Sci & Educ (2007) 16:849–881 DOI 10.1007/s11191-006-9064-4 ORIGINAL PAPER

Definition of historical models of gene function and their relation to students’ understanding of genetics Niklas Markus Gericke Æ Mariana Hagberg

Received: 7 April 2006 / Accepted: 11 November 2006 / Published online: 5 December 2006
Springer Science+Business Media B.V. 2006

Abstract Models are often used when teaching science. In this paper historical models and students’ ideas about genetics are compared. The historical development of the scientific idea of the gene and its function is described and categorized into five historical models of gene function. Differences and similarities between these historical models are made explicit. Internal and external consistency problems between the models are identified and discussed. From the consistency analysis seven epistemological features are identified. The features vary in such ways between the historical models that it is claimed that learning difficulties might be the consequence if these features are not explicitly addressed when teaching genetics. Students’ understanding of genetics, as described in science education literature, is then examined. The comparison shows extensive parallelism between students’ alternative understanding of genetics and the epistemological features, i.e., the claim is strengthened. It is also argued that, when teaching gene function, the outlined historical models could be useful in a combined nature of science and history of science approach. Our findings also raise the question what to teach in relation to preferred learning outcomes in genetics. Keywords Historical models Æ Models Æ Gene Æ Gene function Æ Genetics Æ Students’ understanding of genetics Æ Nature of science Æ History of science Æ Epistemology

N. M. Gericke (&) Department of Biology, Karlstad University Faculty of Social and Life Sciences, Universitetsgatan 2, Karlstad, Varmland 651 88, Sweden e-mail: [email protected] M. Hagberg Teacher Education Faculty office, Karlstad University, Universitetsgatan 2, Karlstad, Varmland 651 88, Sweden

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1 Introduction In science education the expression ‘nature of science’ (NOS) often refers to the epistemology of science, science as a way of knowing, or the values and beliefs include in the progression of scientific knowledge (Lederman 1992). One important aspect of the development of scientific knowledge is models (Leatherdale 1974; Giere 1988). During history scientists have generated different explanations for natural phenomena, which have led to the development of different and/or more elaborate scientific models over time. Models, that later have been extensively elaborated and changed, are referred to as historical models although they might still be in use in science and science education (Gilbert et al. 2000). Historical models are indeed often the products which we use and also want students to learn. By studying the history of science (HOS) it is also possible to learn about different aspects of the NOS, such as the role of models. In this paper we will investigate the historical development of the idea about the gene and its function. We will analyze the significance of this conceptual change from a NOS perspective and relate it to students’ understanding of genetics as it is described in research in science education. The subject has been chosen for two reasons. First research in science education has shown that genetics is one of the most problematic areas of biology education for students to learn (Johnstone and Mahmoud 1980; Bahar et al. 1999) and widespread concept confusion has been documented (Banet and Ayuso 2000; Lewis and Wood-Robinson 2000; Marbach-Ad 2001; Lewis and Kattmann 2004), secondly in modern biology gene function in terms of genetic expression is widely discussed in society, and thus is an important part of the curriculum.

2 Background-Models Science is about describing, predicting and finding explanations for natural phenomena in the world-as-experienced. The outcomes of science can be described as entities of which the world is believed to consist of or be analyzed with (concepts), proposals for how these entities are physically and temporally correlated to each other in the material world (models), and general sets of reasons why these concepts and models can be thought to occur (theories) (Gilbert et al. 2000). The central role that models play as outcomes of scientific enquiry is well established (Giere 1988). Models have been recognized as essential elements in the process of the development of theories (Harre´ 1970; Nersessian 1992; Giere 1994). ‘Indeed, the very term ‘model’ has supplanted the word ‘theory’ in many contexts of scientific inquiry’ (Rosenberg 2000, p. 96). Whether the model explains data, as in a realist view of science, or organizes it, as in an instrumentalist view of science, it is a useful tool for scientists. In both cases applying the model requires a connection to what can be observed or experienced in the world (Rosenberg 2000). There is no unique definition to the term ‘model’ in the literature, and there is no consensus on the use of the term, be it philosophers of science or science educators (Halloun 2004). A model in science is in this paper seen as a representation of a phenomenon initially produced for a specific purpose. A phenomenon is here viewed as an intellectually interesting way of segregating a part of the world-as-experienced for further study. The model is a simplification of the phenomenon intended to be used to develop

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explanations of the phenomenon. The entities from which the model is constructed are concrete or abstract and related within systems or processes. In science education there is an extensive literature about models and modeling (Gilbert 1991; Van Driel and Verloop 1999; Boulter and Buckley 2000; Gilbert et al. 2000; Harrison and Treagust 2000). Models play an important role in communicating science. According to Van Driel and Verloop (1999) individuals may use mental models, which are representations of for instance a natural phenomenon. When a mental model is expressed by an individual through speech or writing the model becomes available for discussion and interpretations also among others. Through comparison and testing, an expressed model, may reach agreement among scientists and become what is called a scientific model (Gilbert et al. 1998). When observational data come in conflict with a model and no easy correspondence can be found between the model and existing data the scientists often are convinced that the anomaly is real and that the model needs revision (Wimsatt 1987; Kuhn 1996). This recognition may prompt solvers to invent entities or processes that did not exist in previous models, or posit new states for ‘old’ entities. The structure of the model is revised until the model again has explanatory power and fits well enough with the empirical world as it is accepted by the scientific community. Hence the entities of the model and/or the relations combining them might be changed to fit the internal- and/ or external-consistency in this process. The internal problems are often resolved by scientists during the research process and do therefore not so often become noticeable in the literature. Accordingly the structure as well as the conceptual meaning of the model can be altered. If the revised model, although similar to its predecessor, replaces it the old model will be regarded out of date. ‘They become ‘‘historical models’’...condemned to be used only for routine enquires and to the graveyard of all science, the school (and university?) curriculum’ (Gilbert et al. 2000, p. 34). However, if both models prove useful in explaining the phenomenon, they may coexist. In genetics the gene is a central concept from which many other concepts in the field are derived. The gene is operationally defined on the basis of four phenomena: genetic transmission, genetic recombination, gene mutation, and gene function. These criteria of definition are interdependent. Thus, we typically cannot for example observe gene function or gene mutation without transmission and vice versa (Portin 1993). Research and applications in genetics has in various degrees focused on the different aspects during history. Scientists have come up with different answers and hypotheses to explain these phenomena and their interrelations. Like in science generally it has led to a change in different scientific models over time. In this paper we have focused on the functional aspects. The gene is the basic biological unit of heredity to which a specific function can be assigned (Cadogan 2000). What the function composes of varies between the models, but it always contributes to an observable characteristic, product or process in an organism. It is not possible to give a single unambiguous view of the idea of gene function in a specific time since competing models and ideas exist simultaneously in a scientific community. Therefore, this study strives to present the most popularized and generally accepted models about the gene and its function during history. Carlson (1966) calls these models ‘straw man’ models, a term which well represent the historical models outlined in this paper. These should be of great interest in science education for multiple reasons. We can only in retrospect judge the relevance of the historical models. In an educational context this is done explicitly or implicitly whether it concerns curricula, textbooks, teacher training or classroom settings by choosing what models to present and not to

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present. In these decision-making processes there might be several important aspects concerning: science; history; pedagogy, and society influencing the choice. Because of the penetrating ability of the ‘straw man’ models in the scientific community as well as in society overall, they will most probably be used as representations in an educational context. Hence, their importance in education should be elucidated. For educational purposes it is considered useful to know about and be able to recognize historical models. According to Roth (1995) science should be taught as ‘authentic’ as possible, e.g., being as faithful to the intellectual structures of the parent disciplines as possible. In the classroom students often encounter descriptions of models as if the models themselves were the phenomenon, without any explicit discussion of their nature and purpose (Grosslight et al. 1991). Often so-called hybrid models are used. These models consist of entities from separate historical models with different theoretical backgrounds. No HOS is then possible since it implies that scientific knowledge grows linearly and is context independent and no progression between the models can be seen and grasped. Instead it implies that different models of a phenomenon constitute a coherent whole, an idea that according to Justi (2000) could lead to concept confusion among students. Therefore, teaching about development and progression of historical scientific models might be a way of improving science education since the context in which a model is built is emphasized. Also the deficiencies and the explanatory capability of a given model will be outlined thus making a contribution to students’ better understanding (Justi 2000). In biology education in secondary school, upper secondary school as well as in undergraduate courses at the university the objective often is to cover all subdisciplines of biology to give an overview. Hence, students encounter different models. If they do that without an ability to think about a model, rather than only think with it, conceptual misunderstandings are to be expected. Yet another problem might be the widespread use of history in genetics education. It is a good idea to bring the historical ideas and models to the students’ notion, but doing this without an explicit framework of NOS could be devastating. As shown by Abd-El-Khalick and Lederman (2000) no improvement of students’ understanding of NOS can be proven from a teaching of HOS without making aspects of NOS explicit. Hence, the conceptual differences and purposes of the historical models should be made explicit when using a HOS approach in genetics education to develop both conceptual learning as well as the learning about NOS. Moreover, the historical and philosophical approach is by many researchers thought to humanize science and stimulate the interest among students (Matthews 1992). The purpose of this study is to identify the major historical models of gene function from different historical contexts and make explicit the differences and similarities between them. Internal and external consistency problems between the models will be identified and discussed as well as their influence on students’ learning and understanding. Students’ understanding of genetics as described in science education literature is examined from a scientific modeling perspective. The relation between scientific models used in genetics and students’ ideas about genetics is investigated. The research questions are: • What major historical models of gene function can be described? • What relations between students’ reported understanding of gene function and the historical models can be identified?

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3 Mode of inquiry An analysis regarding the development and meaning of historical models of gene function from Mendel to today was made in retrospect. A review of contemporary literature of the history and philosophy of genetics was used for the analyses. The literature in the history and philosophy of genetics is very extensive but give quite a coherent view of the historical development. The major contributors for this study were: Carlson (1966, 1991, 2004), Mayr (1982), and Portin (1993). These are authors with personal experiences of research in biology (mainly genetics), and with international recognition as authorities in the field of history and philosophy of genetics. In Sect. 4 we give a brief presentation of the historical development and present our categorization of gene function into five historical models. Later, in Sect. 6, we compare the different models and critically analyze them in a NOS perspective in order to elucidate the epistemological differences. From the literature study our approach was to categorize different meanings and characteristics of the descriptions of gene function into a few main historical models. This was done by a method clarified by Justi and Gilbert (1999) in which important aspects that change over time were determined. Hence, each model represents a significant paradigmatic change in the way the function of the gene was perceived. It is mainly a history of ‘the winning ideas,’ i.e., ‘straw man’ models, and not of ‘sidetrack’ or ‘false’ models because in an educational context it is those that are in use. However, ‘false’ models may still be important in the scientific community as means of improving descriptions and explanations of the world, as recognized by Wimsatt (1987). The presented models in this article are made by us out of modern literature in order to show a historical development. So in that sense the models are not recognized as ‘true historically’ although their component ideas are. In order to explain and compare the most important features in the historical models they are visualized in concept maps. This was done by the aid of a concept-mapping program called Cmap Tools. The most important entities in a model are written in boxes. The meaning of the entities (=concepts) is then explained and arrows show how the entities relate to each other. Entities that belong to boxes with dark background constitute the core idea of gene function in that model and the boxes with a more brighten background are other important entities which specify the characteristics of that model. The method of defining the models relies on a method described by Justi and Gilbert (1999). In order to categorize and show the progression of the historical models the main attribute was determined, which is the fundamental scientific idea in common to all of the models. The secondary attributes of the models are ideas that complement the main attributes to permit a comprehensive characterization of each model. Hence, secondary attributes can differ between the models and be discussed independently of each other, although all of them are related to the main attribute. From the secondary attributes a table of different aspects of gene function was compiled. Further, to define the historical models the following aspects have been systematically investigated to identify and characterize the models: • The main purpose of the model. • The way by which the new model overcame the explanatory deficiencies of its antecedents.

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• The features of the former model that was modified and incorporated into the new model. • The explanatory deficiencies of the new model. Models reside within an extensive disciplinary context that includes reasoning patterns as well as methodological, metaphysical, and epistemological norms (Stewart and Rudolph 2001). Once a model is recognized in a broader context of a discipline two main problems can arise for a user of the model (it might be a scientist or a student): (1) empirical assessment problems, in which the model are either used to (a) solve problems for which the model are assumed to be adequate, or (b) revise existing explanatory models to account for anomalous data. In both of these cases the problem is to fit data with the model. During this process a second problem can arise: (2) conceptual assessment problems, which consider (a) the internal consistency and coherence of the model, i.e., when the model exhibits logical inconsistencies, self-contradictions, conceptual ambiguity, or circularity (b) the external consistency, i.e., if the model fits the extended conceptual context in which it is embedded, including other models, or even with non science worldviews. The conceptual problems are not easily separated from the empirical because they are tightly entwined so when students deal with empirical problems using models in similar ways to scientists, they also enhance their conceptual understanding (Stewart and Rudolph 2001; Passmore and Stewart 2002). In this paper we have analyzed the outlined historical models of gene function from both an empirical assessment problems and a conceptual assessment problems perspective at the same time, but the focus is on the conceptual assessment problems. Both internal consistency problems within the models and/or external consistency problems between them were analyzed. In Sect. 6 this analysis is accounted for in order to problematize the epistemological differences between the models. The results from the analysis are displayed as epistemological features in Sect. 7. Another analysis was then accomplished in order to compare these features of epistemological difficulties with students’ understanding of genetics as described by the research literature in science education. No distinction was made between different age and school form categories of students or teachers. Instead we took an approach in which we wanted to identify as many kinds of views and learning difficulties as possible about gene function, so that we could see in what way these correspond to the epistemological difficulties in the models. A categorization of students’ reported understanding of genetics can be seen in Sect. 5 and the analysis of how these relate to the epistemological features is shown in Sect. 7.

4 The development of the gene function models The fundamental idea in genetics is the idea about a hidden hereditary factor (the gene) that influences a characteristic or a function of an organism (Cadogan 2000). Therefore, this was considered to be the main attribute when developing the gene function models. The secondary attributes of the models, which change between the models, were determined to be ideas concerned with the following factors: • Structure—Ideas about the kind of structure or substance of the genetic factor. • Organization level—Ideas related to which and how different organizational levels are used.

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• Processes—Ideas about the sort of relation between the genetic factors and other entities. • Entities—Ideas about other entities that influence a characteristic. The results of the analysis of the historical review of the gradual change of these secondary attributes were compiled in a list of important aspects of the way the gene and its function have been perceived over history (see Table 1). These aspects have dissimilar meanings in the different models. Hence, the way in which the models were defined from the different aspects can be described as an iterative process, in which determining the models and the meaning of the aspects influence each other. By categorizing the results five different historical models of gene function could be defined; the Mendelian model, the Classical model, the Biochemical-Classical model, the Neoclassical model and the Modern model. These are outlined here below through their different meaning and characteristics together with a short summary of the historical context in which they are founded. These models are what Carlson (1966) denotes ‘straw man’ models, i.e., the cruder and generally accepted models of that time and not of ‘sidetrack’ models. A totally true picture of a scientific view in a specific time is not easy to attain since there are competing models and ideas simultaneously in a scientific community. This is also the case in genetics, in particular during the first half of the twentieth century in the classical era when ‘these concepts of relations between genes and traits were only used on the theoretical level’ (Schwartz 2000, p. 31). Therefore, the availability of empirical evidence was scarce. Instead the indirect data about the gene and its function could be explained in several ways and different explanatory models coexisted. Most of them criticized one or several aspects of ‘the classical gene concept.’ Carlson (1966) clarifies this: ‘Whether Castle, Eyster, Demerec, Goldschmidt, East, Correns, or Muller is cited in the development of the gene concept...the result is the same: each discussed uniquely different models or properties of the gene which were opposed to the ‘straw-man’ model of the classical gene’ (p. 253). It follows that in the scope of this article it is not possible to account for all other contemporary views in a systematic way. Instead only selected examples are mentioned if they in some way highlight important epistemological aspects, which are thoroughly discussed in Sect. 6. 4.1 The mendelian model The idea of biological heredity is an ancient phenomenon based on experience from human mankind as well as her domestic animals and crops. At the time when the work of Mendel was rediscovered, in the early 1900s, there were three theories about the nature of the units of inheritance according to Mayr (1982): (1) (2) (3)

Each unit had all species characters; it was regarded as an entire species homununclus Each unit had the features of a single cell Each unit represented a single species character or trait.

The third theory, which was in line with Mendelian inheritance was later to be embraced as the right one. In these theories no distinction was made between genotype and phenotype in a functional sense since it was more or less taken for granted that the gene, through growth, was directly converted into the phenotype.

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A hypothetical construct (with possible material origin)

A trait: i.e., function ‘top down’

Transmission and function

The phenomenological level

The gene is viewed as

The gene is defined by

The gene is the unit of

The function of the gene is defined from

The model has entities Symbolic level at Phenomenological level

The Mendelian model of gene function

Model

Phenomenological level (enzyme is here seen as a substance)

Cellular level

Transmission, function, mutation and recombination The phenomenological level

A trait: i.e., function ‘top down’

A materiel unit consisting A hypothetical construct with a diverse material of a DNA-segment base consisting of DNA with fixed boundaries segments that take part (at the molecular level) in a developmental process (at the molecular level) A process: the gene A DNA-segment: i.e., A trait: i.e., function exists only when structure ‘bottom up’ ‘top down’ [internal it acts structure and consistency problems function coincides in the model, because there are also immediate direct effect of the gene (production of an enzyme), i.e., ‘bottom up’] Function Transmission, function, Function mutation and recombination The phenomenological The molecular level The molecular level (a polypeptide) level (polypeptides or RNA-molecules) Cellular level Molecular level Molecular level Phenomenological level (enzyme is here seen as a substance)

A particle (with vague material base at the cellular level)

A particle (with vague material base at the cellular level)

The Modern model of gene function

The Biochemical-Clas- The Neoclassical model of gene function sical model of gene function

The Classical model of gene function

Table 1 Important aspects of the way the gene and its function have been perceived in the historical models

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Environmental aspects or epigenesis are

The gene is

The model use

Causal mechanistic Causal idealistic and idealistic relationships relationships between the gene-construct and the trait. No real processes occur Explanatory reduction ‘Reduction’ from from phenomenological phenomenological level to cellular level level to symbolic (internal consistency level problem) Active: it determines a Passive: it only exist characteristic (no real distinction between genotype and phenotype) Not considered in Not considered in the the model model

The processes in the model can be described as

The Classical model of gene function

The Mendelian model of gene function

Model

Table 1 continued

No reduction Explanatory reduction from phenomenological level to cellular level (internal consistency problem) A passive template: Active and producer: it that codes for the produces a substance production of a that determines a polypeptide characteristic Not considered in Not considered in the the model model (although because the model describe biochemical processes it might be implied that the gene is a part of the developmental system)

Not shown direct as entities in the model although implied because the gene only exists in the context of a developmental system that moderate the expression of the gene

Active: producer of molecules in a developmental system

No reduction

Naturalistic biochemical Biochemical reactions reactions that take part that run in a mechanistic in a developmental but naturalistic way process and therefore are context dependent

Biochemical reactions that run in a mechanistic idealistic way

The Modern model of gene function

The Neoclassical model of gene function

The BiochemicalClassical model of gene function

Definition of historical models of gene function 857

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Fig. 1 The Mendelian model of gene function

Hence the genotype was regarded as the phenotype in miniature, not as a homonunclus but as a mosaic of heredity particles (whether called gemmules, pangenes, unit factors, etc.), each responsible for a definite component of the phenotype. No connection was however made to a material unit in the cell. A one-to-one relationship between genetic factor and somatic factor was believed to exist. It was stated by some followers of this so-called unit-factor theory of the early Mendelians that there were as many genetic factors as an organism had characters (Mayr 1982; Schwartz 2000). Thus, the idea of the function of the gene was very dim. From the analysis we have outlined the following model, shown in Fig. 1, which we call the Mendelian model. It describes the main ideas about the gene and its function in a reductionistic and mechanistic way. We have chosen to use the term ‘gene’ although in the early twentieth century several different words were used for the same concept. The term ‘gene’ was coined by Johannsen in 1909 and the term was deliberately created to represent the unit without implying anything of its composition or structure (Carlson 2004). 4.2 The classical model Genetics emerged as a subject of its own when breeding analysis was combined with studies in cytology, embryology and reproduction. The chromosome theory of heredity was established by Morgan in 1911 (Carlson 2004). Later he also demonstrated that coupling could be explained and interpreted through crossing-over. Thus, the same chromosome theory could incorporate linked genes. Sturtevant made a map of the genes on the chromosome from a cross-over index of Drosophila. This map visualized the genes relationship to one another in the chromosome and thus provided a representation of the chromosome as a string of beads where each bead represented a different gene (Portin 1993). Accordingly the classical mapping techniques played an epistemic role as they served to represent genetic structures and fine structures as real objects (Weber 1998; Gaudillie`re and Rheinberger 2004). The years around 1940, at the peak of classical genetics, the gene could be described as an indivisible unit of genetic transmission, recombination, mutation,

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and function. All of these criteria of the gene led to the same unit of genetic material (Portin 1993). The genetic material was considered particulate with long-term stability (‘hard inheritance’) and mutations was a discontinuous change of a gene. Each gene was assumed to be independent of neighboring genes. Definite characters were the product of genes, which were located at well-defined loci on the chromosomes. The genes were linked on the chromosome but could be separated by crossing-over. The principle of diploidy was known, that is each gene is represented in two homologous units at the chromosomes, each derived from, respectively, parents. Strict separation was made between genotype (the genetic material) and phenotype. The phenomena of polygeny (several genes influence a single character) and pleiotropy (a single gene affect several characters) were known to exist, which permitted a much clearer separation between transmission genetics and physiological genetics (Mayr 1982). ‘A contradiction was created however, because the research method was (allegedly) based on a one-to-one relationship between genes and traits’ (Schwartz 2000, p. 28), a fact creating much confusion about this relationship in the classical era. The function of the gene was only in the beginning of being understood in biochemical terms. Many geneticists also suppressed questions of development in favor of chromosomal mechanics, because the latter were susceptible to a quantitative approach (Lawrence 1992). The most widespread idea during the classical era, going back to Weismann among others, was that the genes were enzymes, or acted like enzymes, serving as catalysts for the chemical processes in the body, which resulted in physical traits (Carlson 1966; Mayr 1982). Changing phenotypic effects with position, i.e., position effect, raised questions of whether genes were functional units in the sense of whether or not they carried their function with them (Dietrich 2000). From our analysis we have constructed what we call the Classical model shown in Fig. 2. It describes the main ideas about the gene and its function at the peak of classical genetics. 4.3 The biochemical-classical model In the forties and fifties the classical genetics of breeding analysis and cytology of animals and plants were replaced in the research frontier by microbial experiments on fungi, bacteria, and viruses. The classical view of the gene was then further developed through microbial studies. Beadle and Ephrussi worked out the biochemical pathway for eye color synthesis in fruit flies (Carlson 2004). Later Beadle revealed the biochemical pathways of synthesis of vitamins and that these pathways consisted of ordered series of chemical steps, with a single gene controlling a single step in the chain of reactions. They launched biochemical genetics as a field and gave new incentives for studying one-celled organisms. This change of model organism shifted the emphasis in genetics toward function in general and developmental processes in particular instead of crossing-over and mutation studies, as in the Drosophila research. Although the classical gene concept was constantly questioned during the first half of the nineteenth century, by above all Richard Goldschmidt (Dietrich 2000), it kept its standing as ‘straw man’ model. Tatum proposed in 1941 the one-gene-one-enzyme hypothesis for genetic function (Rheinberger 2000), which is still considered essentially correct for microbial genes. However, these genetic and biochemistry experiments did not explain the nature of the biochemical pathways (Carlson 2004). As expressed with Pontecorvos own words in 1955: ‘The assumptions

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Fig. 2 The Classical model of gene function

behind this model are the ones I proposed some years ago...If we consider stepwise reactions occurring on the surface of the chromosome in an assembly line fashion’ (in Carlson 1966, p. 193). The above mentioned findings were in the field of biochemistry and molecular genetics but they used the conceptual tools of classical genetics. Hence, they did not require the knowledge of the structure of DNA as a double helix, although they did adopt Muller’s central thesis of classical genetics—the gene as the basis of life (Carlson 2004). In the light of the findings from biochemistry we have made a slightly revised model that expresses the ideas about gene function around 1950. The model is outlined in Fig. 3. 4.4 The neoclassical model When the structural model of DNA was suggested in 1953 by Watson and Crick the long search for the material basis of inheritance had ended. Their DNA-model fulfilled the characteristics necessary for the genetic material, that is, auto replication, specificity, and information content. The open questions became increasingly physiological, dealing with the function of genes and their role in ontogeny and physiology. The genotype and phenotype problem could now be stated in definite terms and from 1953 on it was understood that the DNA of the genotype does not itself enter into the developmental pathways but simply serves as a set of instructions. In this molecular model the focus shifts from the particulate atomistic gene to a gene consisting of codes and information. The breakthrough of molecular biology in 1950s coincided with the birth of information sciences and some of the key terms of that field, like program and code, were put to use in molecular genetics (Mayr 1982). In 1950s these metaphors gave rise to two concepts (Fox Keller 2000): ‘the developmental program including the entire cell’ and ‘the genetic program explicitly

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Fig. 3 The Biochemical-Classical model of gene function

identified with the genome.’ But as Fox Keller notes: ‘By 1970s, however, the program for development had effectively collapsed into a genetic program’ (p. 162). A metaphor that encourages a belief that is very deterministic, implying that only the DNA matters. The necessary dependency of genes on their cellular context was easily forgotten (Fox Keller 2000). Benzer’s theoretical division of the gene concept into cistron, muton, and recon, which was done prior to the findings of molecular genetics, proved to be very useful. The cistron being equivalent to a gene (a string of DNA) and muton as well as the recon was considered equivalent to a single base pair in the DNA structure by proving that a nucleotide is the smallest unit of genetic material that can lead to altered phenotype or be separated from other such units in recombination (Carlson 1991). The neoclassical view of the gene peaked at about 1970 and stated that the gene (cistron), defined by a cis-trans test, is a contiguous stretch of DNA that is transcribed as one unit into messenger RNA, coding for a single polypeptide (Portin 1993). From the analysis of this view we have constructed a model, which we call the Neoclassical model. In this model, traits, and phenotype at a macro level are no longer an issue in defining the gene. Instead the explanations are given on the micro and sub-micro-level, i.e., molecular or cell level. The information goes in one direction from the DNA to mRNA to polypeptides (and enzymes). The model is outlined in Fig. 4. 4.5 The modern model In research about gene function after 1970 there has been an increasing amount of anomalies that the Neoclassical model fail in one or more aspects to explain about higher eukaryotic organisms. A number of phenomena have been outlined that contradict the older models, i.e., split genes, alternative splicing, complex promoters,

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Fig. 4 The Neoclassical model of gene function

polyprotein genes, Multiple adenylation, enhancers, overlapping genes, and trans splicing (Rosenberg 1985; Portin 1993; Fogle 2000). Portin summarizes it: The gene is no longer a fixed point on the chromosome, defined by the cis-trans test and producing a single messenger RNA. Rather, most eukaryotic genes consist of split DNA sequences, often producing more than one mRNA by means of complex promoters and/or alternative splicing. Furthermore, DNA sequences are movable in certain respects, and proteins produced by a single gene are processed into their constituent parts. Moreover, in certain cases the primary transcript is edited before translation, using information from different units and thereby demolishing the one-to-one correspondence between gene and messenger RNA. Finally, the occurrence of nested genes invalidates the simpler and earlier idea of the linear arrangement of genes in the linkage group, and gene assembly similarly confutes the idea of a simple one-to-one correspondence between the gene as the unit of transmission and gene function (Portin 1993, p. 207). Thus, in a modern view of the gene and its function it is much more open and complex. It does not longer exist one true and general description; instead it takes different meaning for different scientists. ‘This entity (the gene, authors’ comments) can, and will indeed most often, be endowed with temporary and discontinuous existence, and it will often require a developmental process at its own level of organization for functional expression’ (Gayon 2005, p. 82). From an analysis of the gene definition by Singer and Berg we have constructed what we call the Modern model, as shown in Fig. 5. A eukaryotic gene is a combination of DNA segments that together constitute an expressible unit. Expression leads to the formation of one or more specific functional gene products that may be either RNA molecules or polypeptides. Each gene includes one or more DNA segments that regulate the transcription of the gene and its expression (Singer and Berg 1991, p. 622).

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Fig. 5 The Modern model of gene function

This model finally ends the idea of a gene as a discrete material unit and focus is entirely on the function. The function is no longer solely to produce a polypeptide. Instead there are a number of categories of genes such as enzyme producing genes, genes producing structural (nonsoluble) proteins, regulatory genes, and genes coding for RNA-molecules. The gene is viewed more as a process, which becomes when it acts. The information in the model goes in one direction from DNA to polypeptides or RNA molecules.

5 Analysis of the literature assessing students’ understanding of genetics Extensive work has been done in the field of students’ understanding about inheritance (Wood-Robinson 1994; Knippels 2002). In our analysis of the literature we have focused on the aspects concerning students’ understanding of gene function. In this field less is done, but we consider the following references the best representatives (Hallde´n 1990; Pashley 1994; Martins and Ogborn 1997; Venville and Treagust 1998; Banet and Ayuso 2000; Lewis et al. 2000a, b; Lewis and Wood-Robinson 2000; Marbach-Ad and Stavy 2000; Wood-Robinson et al. 2000; Marbach-Ad 2001; Knippels 2002; Forissier and Cle´ment 2003; Lewis and Kattmann 2004). These studies cover vast categories of students from late compulsory school to undergraduate level at university, including pre-service biology teachers, as well as active primary school teachers. The types of ideas held by the students/teachers, as well as their learning difficulties, seem to be very similar between various categories of students/teachers. What changes is the frequency of how often an idea or a learning difficulty appears in a category. Generally a progression toward a more molecular understanding of genetics is seen in later stages of the educational system. To give a crude, yet vivid picture of students’ understanding of gene function it can be described by the following list of conceptions and learning difficulties (note that some studies also include teachers’ ideas although we do not separate them from the students):

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• There are several categories of views or mental models of the gene described in the literature: a. Genes are transfer bearing particles (Venville and Treagust 1998; Lewis and Kattmann 2004). b. Genes determines characteristics (Marbach-Ad 2001; Lewis and Kattmann 2004). c. Genes are objects with actions that come natural to it, i.e., the gene is thought of as an physical object that take action in a unalterable way in the organism (Martins and Ogborn 1997). d. Genes are transmission of commands that controls characteristics (Martins and Ogborn 1997; Venville and Treagust 1998). e. Genes are active particles that control characteristics (Venville and Treagust 1998). f. Genes are productive sequence of instructions. A connection is being made between the genes and protein synthesis, and protein synthesis and an organism’s phenotype (Venville and Treagust 1998). Most frequent reported view seems to be to look at genes as particles or determining characteristics. To associate genes with protein synthesis is a rare notion. • Students have difficulties in distinguishing between genes and genetic information (Lewis and Wood-Robinson 2000). • Students often make no distinction between genotype and phenotype (Marbach-Ad and Stavy 2000; Marbach-Ad 2001; Lewis and Kattmann 2004). • Students can define single genetic concepts, but show difficulties in relating these concepts (Lewis et al. 2000a; Marbach-Ad 2001). • Students often explain in causal idealistic ways not with biochemical terms or processes (Lewis et al. 2000a, b; Marbach-Ad 2001; Lewis and Kattmann 2004). • Students show difficulties in relating structures and concepts to correct systematic level (Lewis et al. 2000b; Knippels 2002). • Students find it difficult to extrapolate between the different organizational levels (Hallde´n 1990; Marbach-Ad and Stavy 2000). • Students often relate to concepts at a phenomenological (i.e., macro level) and/or cellular organizational level, not to the molecular level (Marbach-Ad and Stavy 2000). • Students seldom investigate environmental influences of characteristics (Forissier and Cle´ment 2003). • Students find it difficult to separate the concept of allele from the concept of gene (Pashley 1994; Wood-Robinson 1994; Lewis et al. 2000a). The described way of understanding genetics is by many researchers looked upon as not adequate or insufficient. But what are the reasons for this result? Knippels (2002) identified five domain-specific difficulties from the literature about genetics education that answers this question: (1) (2) (3) (4)

Domain specific vocabulary and terminology. Mathematical content of genetic tasks. Cytological processes of cell division, which mainly relates to chromosome structure and its processes. Abstract nature due to the sequencing of the biology curriculum, which separate mainly meiosis and genetics.

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The complex nature of genetics: a macro-micro-problem, how to relate concepts and processes from different systematic levels.

Cavallo (1996) reports the lack of reasoning ability as an important factor for insufficient achievements in genetics. Could there be other reasons for students’ understanding of genetics looking like this? In Sect. 7 we will return to the students’ understanding of genetics and give a more extensive description. Further we will relate the historical models, as described in this paper, to students’ reported understanding of genetics in order to shed new light to the question why students speak about gene function the way they do.

6 Interpretation of the models in a nature of science perspective In order to verify the classification of the historical models the following characteristics of the models have been systematically investigated: (1) The main purpose of the model; (2) The way by which the new model overcame the explanatory deficiencies of its antecedents; (3) The features of the former model that was modified and incorporated into the new model; and (5) The explanatory deficiencies of the new model. The results of this investigation strengthen our categorization and are described in Table 2. The most difficult classification was the separation between the Classical- and the Biochemical-Classical model since the latter can be viewed as merely an extension of the former. Nevertheless we find the differences between them important because ‘the function of the genes was first expressed in a workable model by Beadle and Tatum with the one gene-one enzyme theory’ (Carlson 1966, p. 231). An interpretation and description of the models in a NOS perspective follows below. In the Mendelian model the gene is viewed as a hypothetical construct and its main purpose is to explain genetic transmission. The functional aspects are of less importance and the gene is seen as passive and thought of as the phenotype in miniature, hence no environmental aspects are considered. Regarding the function of the gene it is defined ‘top down,’ i.e., from a trait at the phenomenological level which then is extrapolated to the gene at a symbolic level. Because the gene in this model is an idea, not a physical entity, the relation between the gene and its trait can be viewed as causal and idealistic, not naturalistic. These descriptions are in line with many researchers in the history and philosophy of genetics (Mayr 1982; Schwartz 2000; Carlson 2004). The main difference between the Mendelian and the Classical model is that the gene becomes materialized as an indivisible particle on the chromosome. The gene becomes in this model the ‘atom’ of biology, which is the unit of genetic transmission, recombination, mutation, and function although transmission aspects still is in focus. A distinction is made between the gene at the cell level and the trait at phenomenological level. Likewise the function of the gene is determined from a trait at the phenomenological level, but now an explanatory reduction is made to the physical gene at the cell level. This leads to internal consistency problems in the model because polygeny and pleitropy during this epoch are recognized phenomena. The identification of the gene and the separation between genotype and phenotype also leads to a more active gene that determines traits, and there is a vague notion that the gene is an enzyme or acts as one. Still we would consider the relations

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The Classical model of gene function

The BiochemicalClassical model of gene function

The Neoclassical model The Modern model of gene function of gene function

The main purpose of the model was to

Explain the processes Explain the processes Explain in what way or Connect the physical Connect one gene, an by which the gene is by which the gene is by what processes the inheritable traits to abstract entity, with expressed at a expressed at a gene function. This was real materialized one physical character, molecular level. That molecular level. done in the conceptual chromosomal structures which it determines. is to be as naturalistic That is to be as framework of the in the cell. Focus in The main purpose of naturalistic as possible as possible to the classic model genetics at that time genetics in this context biochemical processes. to the biochemical is still at transmission, was to predict the Because of the influence processes. Once and which now is explained outcomes from breeding of developmental for all link a specific at cellular level. Although analysis of hereditary processes this is not function with a unit traits between generations. genetic function is more possible with retaining structure important because the Describing the function the unity of structure gene is now materialized of the gene within an and separated genes individual is only a minor within the individual in consequence of this major which it is expressed aspect The gene is no longer a The model gave a The new model gave a By localizing the genes to The way by which well-defined material and biochemical physical structures, the the new model coherent physical structure of the microscopic chromosomes, explanation of how the overcame structure. Instead it is gene (DNA-segment), defined from a process, it was possible to separate gene functioned within the explanatory a cell. One gene produced which has a single the genotype from the deficiencies of its and it exists only when function (to produce accordingly one enzyme phenotype. Thereby antecedents it acts. This is a more a polypeptide). The transmission genetics could (although the substance plastic and holistic way not the molecular structure model thoroughly be explained (not only to explain because it explains the processes can differ depending on is used as entity) predicted). Also the that relate the entities the context in which the refutation of the unit-factor theory gene is present was accomplished

The Mendelian model of gene function

Table 2 Characterization of important aspects of the historical models

866 N. M. Gericke, M. Hagberg

The Mendelian model of gene function

The Classical model of gene function

The BiochemicalClassical model of gene function

The Neoclassical model of gene function

The Modern model of gene function

The gene still consists of The gene still exists as a The conceptual Physical traits (that The features of the DNA and produces physical entity on the framework from could mutate) former model that polypeptides (but chromosome. The gene classical genetics remained the basis were modified and also RNA) is a producer of parts for defining the gene stayed the same incorporated into of enzymes (polypeptides) the new model It is hard to give explanations Although the model in Still the model could The model did not No correlation to a The explanatory on the macro level with this overall is consistent not explain how the explain how the physical or chemical deficiencies of model. It is also difficult to several anomalies exist biochemical processes processes of gene structure in the cell or the new model understand because the gene for eukaryotic organisms go about when the function worked organism. Therefore, is variable in time and space, gene produces enzymes. (such as split genes, in the cell it does not give a hence no discrete structural alternative splicing, And the connection to physiological unit exist anymore complex promoters, the traits at the macro explanation of gene overlapping genes, etc.) level was even vaguer function (or transmission) that show that this model (leading to internal has external consistency consistency problems) problems. Also the model gives only explanations at the molecular level

Table 2 continued

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between the entities in the model as idealistic without any connection to natural processes. The relations are causal and mechanistic, ‘the gene determines a trait,’ without any influence of environmental factors. Goldschmidt among others were opposed to this model (Carlson 1966; Dietrich 2000), but nevertheless, through the propagation of members in the Drosophila group like Morgan and Muller this became the ‘straw man’ model, which prevailed in that time (Carlson 1966). The Biochemical-Classical model is also in the framework of classical genetics and very similar to the Classical model but there are some crucial dissimilarities: the most important implication is that the main purpose is to describe the functional aspects of the gene. The gene is identified as an active producer of enzymes, which in turn bring about a trait. Thus, the idealistic relations of the Classical model become biochemical reactions in the Biochemical-Classical model. Nevertheless they are still considered as mechanistic and causal. The Biochemical-Classical model contains even more severe internal consistency problems because the connection of the gene to the enzyme is made from a ‘bottom up’ approach, but in the same time the ‘top down’ approach remains that connects the trait with the enzyme by explanatory reduction. This categorization is supported by Gifford’s (2000) division of different criterions for determining genetic traits in differentiating factor (DF) and proper individuation (PI). Gifford (2000) states: ’the PI criterion would fit with a ‘bottom-up’ approach, involving laying out the causal in the individual, rather than looking for patterns in populations. Relatedly, it would seem to fit with the tasks of investigating specific and direct products of a given gene’ (p. 45). The DF concept is by Schwartz (2000) associated to the classical gene and a ‘top-down’ approach in which the gene is defined from mutation and recombination differences at the population level, which is opposed to the introduced ‘bottom up’ approach in the Biochemical-Classical model at the individual level implying genetic determination. In the Neoclassical model we enter the world of molecular genetics in which structures of molecules become important. The most appealing with this model is that the structural aspects (a DNA segment with fixed boundaries) of the gene totally coincides with the functional (producing a polypeptide). It is on one hand very simple and clear-cut and on the other hand it has great explanatory power. The gene is viewed as a passive template that codes for a polypeptide; hence the gene is determined by a ‘bottom up’ approach. The explanations are exclusively about gene function at the molecular level in the model, losing the connection to the phenomenological level, and the relations in the model are described as biochemical reactions with great detail resolution leading to a naturalistic view of these relations. Hence, gene function has become genetic expression, i.e., translation and transcription of genetic information to give a gene product (Cadogan 2000). Although the breakdown of the particulate gene there are similarities between the gene of Biochemical-Classical model and the gene of the Neoclassical model because they describe a one-to-one correspondence between the physical gene and its function in a very mechanic way. Both models use the PI definition of the link between the gene and the phenotype, which gives a deterministic view of how the genes are expressed (Gifford 2000; Schwartz 2000). As a consequence in the Neoclassical model no entities exist that take consideration of environmental aspects of gene function. However, since the function in the model is described as biochemical processes it might be implied that the gene should be part of the developmental system that are influenced by the environment.

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In many ways the Neoclassical model is still valid, this goes for unicellular organisms especially prokaryotes, but for multicellular eukaryotes, such as man, several anomalies have been detected about gene function that give a more complex illustration of the phenomenon. There have been several proposals for giving a universal definition of the gene and its function. Here we have chosen to describe one of the most common ways to portrait the gene and its function, which we call the Modern model. Because it does not exist a one-to-one correspondence between a specific DNA segment and a specific product the mechanistic view of the Neoclassical model must be abandoned. Instead contrasting parts of the DNA strand can cooperate in different ways producing various products in time and space. Therefore, in the Modern model the gene is associated to a process. It exists only when it acts, i.e., producing polypeptides or RNA molecules, and the gene once again becomes an active producer. The relations in the Modern model is described as naturalistic biochemical reactions as in the Neoclassical, but because genes operates differently in various contexts (i.e., in different cells and organisms) the processes are viewed as more dynamic and not deterministic. As a consequence the functional aspect of the gene widens from a mere molecular expression in the Neoclassical model (what does it produce) to a more general functional context, which Dutilh et al. (2006) describe as the context of the encoded protein and the regulation process of its expression in time and space. This also implies the importance of the environment in which the gene is placed; nevertheless no direct entities representing environmental aspects are present in the model. This way of perceiving genetics has become more widespread in philosophy of genetics. Griesmer (2000) argues: ‘I urge a change of perspective on genetics and gene concepts. The fundamental entities of biology are processes rather than structures and functions’ (p. 240). As described above and in Tables 1 and 2 there are external consistency problems between the different models and in some cases internal consistency problems within them. The vocabulary differs only slightly or not at all, albeit the meaning of terminology does. The entities and the relations in the models should be interpreted differently. Hence the models do not fully coincide which make it difficult to extrapolate between entities from the different models. The problem of reduction is addressed to by many antireductionists in genetics, whom in various degrees declare the difficulties of reducing classical genetics into molecular (Kitcher 1982; Mayr 1982, 1997; Rosenberg 1985; Kincaid 1990). Rosenberg (1985) sums this up: ‘it is true that the required connections between Mendelian terms, like gene, phenotype, dominant, recessive, mutation, etc., and their molecular realizations, may be far too complicated...for limited creatures like us’ (p. 110). This is a central part of the problem but not the whole explanation. Since genetics evolved in different scientific communities with different model organisms and practices, different concepts and meaning of terminology evolved (Weber 2004, p. 63). The historical models as represented in this paper is a way of systematically describe and reflect these different conceptual frameworks. In Sect. 7 we will highlight the results from the consistency analysis of the models as epistemological features. The practice and focus of genetics have changed over time from breeding analysis of inheritance at macro level to the function of biological processes at molecular level. This in turn influences the focus of the models from macro level to molecular level. The intention has through history been almost constant; to outline the biological determinants influence on physical traits (the main attribute); leading to that all the models neglect environmental factors. Although through the history of

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genetics there has always been acknowledged that environmental factors influence the phenotype. A gene can be said to have a structural and a functional component. The first Mendelian model mediates a vague picture of both these components. In the forthcoming three models structure as well as function becomes more clear-cut because of the more reductionistic approach ignoring developmental biology and hence environmental factors. In The Modern model a return of a more holistic approach can be seen, as developmental and physiological issues no longer can be totally ignored, thus making the model much more complex. The advances in our understanding in the modern molecular views come from better understanding properties arising from relations within the genetic system and not from understanding just those properties the parts would exhibit in isolation. A development to a more holistic way of looking at gene function can be seen although this understanding arises from research and explanations at the molecular level, the ‘most reduced level.’ It is a paradox that as our understanding of gene function has increased through more detailed knowledge about biochemical processes; a reduction in study object, our notion of gene function is more holistic and flexible than ever. This is seen in The Modern model of gene function. It tries to be as ‘naturalistic’ as possible in order to be as authentic as possible to the biochemical processes (the natural phenomena). Therefore, molecular genetics has abandoned the abstract concepts of classical genetics. The problem is that the structure and function, which are the constituents these models try to link together and explain do not coincide. To solve this problem Falk (2000) have proposed following solutions to cope with the molecular gene: (1) Abstract away from the complexities of molecular biology and define terms of some role they play in evolution. (2) Continue to seek a structural definition at the molecular level (a quest Falk regards as hopeless). (3) Look for a functional account of the gene in molecular development biology, relying on a broadening focus from the DNA to the wider developmental system in which the gene concept is embedded. (4) To treat genes as ‘generic operational entities’ defined by experimentalists to suit changing needs in different contexts. This is of course suggestions for the research community in biology and genetics but nevertheless a relevant question also to an educator in genetics. When teaching gene function you have to adapt to one of these four strategies in order to teach the subject authentically. We would argue for the forth position by Falk because in science education you have to comply with the setting in the surrounding world. There the different historical models are used depending on context. Kitcher (1982) argues that to reach the best naturalism and clarity one must be prepared to have different definitions of the gene for different purposes. This is a view that many researchers in the history and philosophy of science agree upon (Fogle 1990, 2000; Carlson 1991; Waters 1994) and we comply with it too because although our knowledge about the structure and organization of the genetic material has increased tremendously our notion of the gene is now much more complex, general and open. It is hard to construct one model that takes into account all the aspects of the gene, which can be considered as ‘the true description of the gene.’ Instead as Carlson (1991) concludes, the gene can mean different things in different contexts. Different models can be used for different purposes and this includes historical models. Carlson (1991) claims that for most students not engaged in genetic research, the gene in its classical functional sense is more helpful than the gene in its complex biochemical or molecular sense. Also Rosenberg (1985) argues for classical genetics in education: ‘the practical applicability that they (Mendelian laws, authors’

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comments) do have is more than enough to ensure the permanent entrenchment of Mendelian genetics, both in textbook presentation of biology and in the theoretical edifice of life science’ (p. 109). Hence, it is not always obviously best to use the Modern model in teaching genetics but one should be clear of what model being used and the purpose of it. Which aspect of the gene is of interest, its function, transmission, recombination or mutation? Which level of biological organization is to be described molecular, cellular, individual or populations? Due to different levels and aspects of the gene that is to be perceived, accordingly different models can and should be used.

7 Students’ understanding of gene function in relation to the historical models From the exposition of the critical aspects that differ between the models we will now identify the important epistemological features of gene function that the internal- and external-consistency problems of the models have bearing on. Below we identify seven features, which of course are tightly intertwined and not possible to separate completely, but by doing just that it is possible to address these important features when teaching genetics in a systematically way and make them explicit. If not, we would expect difficulties among students’ understanding to emerge. The underlying assumption is that where we can find internal consistency problems within a model or external ones between models, we might presume learning difficulties among students. Especially from students in primary-, secondary-, and upper secondary-school, in which an approach with models in a NOS perspective in teaching is less likely to occur. Below we will first present each feature and argue for its importance, and second relate these epistemological features to the research done about students’ and teachers’ understanding and alternative conceptions. 7.1 The structure and function relation of the gene This feature addresses the issue about what a gene really is. The question can be answered in various ways depending on to what extent structure, respectively, function contribute to the answer. Different explanations can be deduced according to which model being used. Hence, there are external consistency differences between the models. The relation between structure and function is recognized as of central importance in biology (Mayr 1982; Hoffmeyer 1988) and in the history and philosophy of genetics their shifting meaning has been addressed by many authors (Carlson 1966; Rosenberg 1985; Schwartz 2000; Griffiths 2002). Carlson (1966) describes this problem in a compelling way: ‘the quest for the structure and function of the hereditary units is as old as the rediscovery of Mendelism’ (p. 244), and as explained by Falk (2000) still a central problem. How then is this feature related to teachers’ and students’ conceptions? In a study of 14–16-year-old students’ understanding of genetics at late compulsory school (Lewis et al. 2000a) it was concluded that: ‘what they (students, authors’ comments) appeared to lack was a basic understanding of what a gene is—Its basic function, where it might be found and how it relates to other structures’ (p. 76). Similar results can be seen in an interview study by Martins and Ogborn (1997) in which they

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identified two different metaphoric models for genes used by primary school teachers: ‘genes are objects with actions come natural to it’ and ‘transmission of commands,’ the first being more frequently used. From these metaphoric models they recognized the identification problem—is the gene an object or an action? Is the gene part of DNA or is DNA part of the gene? Here we are at the core of the problem. The scientific models use different structural and functional explanations of what a gene is and the students’ and teachers’ answers are reflections of this. The question is what model or models do we want the students to learn and for what reason? The educational aims must be decided before teaching, and evaluations of students’ and teachers’ conceptions are meaningful only in comparison to the goals of teaching. Otherwise how can we assess their knowledge? Venville and Treagust (1998) extended the picture of students’ conception by looking at conceptual change among grade ten students and they defined four different ontological mental models held by the students: passive particle gene—the gene is viewed as a particle that are passed from one parent to offspring; active particle gene—the gene is more active and controls characteristics; sequence of instruction gene—a code or message that controls characteristics and productive sequence of instructions gene—a connection is being made between the genes and protein synthesis, and protein synthesis and an organism’s phenotype. A majority of the students had an active particle gene conception after attending a genetics course. Another evident result from their study was that few students connected protein synthesis to the gene and could not explain the function of the gene in that sense. Marbach-Ad (2001) probed 9th graders, 12th graders, and pre-service teachers at college and university, understanding of the relationship between genetic concepts. The most common notions among 9th and 12th graders were that: ‘a gene is a trait,’ ‘the gene determines a trait’ and for pre-service teachers ‘a gene is a code or template for traits or proteins’ (p. 185). Also here we can see a predominant particulate view of the gene. A result also found in a study by Lewis and Kattmann (2004) with students aged 15–19, in which the students’ understanding of the processes and mechanisms of inheritance was investigated. Their result shows convincing evidence that genes are viewed as small particles containing a trait or characteristic in miniature and no clear distinction between the genotype and phenotype is made. The students often had a notion of ‘transfer of trait bearing particles’ (p. 200) between generations. Another finding by Lewis and Kattmann (2004) was that the students stated that genes determined characteristics, but they did not hold any coherent understanding of the biological mechanisms that explains how this might be achieved. One of the most common alternative conceptions in genetics, which might be connected to the relation between structure and function of the gene, is the difficulties in separating the concept of genes from the concept of alleles (Pashley 1994; Wood-Robinson 1994; Lewis et al. 2000a). If the foundation in understanding the gene is structural, as in the Classical-, Biochemical-Classical- and Neoclassical model, there are no differences between a gene and an allele. Instead the concept of gene and allele belongs to different categories if functional aspects are the foundations of definition. Schwartz (2000) stresses a related issue; the difference between the DF concept of the gene (e.g., used in the Classical model), which covers the relations between alleles and determinable traits, and the PI concept (e.g., used in the Neoclassical model), which covers one-to-one relations between genes and their determinable traits. If the former definition is being used interchangeable with the

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latter consistency problem emerge between the use of the concepts of gene and allele. 7.2 The relation between organization level and definition of gene function This feature addresses the issue that the various models explanatory power of gene function stretches to different organizational levels. In the first three models the function is defined from the phenomenological level and in the last two from molecular level. Therefore, there are external consistency problems between the models. The difference in explanatory power between the models may seem obvious, but perhaps not the consequences. According to Rosenberg (1985, p. 112) the Mendelian phenomena on the macro level are supervenient to molecular interactions, i.e., not connectable to one another in a manageable way. A problem also identified in teachers’ explanations by Martins and Ogborn (1997) as the the localizability problem—that is the systematic level at which to locate genes and their effects. Lewis et al. (2000a) and Wood-Robinson et al. (2000) investigation of 14–16-year-old students understanding of genetics at late compulsory school found that students think of genes as determining characteristics or provide information, not as a protein producer. Marbach-Ad and Stavy (2000) found that students gave answers rather on a cellular than a molecular level: ‘almost all micro-level explanations were on the microscopic level (the students especially used the concept of gene) rather than the submicroscopic [molecular, authors’ comments] level’ (p. 202). The tendency is that students predominantly use explanations from the first three models. 7.3 The ‘real’ approach to define the function of the gene: top down/bottom up This feature addresses the issue of whether constructing the models by either extrapolating a relation from the gene to an entity that defines its function, or vice versa from an entity that defines the function back to the gene itself. This feature is closely related to the former ‘The relation between organization level and definition of gene function.’ The difference is that the former feature addresses what the models intend to explain, i.e., how the gene operates, and this how the gene is ‘really’ defined when constructing the models. External consistency problems arise between the models depending on if a ‘top down’ or a ‘bottom up’ approach is used due to it is not necessarily a total coherence between the function of the gene in the different models. Gifford (2000) argues in a similar manner about the defining of genetic traits with DF (top-down) criterion or PI (bottom up) criterion: ‘the fact that traits are at various degrees of directness to or remoteness from the genes will individuate the genes differently’ (p. 45). In the Biochemical-Classical model there are also internal consistency problems because there is a ‘top down’ approach in determining the gene but at the same time a ‘bottom up’ approach is introduced when a gene is said to be an enzyme producer. From the literature it can be seen that students tend to think of genes as determining characteristics or provide information not as a protein producer (Lewis et al. 2000a; Wood-Robinson et al. 2000), hence a view with a ‘top down’ approach is most often reported. As stated by Wood-Robinson et al. (2000): ‘All but one of the groups referred to chromosomes or genes being involved in determining characteristics—specific characteristics being mentioned by a number of them’ (p. 31).

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7.4 The relation between genotype and phenotype This feature addresses the issue if and how the models describe the separation between the genotype and the phenotype. Also here there are external consistency problems between the models. In the Mendelian model no separation is made, i.e., a preformationist view (Mayr 1982; Schwartz 2000). In the Classical- and BiochemicalClassical models a separation is made but not explained. In the Neoclassical and Modern models there is a separation, which is fully explained by biochemical processes. Gifford (2000) call upon the difficulties about this relation: A challenge to the connection (between genes and genetic traits, authors’ comments) is simply that the one case is about genes and the other about phenotypic traits. One-way of seeing how these are very different is to note that one is about causes, the other about effects. A second is to note that the gene case involves entities, while the trait involves features and properties (Gifford 2000, p. 50). Further, Gifford (2000) and Schwartz (2000) problematize about the differences in this relation according to if a DF or PI criterion are used in the definition of genes and genetic traits. Hence, from a scientific viewpoint this relation can be described differently as presented in the models. From several studies the problem that students see no difference between genotype and phenotype has been identified (Marbach-Ad and Stavy 2000; Marbach-Ad 2001; Lewis and Kattmann 2004). Lewis and Kattmann (2004) conclude: ‘the terms ‘‘gene’’ and ‘‘character’’ may be considered equivalent and students make no distinction between the genotype and phenotype’ (p. 199). It is also common for students to have a notion from the Classical model that ‘genes determines a trait’ (Lewis et al. 2000a; Wood-Robinson et al. 2000) without explaining how. 7.5 The idealistic versus naturalistic relations in the models This feature addresses the issue if the relations between the entities in the models are viewed as idealistic or naturalistic. As described in Table 1 this varies between the models and so we have external consistency problems. The evolution of the relations in the models is toward a more naturalistic view, with the BiochemicalClassical model as a transient stage. Gayon (2000) express this change: ‘the molecular phase of the science of heredity developed in the philosophical mood of realism’ (p. 80). The mechanistic description of the relations emerged when the gene became materialized in the Classical model and survived through the molecularization to the Neoclassical model as Carlson (1966) manifests with the description of the operon theory in the 1960s: ‘the operon became an extreme example of a mechanistic system of circuits, feedbacks, and blueprints’ (p. 229). How then is this feature reflected in students’ understanding? Students show a lack of understanding of biochemical processes (Lewis et al. 2000a, b; Marbach-Ad 2001; Lewis and Kattmann 2004) and tend to favor idealistic and mechanistic explanation patterns. Marbach-Ad and Stavy (2000) describes it as follows: ‘although many pupils (12th graders) used concepts and terms from the micro-level..., such as, ‘Gene/DNA is responsible for the production of a trait’ or ‘Gene/DNA is encoded for a trait,’ they were unable to explain the mechanisms and the intermediate stages involved in this link’ (p. 204).

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7.6 The reduction explanatory problem This feature addresses the issue of internal consistency problems in the Classical model and particularly in the Biochemical-Classical model, and external consistency problems if you reduce between entities from the different models and/or different systematic level. The problem of reduction in genetics, and in particular from Mendelian/Classical genetics to molecular genetics, is well documented. Although there is a connection it is almost impossible to make this connection explicit (Kitcher 1982; Mayr 1982, 1997; Rosenberg 1985; Kincaid 1990). Several studies have shown that students have difficulties in relating structures and concepts to correct systematic level (Lewis et al. 2000b; Knippels 2002). Students find it also difficult to extrapolate between the different organizational levels (Hallde´n 1990; Marbach-Ad and Stavy 2000). Hallde´n (1990) deduces these difficulties to the subject matter. He claims that to identify a trait with its genetic counterpart would be an example of category mistake, the two categories being macro- respectively, micro-level. The students in his study realized that they had problems learning the subject but did not apprehend that this was part of the problem of genetics, the macro phenomena not always being possible to reduce to micro-explanations. Another way of explaining this is that it is also about different scientific frameworks; classical genetics and molecular genetics. The macro- and cell level belongs to the Classical and Biochemical-Classical models and the molecular level belongs to the Neoclassical and Modern models. Therefore, students, which often tend to give explanations at cellular level (Marbach-Ad and Stavy 2000), might have difficulties to extrapolate to the molecular level, which is dealt with in a different scientific framework apart from the cellular level. 7.7 The relation between genetic and environmental factors This feature addresses the issue of how the models describe environmental influences on gene function. All the historical models of gene function presented in this study point in one direction from the gene toward its product, either it is traits at macro level or polypeptides at the micro-level. In that way these models emphasize the hereditary factors and no consideration is taken over that developmental factors moderate the genetic expression according to environmental influences. In the first three models environmental issues could not even have been addressed, since unless researcher directly addressed environmental factors in the classical era, they could be ignored in the laboratory. Thus, conceptual tools that relates to the environment was ignored in Western genetic literature until 1950 (Sarkar 1999; Schwartz 2000). The idea that developmental biology could be replaced by going straight to the genes and reading the instructions for development has been called ‘neo-preformationism’ (Griffith and Neumann-Held 1999). This view is expressed in the Neoclassical model although the environmental factors could be implied from the developmental context. In the Modern model environmental factors are implied because the gene only exists in the context of a developmental system that moderates the expression of the gene. This issue has been attended to by Forissier and Cle´ment (2003) who have found a poor understanding of the influence of environmental factors on traits among trainee teachers. Also Lewis and Kattmann (2004) address this issue: ‘students need to be taught explicitly that genes are switched on and off according to need’ (p. 204).

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From the analysis of the external and internal consistency problems of the models we have shown seven epistemological features that differ between the historical models in such ways that we believe learning problems might be the result if these are not explicitly addressed when teaching genetics. We argue that this claim is strengthen by our review of the research literature of students’ understanding of genetics, which shows extensive parallelism between students’ understanding and the epistemological features.

8 Implications for teaching and educational research We have described five major historical models of gene function, and showed different important aspects of them that differ in such ways that learning difficulties might be expected. What implications might this have on teaching genetics? The most important conclusions an educator in the field should be aware of could be summarized as follows: the different models do not portrait the same gene. Molecular genetics is not a mere reduction of classical genetics. The implication of this is that the different models do not fully coincide. All the models are constructed for a certain purpose in a historical context. We therefore would like to stress that the reported learning problems among students to integrate and extrapolate between concepts in genetics might be due to the character of the subject. When teaching gene function specific attention should be drawn to the seven epistemological features identified in this study and the student conceptions that might be related to these features. This is also indeed an important implication for teacher education and training since teachers in turn will influence future generations’ ideas about genetics. Another possible implication of this study is to use the described historical models in teachings of genetics. In order to fully comprehend and have a coherent knowledge of both classical and molecular genetics you need to have knowledge about the meaning and relations between the models, i.e., different aspects of NOS. We suggest a combined NOS and HOS approach when teaching genetics and gene function, in which the models and their similarities and limitations as outlined in this paper, may be used for creating better understanding among students. In that way the different aspects of the models serve as a NOS approach, which can be framed in a HOS approach by giving the historical development. With this teaching approach, the findings by Abd-El-Khalick and Lederman (2000) that combined NOS- and HOS-teaching approaches might enhance students’ knowledge about NOS, can be fulfilled. One way to do this in practice might be through Model-revising problem solving, which occurs when existing models are no longer sufficient to explain data, and must be revised regarding the entities or processes of the model (Finkel 1996). Model revising requires a problem solver to be aware of a model from a NOS perspective (Kuhn et al. 1988; Stewart and Hafner 1991). The level or degree of the NOS aspects to use when teaching genetics must of course be adjusted and made relevant to the age and school form. Models are man made constructs with a specific purpose (Rosenberg 2000). This should come as no surprise to a person with insight in NOS. In science education however this is not always well known. Research has shown (Grosslight et al. 1991; van Driel and Verloop 1999) that many teachers have a positivistic view of science and treat the different models as if they have the same scientific framework. Many

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teachers are not aware of that the meaning and purpose of the models can shift. Instead they treat them as they were fully compatible only with different levels of generalization. In genetics as well as in genetics education (as in most fields of science) historical scientific models do not completely replace each other. Instead they exist in parallel and are used for different purposes. A problem in communication and teaching of genetics could be that the described historical evolution of models is not widely known to either teachers or students probably due to recognition of models in biology is not always obvious (Mayr 1997, p. 60). In biology scientific development is often viewed as gradual without discrete steps. Hence, the classification of the historical models of gene function as performed in this study is probably not generally fully recognized, which in turn makes it even more probable that hybrid models are constructed and used in education. This use might bring about misunderstandings and concept confusion among students (Justi 2000). We therefore emphasize the importance of educators in genetics recognizing the existence of the models and their differences. We also believe that the historical context in which the models were made are important in getting a deeper understanding. Kinnear claims that: A valuable experience for students is to explore the development of a concept or model over time, and to note its maturation from initial observation, through descriptive statements, and finally to an explanatory model with predictive power that is generally accepted by the relevant community of scholars (Kinnear 1991, p. 71). The experience of tracing the development of an explanatory model might clarify students’ own understanding of the concepts involved, particularly when it exist several rivalry models. Also historical perspectives can sensitize students to the development of historical models, the constraints imposed on the model by its underlying assumptions, and to the effects of scientific methodology. An historical approach can confront the view that the ‘right’ model exists, and is waiting to be discovered like an archeological founding. Also a historical approach can help students recognize that explanatory models are constructs developed over time for a specific purpose and that it indeed can be flawed or inadequate in a variety of ways (Kinnear 1991). Nevertheless we again stress that consideration must be taken to the age of the students, as well as the level of education they participate in, if the suggested teaching approach with models in a combined HOS and NOS framework should be applied. We believe such an approach is suitable for upper secondary school as well as college and university level. The students’ understanding as reviewed in this paper correlates mainly to the first three models, hence, are mostly founded in classical genetics and very similar to the scientific view of the first half of the twentieth century. A progression toward a more molecular understanding of genetics can be observed among students at later stages in the educational system (i.e., university level). The most logical explanation to this trend is that it reflects the content of which the students encounter in school genetics. The similarities between the epistemological aspects of the models and the conceptions of students are far too great to be due to chance. Hence, we mean that students’ reported alternative conceptions in many cases are not necessarily the result of everyday thinking, but a result from teaching implicitly with historical models. This illustrates the didactical question: What to teach about? What is the objective of the genetic education in question and what do we expect the students to

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know after participating in a specific course? These are relevant and adequate issues for teachers as well as researchers to address. With a NOS perspective it becomes clear that it is not the phenomenon we are teaching but representations of it in the form of historical models. The question then becomes which model or models should be used and why? It is a way for teachers to make explicit what to teach about and what not to teach about. Several studies in science education found that students’ ideas are too restricted to rules and patterns of inheritance, than processes and mechanisms, and an urge for a better ability among students to integrate concepts and biochemical processes from molecular genetics with those of classical genetics can be noted (Venville and Treagust 1998; Lewis et al. 2000a; Marbach-Ad 2001; Lewis and Kattmann 2004). Accordingly, students’ understanding can be scientifically correct but nevertheless not be considered desirable or regarded insufficient. Future research about students’ and teachers’ understanding about genetics should be done in relation to the objectives of genetics education. Why does this discrepancy exists between what students’ know, i.e., classical genetics, and what they ought to know, i.e., molecular genetics? Is it a problem of what is actually taught or how students profit from the teaching? We also find it important that future research address how models are used in genetics education and how a combined NOS and HOS approach might influence learning outcomes?

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