Download Definition of historical models of gene function and their relation to

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 (Harré 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; Gaudillière 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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Definition of historical models of gene function
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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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Definition of historical models of gene function
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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 (Halldén 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 Clément 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
(Halldén 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 Clément 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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(5)
865
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
123
123
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
Definition of historical models of gene function
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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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Definition of historical models of gene function
869
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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871
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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N. M. Gericke, M. Hagberg
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
(Halldén 1990; Marbach-Ad and Stavy 2000). Halldén (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 Clément (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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