bases that can be arranged in various sequences. The code must be able to specify the 20 different amino acids found in proteins. Mathematical principles predict the following about the genetic code:one-base code specifies 4 amino acids at most
two-base code specifies 16 amino acids at most
three -base code specifies 64 amino acids at most
four-base code specifies 256 amino acids at mostgenetic code
Evidence B:
Investigators found that removal of three nucleotides from a gene causes the resulting protein to lose one amino acid. However, removal of one or two nucleotides from a gene causes much more disruption in the resulting protein structure. Other scientists quickly repeated these experiments and got the same results.genetic code
Evidence C:
Many types of chemical analysis have shown that DNA contains about equal amounts of four different components (the nucleotides, which contain bases abbreviated A, G, C, and T). In contrast, proteins are made of 20 different components (amino acids), and they vary in amount in different proteins.genetic code
Evidence D:
A shampoo is advertised as containing DNA and able to enrich hair.
MILESTONES IN UNDERSTANDING GENETICS
Question: How does a new, heritable trait appear in a population?
Milestone Explanation: Mutations change the structure of DNA in reproductive cells (gametes).mutations
Evidence A:
People who build large muscles through exercise will have children who also have large muscles.mutations
Evidence B:
A scientist named Hermann J. Muller exposed fruit flies to increasing doses of radiation in the form of X rays. He kept careful records of the number of mutant traits that appeared in their offspring. Muller found that there was a direct correlation between the number of mutations and the amount of radiation: more X rays produced more mutant offspring. (Later research showed that X rays damage chromosomes.) Other scientists have repeated this experiment, and similar experiments have been repeated many times.mutations
Evidence C:
Investigators found that removal of three nucleotides from a gene causes the resulting protein to lose one amino acid. However, removal of one or two nucleotides from a gene causes much more disruption in the resulting protein structure. Other scientists quickly repeated these experiments and got the same results.mutations
Evidence D:
Investigators at the Toronto Hospital for Sick Children studied the DNA of children who have cystic fibrosis (CF). The investigators also studied the parents of these children. They found that these children and their parents have changes in their DNA that are not present in unaffected children or in persons who do not carry the CF gene. Further investigation has revealed more than 600 different DNA mutations in the CF gene.