Gregor Mendel's classic paper and the nature of science in genetics courses
Julie F. Westerlund, 601 University Drive, Texas State University, San Marcos, TX 78666, USA. E-mail: email@example.com
The discoveries of Gregor Mendel, as described by Mendel in his 1866 paper Versuche über Pflanzen-Hybriden (Experiments on plant hybrids), can be used in undergraduate genetics and biology courses to engage students about specific nature of science characteristics and their relationship to four of his major contributions to genetics. The use of primary source literature as an instructional tool to enhance genetics students’ understanding of the nature of science helps students more clearly understand how scientists work and how the science of genetics has evolved as a discipline. We offer a historical background of how the nature of science developed as a concept and show how Mendel's investigations of heredity can enrich biology and genetics courses by exemplifying the nature of science.
“One can no longer teach genetics only as genetics but must teach genetics also as a science. The reason for this is that our nation and the world seem to be pitted against an ever-increasing galaxy of problems that relate to our interactions with one another and with the natural world […] Whatever decisions are made depend on one's mode of thought and whether one thinks of today only or of the future as well.”
John Moore 1986, p. 573
These perceptive words of John Moore, a quarter of a century ago, concerning genetics courses and echoed more recently by Bruce Alberts, current Editor-in-Chief of the journal Science and past president of the Natural Academy of Sciences (NAS) in the United States, stress the need for undergraduate science courses that focus on the ‘nature of science and development of scientific knowledge’.
“An objective analysis of a typical introductory science course in colleges and universities around the world […] would probably conclude that its purpose [goal] is to prepare students to ‘know, use and interpret scientific explanations of the natural world […] The other three goals of equal merit are to prepare students to generate and evaluate scientific evidence and explanations, to understand the nature and development of scientific knowledge, and to participate productively in scientific practices and discourse. Scientists would generally agree that all four types of science understanding are critical not only to a good science education but also to the basic education of everyone in the modern world. Why then do most science professors teach only the first one?”
Bruce Alberts 2009
Science curricula that include the nature and development of scientific knowledge (nature of science) exist in many countries worldwide. Examples of nature of science content are embedded in curricula in the United States, United Kingdom, Canada, Denmark and Spain (Matthews 1998, 2000). Science agencies in the United States, such as the NAS, the National Science Foundation (NSF), the National Research Council (NRC) and the American Association for the Advancement of Science (AAAS), have promoted reform in basic science courses to ensure that undergraduate students are better able to understand such complex scientific issues as climate change, world hunger, human diseases, rainforest depletion, overpopulation, evolution and pollution. Thereby, they may influence rational decisions about national policies. The NRC stresses that science courses should not only include science content, but also an “understanding of the nature and structure of scientific knowledge and the process by which it is developed.” (NRC 2007, p. 168). For teachers of genetics, it may be daunting to expand an already crowded course schedule. In our genetics courses, we discuss the nature of science in the context of Mendel. And, as this paper demonstrates, the time-honored lecture topic concerning the work of Gregor Mendel, can be quite useful in illustrating the “nature and development of scientific knowledge.”
The purpose of this essay is two-fold: 1) to offer a brief historical background concerning the development of the concept of the nature of science and its relevance to undergraduate science curricula and 2) to show how Mendel's investigations of heredity can enrich biology and genetics courses by exemplifying the nature of science.
NATURE OF SCIENCE (NOS)
The nature of science (NOS) concerns the actual practice of science and the development of scientific knowledge. The practice of science includes, the “scientific method,” but also extends to include all legitimate means for conducting scientific inquiry. Scientific inquiry is characterized by the AAAS (1989) as:
“Scientific inquiry is not easily described apart from the context of particular investigations. There simply is no fixed set of steps that scientists always follow, no one path that leads them unerringly to scientific knowledge. There are, however, certain features of science [nature of science] that give it a distinctive character as a mode of inquiry. Although those features are especially characteristic of the work of professional scientists, everyone can exercise them in thinking scientifically about many matters of interest in everyday life.”
The traditional scientific method was first formally described and promoted by philosopher Francis Bacon in the early 1600s (Gribben 2002). This method is based upon induction in which all ideas are gathered together by an investigator prior to the formation of a hypothesis. Deductions that can be tested are then made from the hypotheses. However, not all scientific discoveries and knowledge are created from the crafted step-wise process outlined by Bacon (McComas et al. 2000). In the 20th century, Karl Popper described science as having the criterion of falsifiability. He stated, “The criterion of falsifiability says that statements or systems of statements, in order to be ranked as scientific, must be capable of conflicting with possible, or conceivable, observations” (Popper 1963). A companion distinction of the falsifiability criterion is “nothing is ever proven true in science”. An extension of Popper's characterization of science was the idea of ‘Science as a way of knowing’ (Moore 1993; Union of Concerned Scientists 2010). This popular phrase embodies the idea that science is experimental and based on observations of the natural world and that science is founded on hypotheses/theories that are falsifiable and cannot be proven but can be confirmed or refuted. It also asserts that scientific knowledge is subject to revision and change (Moore 1993). The nature of science is an expression more commonly used today than ‘Science as way of knowing’ and is an expanded perspective of the practice of science. For further reading in this area, Abd-El-Khalick and Lederman (2000) provide a detailed history of the development of NOS through the twentieth century.
CHARACTERISTICS OF THE NATURE OF SCIENCE
Characteristics of NOS were originally described in the 1950s by philosophers of science James Conant and J. Bronowski (Kimball 1967) and others in a variety of disciplines in the philosophy of science, history of science and science education. Although there is not a specific ‘list’ of nature of science characteristics, there are specific characteristics that are commonly agreed upon by those in the field. Abd-El-Khalick et al. succinctly listed the characteristics of NOS:
“…scientific knowledge is tentative (subject to change); empirically based (based on and/or derived from observations of the natural world); subjective (theory-laden); partly the product of human inference, imagination, and creativity (involves the invention of explanation); and socially and culturally embedded. Two additional important aspects are the distinction between observations and inferences, and the functions of, and relationships between scientific theories and laws…. At this level of generality, virtually no disagreement exists among historians, philosophers, and science educators.”
Abd-El-Khalick et al. 1998, p. 418
Alters (1997) and McComas et al. (2000) reviewed commonly held NOS tenets or characteristics from various science education organizations and researchers. Most contain the general ideas expressed in the passage above. Furthermore, US national science education reform documents, such as ‘Science for all Americans’ (AAAS 1989) and the ‘National Science Education Standards’ (NRC 1996), emphasized most of these NOS characteristics. One fundamental characteristic of the nature of science is that science is not dogma. Dogma represents a system of beliefs that is authoritarian, difficult to dispute and often used in a religious context. It is most important to stress to students and to society that scientific knowledge is not dogmatic but rather durable with gradual modifications as understanding of natural phenomena grows. Further, that scientific knowledge is durable rather than tentative is an important distinction in the NOS characteristics. As stated by the AAAS:
“Although scientists reject the notion of attaining absolute truth and accept some uncertainty as part of nature, most scientific knowledge is durable. The modification of ideas, rather than their outright rejection, is the norm in science, as powerful constructs tend to survive and grow more precise and to become widely accepted. Continuity and stability are as characteristic of science as change is, and confidence is as prevalent as tentativeness.”
For the benefit of our genetics courses, we have distilled the reported NOS characteristics into six major nature of science characteristics. These six NOS characteristics should help guide instructors and students in their own understanding of NOS in relation to science history. The following NOS characteristics need not be memorized by students, but serve as ‘tools’ when examining the history of science and development of scientific knowledge. The six NOS characteristics are that scientific knowledge: (1) may be considered as scientific laws or theories according to the type of knowledge (i.e. mathematical and/or explanatory), (2) is based upon evidence from observations of the natural world, (3) is embedded in the culture in which science is conducted, 4) is developed within prevailing scientific concepts (theory-laden observations and interpretations) and within scientists’ values, knowledge, and prior experiences, (5) originates from imaginative and creative processes, and lastly, (6) is not dogmatic but considered durable and is to be modified or replaced as further evidence requires, (inherently tentative) (Lederman et al. 2002; Abd-El-Khalick et al. 1998; AAAS 1989; NRC 1996, 2000). Hereafter, we will refer to these six characteristics as NOS-1 through NOS-6.
VIEWS OF SCIENTISTS ON THE NATURE OF SCIENCE
Those who teach undergraduate science courses, often practicing scientists, should be familiar with the characteristics of NOS so they are able to engage their students in discussions concerning it. As stated by the late Stephen Jay Gould:
“Science is inevitably socially embedded. As a practicing scientist, I believe, as we must, that there is an external truth out there. I also believe that science bumbles along fitfully towards knowledge of that external reality. And that is socially embedded and is inevitably so because it is done by human beings and not robots.
“Nature is objective, and nature is knowable, but we can only view her through a glass darkly – and many clouds upon our vision are of our own making; social and cultural biases, psychological preferences, and mental limitations in universal modes of thought…”
Gould 1996, p. 8
In this section, we highlight views concerning the nature of science held by practicing scientists in order that the reader may see how the views of practicing scientists correspond to the list of six nature of science characteristics.
Early studies (Kimball 1967; Pomeroy 1993) of scientists’ understanding of the nature of science used survey methods and indicated limited understanding of the nature of science. To illustrate, Pomeroy (1993) studied 71 scientists from various disciplines and found that many of them expressed traditional views concerning the practice of science. Their views of science were characterized by strict objectivity and observation-based evidence.
More recently, Schwartz and Lederman (2008) interviewed 24 veteran research scientists from four different disciplines who had an average of 25 years of active research. Concerning the differences between theories and laws (NOS-1), more than one-half of the scientists held an hierarchical view that theories will develop into laws with time or more testing. The hierarchical view of theories and laws is a commonly held misconception. This misconception implies that scientific theories are not as well supported by evidence as scientific laws. It is one reason why the general public in the United States has resisted the theory of evolution although this theory is based on an enormous body of evidence. Unlike most hierarchical beliefs held by the public, scientists recognized that scientific laws may evolve as further evidence is accrued. (Schwartz and Lederman 2008).
The majority of these scientists acknowledged that the development and justification of scientific knowledge is based upon evidence from observation (NOS-2), although a few indicated that scientific knowledge could be based on strictly theoretical or mathematical ideas (for example, Albert Einstein's “thought experiments” that led to the theory of relativity) (Schwartz and Lederman 2008).
The majority of scientists (15) felt that today the direction of science is largely determined by funding and grants and, because of that; it is embedded in the culture (NOS-3). As one surveyed scientist indicated,
“You try to match your research idea with the funding agency. Sometimes you put a spin on it, or rationale that will convince the funding agency you want to work on a problem here that has implications to these problems the agency has interest in … Just doing research for research sake is gone … You have to be flexible with what you are willing to research. Be more dynamic and responsive or else you are not going to make it.
Schwartz and Lederman 2008 p. 753
Fifteen of the 24 scientists reported that their “theoretical framework, or that of other scientists” or current scientific theories determined the questions asked in their investigations and how they interpret their data (NOS-4). Schwartz and Lederman (2008, p. 747) stated that the scientists, “recognized theory-laden observations and investigations from within their research contexts and provided examples.”
Although the majority (16) of the surveyed scientists felt that scientific knowledge was partly created by reasoning and not solely from empirical data, several indicated that creativity was only involved in data interpretation and in design (NOS-5). To illustrate, one scientist remarked,
“You just look at the data and, you know, … interpretation … there is a lot of art … in the interpretation, there is a lot of creativity in how you choose to interpret the data, as well as in how you choose to design the experiment in the first place”
Schwartz and Lederman 2008, p. 751
Three of the scientists indicated that creativity is restricted to the initial stages of research during the design (Schwartz and Lederman 2008).
Of these scientists, only 46% acknowledged that scientific knowledge is inherently tentative (NOS-6). Scientists differed in their ideas concerning the certainty of scientific knowledge. The following illustrate differing views.
“An atmospheric scientist responded, when asked about the certainty of the model of the atom, “Certain. It's the way nature is” This view of certainty is in contrast to one of the theoretical physicists who stated in reference to scientists understanding of the atom, “As certain as we can be”
Schwartz and Lederman 2008, p. 742
“The aquatic ecologist commented: Scientific knowledge changes as better approximations of nature are realized, while religious knowledge is dependent on established (or accepted) elements …All scientific knowledge is subject to question, doubt and criticism (a further distinction from religion) … Nonetheless, someone will eventually challenge an accepted scientific finding and take a fresh look at it. … That is the self-corrective nature of science. Does science lead to universal truths? It leads to close approximations of universal truths.”
Schwartz and Lederman 2008, p. 742
RELEVANCE OF NATURE OF SCIENCE INSTRUCTION IN SCIENCE COURSES
Students in courses that are taught NOS in an explicit and reflective manner develop scientific literacy (Rutherford and Ahlgren 1991). Indicators of a scientifically literate person include: “A good understanding of basic scientific terms, concepts, and facts; an ability to comprehend how science generates and assesses evidence; and a capacity to distinguish science from pseudoscience” (Science Engineering Indicators p. 7–17). As Maienschein noted,
“Increased scientific literacy …produces skeptical habits of mind to keep seeking to know more and a willingness to accept change and revision. What is known one day may be replaced the next day with something quite different and even apparently contradictory. Scientific literacy teaches us to expect such change and difference, and gives us approaches for sorting through and selecting alternative accounts.… Science is useful in particular ways as a basis for informed decision making.”
Maienschein 1999, p. 83
By studying science history, such as Mendelian history, and the development of scientific knowledge, undergraduate students increase their understanding of how science is practiced, i.e. the nature of science, and their critical thinking skills. As Matthews (1998, p. 169) stated: “introductory philosophical analysis [study of nature of science] allows greater appreciation of the distinct empirical and conceptual issues involved when, for instance, Boyle's law, Dalton's model, or Darwin's theory is discussed. It also promotes critical and reflective thinking”.
Ignorance of science has serious consequences. Not being scientifically literate, and thus not being able to understand the evidence of global warming, may prevent efforts to abate catastrophic climatologic effects, such as rising sea levels, inundation of islands and shorelines, and severe weather and droughts worldwide. As Matthews stated, “The ability to distinguish good science from parodies and pseudoscience depends on a grasp of the nature of science. (Matthews 2000, p. xiv) Those who are better educated in science are better equipped to understand the nature of scientific inquiry, to distinguish science from non-science, and to examine more carefully “research processes” when determining whether scientific assertions are valid (Science and Engineering Indicators 2010, p. 7–27, p. 7–35), and are more likely to make informed decisions about scientific issues (AAAS 1989; NRC 1996; Kenyon and Resier 2006). An example of this is the performance of US residents on the 2008 General Social Survey (GSS). Of those who correctly answered the questions concerning scientific inquiry on the 2008 GSS, 74% recognized that astrology as being “not at all scientific” or being a non-science. Of those who did not correctly answer the inquiry question, only 57% recognized astrology as “not at all scientific” or a non-science. Thus, an understanding of the nature of science was associated with the correct perception that astrology is not a science. This correlation has been shown on all six GSS surveys administered since 1995 (Science and Engineering Indicators 2010, Table 7–16).
To train students to be able to distinguish science from non-science, science courses for undergraduate students, must inculcate “fundamental knowledge of what science is, and what it is not [nature of science], along with some key concepts” stated Alan Lesher, AAAS CEO (AAAS 2009). This requires consideration of science courses that better prepare students to become world citizens who are able to make informed decisions on issues of science.
EFFECTIVE NATURE OF SCIENCE INSTRUCTION IN UNDERGRADUATE SCIENCE COURSES
The most effective way to teach nature of science is through an explicit reflective approach (Abd-El-Khalick and Lederman 2000; Bell 2001; Bartholomew et al. 2004, Matthews 1998). As Bell states
…. the scientists who teach college level science courses believe that students will pick up current conceptions of the nature of science by ‘osmosis’ [implicitly] by listening to lectures about science, engaging in discussions about science, or by ‘doing’ science, including hands-on, inquiry-based activities. Yet the nature of science is a complex topic, and students’ misconceptions about the nature of science have proven as resistant to change as their misconceptions about other science content.”
An explicit approach is purposeful and “addresses the nature of science head-on” (Bell 2001). Just as the concepts of science content are explicitly taught in science courses, so should the concepts of the nature of science be explicitly taught. However, Matthews (1998) recommends that explicit teaching of the nature of science must not resemble “indoctrination” of specific concepts. Instead, he suggests that teachers should question their students and engage them in discussions. As Matthews states:
“At a most basic level any text or scientific discussion will contain terms such as law, theory, model, explanation, cause, truth, knowledge, hypothesis, confirmation, observation, evidence, idealization, time, space, fields, and species. Similarly history—minimally in the form of names such as Galileo, Newton, Boyle, Darwin, Mendel, Faraday, Volta, Dalton, Bohr, Einstein, and so on – is unavoidable. A professional teacher should be able to elaborate a little on these matters – Philosophy begins when students and teachers slow down the science lesson and ask what the above terms mean and what the conditions are for their correct use… Students and teachers can be encouraged to ask the philosopher's standard questions: What do you mean by… ? and How do you know … ? It is preferable for students genuinely to struggle to grasp the simple questions than just repeat popular nostrums, or their teacher's prejudices, about the complex questions.”
Matthews 1998, p. 168–169
Explicit historical approaches to teaching the nature of science may involve introducing students to a specific list of commonly held nature of science characteristics that can be illustrated through the history of science (Clough 2007; Metz et al. 2007; McComas 2008). As noted above, students should be given a list not to memorize but rather to engender reflection, questions, and discussion about the development of scientific knowledge. For example, in the case of Mendel, historical vignettes and class discussions have been used successfully to illustrate the nature of science (Lonsbury and Ellis 2002; Clough et al. 2007). Lonsbury and Ellis (2002) introduced Mendelian history in a high-school biology classroom through discussion of scientific inquiry as “thinking outside of the box,” with Mendel as the example of an original thinker. Also, in this study, students discussed why Mendel's paper went unnoticed until 1900 (Lonsbury and Ellis 2002). This study revealed that the history-integration group, which had discussed Mendelian history, significantly outperformed the normal-instruction group on a total ‘Nature of scientific knowledge scale’ (NSKS), which is based on nature-of-science characteristics. Clough et al. (2007) developed a historical short story about Mendel (<http://science-stories.org/>) that had questions embedded within the story to engage the students in thinking about the nature of science as they read. Clough et al. (2007) demonstrated that as a result of their reading and reflecting on Mendel's story, undergraduate biology students improved their understanding of the creative and subjective aspects of the nature of science, Clough (2007), like Matthews (1998), stressed the importance of asking questions rather than simply listing the nature-of-science characteristics. Clough (2007) stated: “the key [for nature of science instruction] is to explore the nature of science as questions, so that science teachers and students come to a deeper understanding of the nature of science. […] ‘tenets’ can easily be turned into questions such as: 1) In what sense is scientific knowledge tentative? In what sense is it durable? etc.”
NATURE OF SCIENCE INSTRUCTION AND MENDEL'S SCIENTIFIC RESEARCH
An historically accurate depiction of Mendel's research, supported by Mendel's own words from his paper, offers an excellent example to students of the nature of science. Mendel's seminal 1866 paper Versuche über Pflanzen-Hybriden (Experiments on plant hybrids) and his letters to Carl Nägeli, can engage students about specific nature of science characteristics and their relationship to four of Mendel's major contributions to genetics. The use of primary source literature as an instructional tool helps students more clearly understand how scientists work and how the science of genetics has evolved as a discipline.
We include explicit nature of science instruction in our genetics courses during the lecture on Gregor Mendel. We do this by discussing selected passages from Mendel's classic 1866 paper to show correlations with NOS-1 through NOS-6. We ask them to examine a poster that we have prepared entitled ‘An understanding of Gregor Mendel's contributions to science from a nature of science perspective’ (Westerlund and Fairbanks 2005). We then discuss the manner in which Gregor Mendel's study exemplifies the six nature of science characteristics. We are able to demonstrate the success of this approach by analyzing the responses of students to open-ended essay questions.
In the following section, we provide a supplemental reading for genetics or biology students that includes a short history of Mendel's scientific research and passages from Mendel's 1866 paper that correlate with NOS-1 through NOS-6. We suggest that instructors provide the reading to their students prior to the traditional Mendel lecture. The reading correlates each of the six NOS characteristics with Mendel's scientific research. After an adequate time for student reflection on the reading, instructors may choose to hold a class discussion based upon questions conducive to effective nature of science instruction as mentioned earlier. Using this approach, the time-honored lecture topic concerning the work of Gregor Mendel, can be quite useful in illustrating the “nature and development of scientific knowledge.”
STUDENT READING- CORRELATION OF GREGOR MENDEL'S DISCOVERIES TO THE NATURE OF SCIENCE
The nature of science [NOS] concerns the practice of science and the development of scientific knowledge. This reading guides you through a study of Gregor Mendel's discoveries in heredity correlated to six major characteristics of the nature of science [NOS-1 through NOS-6]. An example of each NOS characteristic is presented and noted in tables throughout the reading.
Over an eight-year period (1856–1863), Gregor Mendel conducted a series of extensive hybridization experiments with the garden pea, Pisum sativum, in his monastery garden and greenhouse. From the results of these experiments and his interpretations of them, he developed a theory to describe the fundamental mechanisms of heredity. He is recognized for his creative genius as being the first scientist to bring together three different disciplines in biological experimentation: mathematics, the fertilization of gametes and probability theory, to investigate heredity (Orel and Hartl 1994). For his theory, which became the foundation of hereditary principles, Mendel is commonly referred to as the founder of genetics. Mendel described a wide range of genetic phenomena in his paper, including plant reproduction, artificial cross-pollination, segregation, independent assortment, dominance, parental equivalence, pleiotropy, epistasis, the role of hybridization in speciation, and statistical analysis of genetic data. However, his so-called laws of segregation and independent assortment are his most cited discoveries. According to the National Academy of Science, (NAS), a scientific law is defined as “a descriptive generalization about how some aspect of the natural world behaves under stated circumstances,” and a theory as “a well-substantiated explanation of some aspect of the natural world that can incorporate facts, laws, inferences, and tested hypotheses” (NAS 2008). According to these definitions, segregation and independent assortment, as described by Mendel, may be stated as laws. Mendel himself referred to his discoveries as the “law of development” (Entwicklungs-Gesetz) and the “law of combination” (Gesetz der Combinirung). Moreover, his interpretation and explanations of those laws, which have been substantially confirmed and augmented, qualify as the fundamental theory of heredity (Table 1).
Table 1. Correlation of NOS-1 to Mendel.
|NOS-1 Scientific knowledge may be considered as scientific laws or theories according to the type of knowledge (i.e. mathematical and/or explanatory).||Mendel's law of segregation|
|Mendel's law of independent assortment|
|Fundamental theory of heredity.|
Carl Correns (1900), Hugo de Vries (1900a, 1900b) and Erich Tschermak (1900), rediscovered Mendel's law of segregation in 1900, and Correns, in the same 1900 paper, rediscovered the law of independent assortment (Fairbanks and Rytting 2001). Before Mendel, the mechanisms by which traits are inherited were incorrectly or vaguely explained. Mendel presented abundant experimental evidence and offered a well-supported theoretical framework to explain the patterns of inheritance that he observed in a detailed and unambiguous way. Mendel did not title his laws as “segregation” and “independent assortment” as textbooks typically describe them; instead various biologists attributed them to him (Fairbanks and Rytting 2001; Orel and Hartl 1994). For example, de Vries (1900a, 1900b) titled his rediscovery papers, published separately in German and French, “The law of segregation of hybrids” (Das Spaltungsgesetz der Bastarde, and Sur la loi de disjonction des hybrides). However, as several authors have pointed out (Orel and Hartl 1994; Orel 1996; Fairbanks and Rytting 2001; Franklin et al. 2008), and as we will reiterate momentarily, Mendel clearly stated these laws in his paper.
Mendel's development of his laws can be correlated to the nature of science characteristics. For example, NOS-2 is clearly evident from his observations of the different phenotypes of Pisum sativum growing in his experimental gardens (Table 2).
Table 2. Correlation of NOS-2 to Mendel.
|NOS-2 Scientific knowledge is based upon evidence from observations of the natural world.||Observations of Pisum sativum, the garden pea in the experimental gardens.|
Another example, NOS-3, is illustrated in his paper by the manner in which Mendel interpreted the results of his experiments using the terminology and the available technology of the 1850s; primarily true-breeding pea varieties and cross-fertilization techniques (Table 3).
Table 3. Correlation of NOS-3 to Mendel.
|NOS-3 Scientific knowledge is embedded in the culture in which science is conducted.||Mendel's design and interpretation of the results of his experiments.|
Furthermore, Mendel's decision to conduct experiments on Pisum sativum were based upon the considerable discussion in the scientific literature of his time regarding heredity and the role of hybridization in the development of new species (Roberts 1965). The problem of not being able to predict offspring from sheep breeding in the region where Mendel lived had “serious economic implications that was crying out for experimental investigations” (Orel 1996, p. 32). The Sheep Breeding Society of Brno in the 1830s was very active in encouraging further study of the problem of heredity (Orel 1996). NOS-3 is also illustrated by the scientific research questions that Mendel posed about heredity were embedded in his culture.
We now examine the development of four of Mendel's discoveries from a nature of science perspective. These discoveries, formulated in modern terminology, include: 1) inheritance of traits is particulate rather than blended; 2) for every diploid organism, inherited traits are governed by genes, and each gene has a pair of alleles that influence variation; 3) paired alleles segregate during the formation of gametes (Mendel's law of segregation) and 4) alleles of different genes assort independently during the formation of gametes (Mendel's law of independent assortment).
The idea of blending inheritance derives from the observations that hybrids often display a phenotype intermediate between their two parents, and that the intermediate hybrid phenotype should be maintained in the progeny of hybrids, implying that the hereditary material of both parents is somehow blended in the hybrid. However, this notion of blending inheritance was already in dispute when Mendel began his experiments. Although some researchers had claimed observation of an intermediate phenotype being maintained in the offspring of hybrids, as blending inheritance predicts, numerous published experiments, which Mendel had studied extensively, argued otherwise. At the outset of Versuche, Mendel specifically mentioned the work of “Kölreuter, Gärtner, Herbert, Lecoq, Wichura and others” (Stern and Sherwood 1966, p. 1). A perusal of the experimental results of these researchers, as summarized by Roberts (1965), shows, as Mendel noted, that hybrids do not always display a phenotype that is exactly intermediate between their parents, and the offspring of hybrids tend be highly variable. Darwin likewise mentioned in ‘The Origin of Species’ that “the slight degree of variability in hybrids from the first cross or in the first generation, in contrast with their extreme variability in the succeeding generations, is a curious fact and deserves attention” (Darwin 1861, p. 296), a passage that Mendel marked with double lines in his personal copy of ‘The Origin of Species’ (Orel 1996; Fairbanks and Rytting 2001).
Although Mendel is often credited as being the first to dispel the notion of blending inheritance, it is clear that others had already done so. For example, Charles Naudin received the 1861 grand prize in physical sciences from the Paris Academy for Sciences for his studies of plant hybrids and their offspring. The Academy noted in its review that Naudin “has confirmed that which Sageret already knew, that in a hybrid the characters of the two parents are often shown not blended” (Roberts 1965, p. 130). Following Naudin, Darwin elaborated in his two-volume work, ‘The Variation of Plants and Animals under Domestication’:
“As a general rule, crossed offspring in the first generation are nearly intermediate between their parents, but the grandchildren and succeeding generations continually revert, in a greater or lesser degree, to one or both of their progenitors. Several authors have maintained that hybrids and mongrels included all the characters of both parents, not fused together, but merely mingled in different proportions in different parts of the body, or as Naudin has expressed it, a hybrid is a living mosaic-work, in which the eye cannot distinguish the discordant elements, so completely are they intermingled.
Darwin 1868, vol. 2, pp. 48–49
Mendel recognized his contribution as uniquely augmenting and explaining a concept already well studied in his day – the uniformity of hybrids compared with the variability of their offspring. In his words, “among the numerous experiments not one has been carried out to an extent or in a manner that would make it possible to determine the number of different forms in which hybrid progeny appear, permit classification of these forms in each generation with certainty, and ascertain their numerical interrelationships” (Stern and Sherwood 1966, p. 2). In other words, Mendel perceived his work as breaking new ground by offering an experimentally supported and mathematically coherent model that explained how variation in the offspring of hybrids was transmitted through inheritance. Although Mendel's work was clearly new, and was the first to deduce in detail the fundamental principles of inheritance, it fit, as Olby (1979, pp. 53–54) put it, “squarely within the context of mid-nineteenth century biology.”
From the historical evidence, we see in Mendel's work an example of NOS-4 wherein Mendel's science was influenced by his readings in current scientific literature and the scientific context of his time (Table 4).
Table 4. Correlation of NOS-4 to Mendel.
|NOS-4 Scientific knowledge is developed within prevailing scientific concepts (theory-laden observations and interpretations) and within scientists’ values, knowledge and prior experiences.||Mendel believed that inheritance of traits is discrete, rather than blended from studying the work of his predecessors and designed his experiments based upon it.|
He doubted previously held notions concerning blending inheritance and designed his research to augment the work of his predecessors. The design of his experiments and the interpretation of his data were influenced by the scientific thinking of his time.
Mendel realized that one pollen grain must fertilize one ovule. This was a new and controversial idea in germ cell theory during Mendel's time (Orel 1996, p. 194). Mendel understood that development is
“initiated by the union of one germinal cell and one pollen cell into one single cell, which is able to develop into an independent organism through incorporation of matter and the formation of new cells. This development proceeds in accord with a constant law based on the material composition and arrangement of the elements [Mendel's term for genes] that attained a viable union in the cell”
Stern and Sherwood 1966, p. 41–42
Mendel had studied plant physiology with Professor Franz Unger at the University of Vienna, who introduced Mendel to the concept of one pollen grain fertilizing one ovule. In this, we see another example of NOS-4 in the development of ideas in a scientist as deriving from his educational background. Mendel even conducted experiments with Mirabilis to specifically demonstrate that a single pollen grain is sufficient to fertilize an ovule (Stern and Sherwood 1966; Orel 1996). Mendel's understanding that a fertilized germ cell arises from the mutual and equal contribution of a pair of parental units set the stage for his further understanding that an outward phenotypic variation is a result of variation for a pair of alleles, and that 3:1 ratios occur in the F2 offspring as a result.
Gregor Mendel is generally remembered for his work with peas and his discovery of the 3:1 ratio of traits exhibited by hybrids. His recognition of a 3:1 ratio of dominant to recessive phenotypes in the F2 generation, and the theoretical implications derived from this observation, was a seminal achievement in his analyses that set him apart from his contemporaries (Orel 1996). For example, his contemporary Charles Darwin crossed the peloric snapdragon with the common form and allowed the F1 offspring to self-fertilize. The results from this experiment yielded F2 offspring – 88 in the common and 37 in the peloric form – a ratio of 2.38:1, which approaches a 3:1 ratio. However, Darwin apparently did not notice the significance of this ratio (Darwin 1868, vol. 11, p. 70), and did not continue the experiment into the F3 generation. Perhaps Darwin's preconceptions concerning multiple pollen fertilization of an ovule and mosaicism compromised his ability to recognize the significance of the ratio he observed in the F2 offspring. Or, since Darwin did not examine a large series of experiments as Mendel did, he may not have observed a consistent and repeated pattern that demanded an explanation.
Mendel's insight from his formal studies concerning germ cell theory and discrete inheritance, allowed him to recognize not only the significance of the F2 phenotypic 3:1 ratio but also to see that this 3:1 ratio is a consequence of a genotypic ratio of 1:2:1, in other words, 1/4 homozygous dominant, 2/4 heterozygous, and 1/4 homozygous recessive. The 1:2:1 genotypic ratio meant that of the plants in the F2 generation with the dominant phenotype, 1/3 should be homozyous and 2/3 heterozygous, and all of the recessive F2 individuals were homozygous.
To explain his consistent 3:1 ratios in the F2 offspring, Mendel did something different from his contemporaries and very creative, which is an example of NOS-5: he allowed the F2 offspring to naturally self-fertilize and thus was able to resolve the phenotypic 3:1 ratio into the genotypic 1:2:1 ratio. As Mendel stated,
“The ratio of 3:1 in which the distribution of the dominating trait and recessive traits takes place in the first generation therefore resolves itself into the ratio of 2:1:1 in all experiments if one differentiates between the meaning of the dominating trait as a hybrid trait and as a parental character.”
Stern and Sherwood 1966, p. 15
Mendel presented his ideas in mathematical terms and used the designation of Aa to indicate a heterozygote and A or a to indicate a homozygote. Mendel used notation resembling fractions to symbolize the contributions of hereditary elements in the pollen and ovule cells. He represented elements contributed by pollen cells in the numerator and those in ovule cells in the denominator. This notation indicates how Mendel “placed the average course of fertilization in the context of the simple series” (Orel 1996, p. 115).
Stern and Sherwood 1966, p. 30
Mendel's decision to allow the F2 offspring to self-fertilize so that he could identify the 1:2:1 genotypic ratio, and his decision to apply mathematics to his theory clearly illustrate that his scientific knowledge originated from imaginative and creative processes (Table 5).
Table 5. Correlation of NOS-5 to Mendel.
|NOS-5, Scientific knowledge originates from imaginative and creative processes.||Mendel's decision to allow the F2 offspring to self-fertilize and to apply mathematics to his theory.|
Mendel's laws of segregation and independent assortment are invariably stated in the chapter on Mendelian genetics in general biology and genetics textbooks. For example, Mendel's law of segregation may be stated as, “the two alleles for each trait separate (segregate) during gamete formation” (Hartwell et al. 2004, p. 20), and independent assortment as ”during gamete formation, different pairs of alleles segregate independently of each other” (Hartwell et al. 2004, p. 26). Although Mendel did not state these laws in the same way as they are now stated, he did state them quite clearly in several passages using the terminology of his day, as in the following, which is his statement of the law of segregation:
“One could perhaps assume that in those hybrids [heterozygotes] whose offspring are variable a compromise takes place between the differing elements of the germinal and pollen cell great enough to permit the formation of a cell that becomes the basis for the hybrid, but that this balance between antagonistic elements is only temporary and does not extend beyond the lifetime of the hybrid plant. Since no changes in its characteristics can be noticed throughout the vegetative period, we must further conclude that the differing elements succeed in escaping from the enforced association only at the stage at which the reproductive cells develop. In the formation of these cells, all elements present participate in completely free and uniform fashion, and only those that differ separate from each other. In this manner the production of as many kinds of germinal and pollen cells would be possible as there are combinations of potentially formative elements.”
Stern and Sherwood 1966, pp. 42–43 (italics in the original)
Mendel's “potentially formative elements” were in today's terminology “genes or alleles,” and inherited determiners of traits carried within the pollen and egg cells. His reference to “the differing elements succeed in escaping from the enforced association only at the stage at which the reproductive cells develop” demonstrates his understanding that the different alleles (A and a) are paired in heterozygotes (Aa). He viewed the elements as being paired until they separate from one another when “the reproductive cells develop,” This is a clear description of segregation of paired alleles during meiosis, or Mendel's law of segregation.
Mendel also quite clearly stated the principle of independent assortment in his paper, albeit not with the same terms commonly used today. After summarizing the results of his dihybrid and trihybrid experiments, Mendel concluded the section stating this law as, “the behavior of each pair of differing traits in a hybrid association is independent of all other differences in the two parental plants” (Stern and Sherwood 1966, p. 22, italics in the original).
Mendel's laws are durable in that they have been repeatedly demonstrated over a period of more than a century in numerous species, although they have been modified on the basis of further evidence (NOS-6). For example, our understanding today of segregation is that alleles in both the heterozygous and homozygous states separate during anaphase I of meiosis. That alleles in both the heterozygous and homozygous states separate from their “enforced association” is not indicated in Mendel's paper. Mendel viewed segregation as being restricted to heterozygotes (Fairbanks and Rytting 2001). NOS-6 is clearly evident in this example that scientific knowledge, (the segregation of alleles) is durable and can be modified (now includes alleles in the homozygous state) with further evidence (Table 6).
Table 6. Correlation of NOS-6 to Mendel.
|NOS-6 Scientific knowledge is not dogmatic but considered durable and can be modified or replaced with further evidence||Mendel viewed segregation of alleles as being restricted to heterozygotes, which has been modified to include homozygotes|
Moreover, the discoveries that genes are located on chromosomes, and that unlinked genes assort independently whereas linked genes do not is an important modification of Mendel's law of independent assortment.
Mendel demonstrated his laws of segregation and independent assortment mathematically, but ventured beyond mathematical demonstration to propose an explanatory theory of inheritance based on the cell theory of his day (Table 1). As stated earlier, this is an example of NOS-1, that concerns the difference between scientific theories and laws. His theory of inheritance remains durable, an example of (NOS-6), as the modern foundational theory of inheritance.
End of Student Reading
We have used a nature of science perspective in our analysis of Mendel's experiments to explore his contributions to genetics. In our genetics courses, we have discussed the nature of science in relation to Mendel's discoveries. We present it here as a way for genetics instructors to enrich and expand their traditional presentation on Mendel by including characteristics of the nature of science, with references to original source material and accurate historical context. As McComas (2008, p. 261) stressed, “If students do not have an explicit opportunity to link the historical example with an NOS principle [characteristic], they will likely hear these accounts of science and consider them interesting but not particularly enlightening stories.” Thus, students need explicit instruction on the nature of science, and opportunities through questioning to reflect upon them. We suggest that instructors ask their students questions about the nature of science based upon the provided student supplemental reading so that they have opportunities to reflect upon and fully engage in the material. Instructors also may find Clough's (2007) nature-of-science questions beneficial for discussions. These questions can accompany any historical vignette, including Mendel's story and stories about other geneticists (e.g. Bateson, Morgan, Fisher, McClintock, and others) in later lectures. These questions should encourage classroom discussions that are needed to create meaningful learning about the nature of science.