Common misunderstandings of genetics
Encyclopedia
During the latter half of the 20th century, the fields of genetics
and molecular biology
matured greatly, significantly increasing understanding of biological
heredity
. As with other complex and evolving fields of knowledge, the public awareness
of these advances has primarily been through the mass media
, and a number of common misunderstandings of genetics have arisen. Common misunderstandings include the following ideas:
, are rigidly genetically determined. There is good evidence that some basic aspects of human behaviour, such as circadian rhythm
s are genetically-based, but it is clear that many other aspects are not.
In the first place, much phenotypic variability does not stem from genetics. For example:
onwards inevitably focused on genes whose effects could be readily identified; partly that it was easier to teach science that way; and partly because the mathematics of evolutionary dynamics is simpler if there is a simple mapping between genes and phenotypic characteristics.
These have led to the general perception that there is "a gene for" arbitrary traits,
leading to controversy in particular cases such as the purported "gay gene
". However, in light of the known complexities of gene expression networks (and phenomena such as epigenetics
), it is clear that instances where a single gene "codes for" a single, discernible phenotypic effect are rare, and that media presentations of "a gene for X" grossly oversimplify the vast majority of situations.
or mechanical engineering
blueprints describe buildings or machines. At a superficial level, genes and conventional blueprints share the common property of being low dimension
al (genes are organised as a one-dimensional string of nucleotide
s; blueprints are typically two-dimensional drawings on paper) but containing information about fully three-dimensional structures. However, this view ignores the fundamental differences between genes and blueprints in the nature of the mapping from low order information to the high order object.
In the case of biological systems, a long and complicated chain of interactions separates genetic information from macroscopic
structures and functions. The following simplified diagram of causality illustrates this:
Even at the small scale, the relationship between genes and proteins (once thought of as "one gene, one polypeptide") is known to be complicated, with approximately 5 proteins in the human body for each gene. More significantly, the causal chains from genes to functionality are not separate or isolated but are entangled together, most obviously in metabolic pathway
s (such as the Calvin
and citric acid
cycles) which link a succession of enzyme
s (and, thus, gene products) to form a coherent biochemical
system. Furthermore, information flow in the chain is not exclusively one-way. While the central dogma of molecular biology
describes how information cannot be passed back to inheritable genetic information, the other causal arrows in this chain can be bidirectional
, with complex feedbacks ultimately regulating gene expression
.
Instead of being a simple, linear mapping, this complex relationship between genotype and phenotype is not straightforward to deconvolute
. Rather than describing genetic information as a blueprint, some have suggested that a more appropriate analogy
is that of a recipe
for cooking
, where a collection of ingredient
s is combined via a set of instructions to form an emergent structure, such as a cake, that is not described explicitly in the recipe itself.
It is popularly supposed that a gene is "a linear sequence of nucleotides along a segment of DNA that provides the coded instructions for synthesis of RNA" and even some current medical dictionaries define a gene as "a hereditary unit that occupies a specific location on a chromosome, determines a particular characteristic in an organism by directing the formation of a specific protein, and is capable of replicating itself at each cell division."
In fact, as the diagram illustrates schematically, genes are much more complicated and elusive concepts. A reasonable modern definition of a gene is "a locatable region of genomic sequence, corresponding to a unit of inheritance, which is associated with regulatory regions, transcribed regions and/or other functional sequence regions." One of the major complicating factors is that the exons which code
for the proteins are often separated by many introns, which used to be called "junk DNA" but appear to have various as-yet-ill-understood purposes. The exons can be combined in different orders (splice variants) to produce different proteins. For example the gene called Dscam
in Drosophila
has 110 introns and therefore tens of thousands of possible splice variants.
This kind of misperception is perpetuated when mainstream media report that an organism's genome has been "decyphered" when they mean that it has simply been sequenced
.
A related misconception is that the sole function of genes is to code for proteins, with the non-coding remainder being "junk DNA". However, it now appears that, although protein-coding DNA makes up barely 2% of the human genome
, about 80% of the bases in the genome may be being expressed, so the term "junk DNA" may be a misnomer.
Genetics
Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....
and molecular biology
Molecular biology
Molecular biology is the branch of biology that deals with the molecular basis of biological activity. This field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry...
matured greatly, significantly increasing understanding of biological
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...
heredity
Heredity
Heredity is the passing of traits to offspring . This is the process by which an offspring cell or organism acquires or becomes predisposed to the characteristics of its parent cell or organism. Through heredity, variations exhibited by individuals can accumulate and cause some species to evolve...
. As with other complex and evolving fields of knowledge, the public awareness
Public awareness of science
Public awareness of science , also public understanding of science , is a term relating to the attitudes, behaviours, opinions and activities that comprise the relations between the general public or lay society as a whole to scientific knowledge and organisation.It is a comparatively new approach...
of these advances has primarily been through the mass media
Mass media
Mass media refers collectively to all media technologies which are intended to reach a large audience via mass communication. Broadcast media transmit their information electronically and comprise of television, film and radio, movies, CDs, DVDs and some other gadgets like cameras or video consoles...
, and a number of common misunderstandings of genetics have arisen. Common misunderstandings include the following ideas:
- Every aspect of the biology of an organismOrganismIn biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...
can be predicted from its geneGeneA gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
s - Single genes code for specific anatomicalAnatomyAnatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy , and plant anatomy...
or behaviouralEthologyEthology is the scientific study of animal behavior, and a sub-topic of zoology....
features - Genes are a blueprintBlueprintA blueprint is a type of paper-based reproduction usually of a technical drawing, documenting an architecture or an engineering design. More generally, the term "blueprint" has come to be used to refer to any detailed plan....
of an organism's form and behaviour - Genes are uninterrupted sections of DNADNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
that only code for a single proteinProteinProteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
Genetic determinism
While there are many examples of animals that display certain well-defined behaviour that is genetically programmed, these examples have been extrapolated to a popular misconception that all patterns of behaviour, and more generally the phenotypePhenotype
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...
, are rigidly genetically determined. There is good evidence that some basic aspects of human behaviour, such as circadian rhythm
Circadian rhythm
A circadian rhythm, popularly referred to as body clock, is an endogenously driven , roughly 24-hour cycle in biochemical, physiological, or behavioural processes. Circadian rhythms have been widely observed in plants, animals, fungi and cyanobacteria...
s are genetically-based, but it is clear that many other aspects are not.
In the first place, much phenotypic variability does not stem from genetics. For example:
- Epigenetic inheritance. In the widest definition this includes all biological inheritance mechanisms that do not change or involve the genomeGenomeIn modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
. In a narrower definition it excludes biological phenomena such as the effects of prions and maternal antibodies which are also inherited and have clear survival implications. - Learning from experienceLearningLearning is acquiring new or modifying existing knowledge, behaviors, skills, values, or preferences and may involve synthesizing different types of information. The ability to learn is possessed by humans, animals and some machines. Progress over time tends to follow learning curves.Human learning...
. This is obviously a very important feature of humans, but there is considerable evidence of learned behaviour in other animal species (vertebrateVertebrateVertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s and invertebrateInvertebrateAn invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
s). There are even reports of learned behaviour in DrosophilaDrosophilaDrosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...
larvaLarvaA larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
e.
A gene for X
In the early years of genetics it was suggested that there might be "a gene for" a wide range of particular characteristics. This was partly because the examples studied from MendelGregor Mendel
Gregor Johann Mendel was an Austrian scientist and Augustinian friar who gained posthumous fame as the founder of the new science of genetics. Mendel demonstrated that the inheritance of certain traits in pea plants follows particular patterns, now referred to as the laws of Mendelian inheritance...
onwards inevitably focused on genes whose effects could be readily identified; partly that it was easier to teach science that way; and partly because the mathematics of evolutionary dynamics is simpler if there is a simple mapping between genes and phenotypic characteristics.
These have led to the general perception that there is "a gene for" arbitrary traits,
leading to controversy in particular cases such as the purported "gay gene
Biology and sexual orientation
Biology and sexual orientation is the subject of research into the role of biology in the development of human sexual orientation. No simple, single cause for sexual orientation has been conclusively demonstrated, but research suggests that it is by a combination of genetic, hormonal, and...
". However, in light of the known complexities of gene expression networks (and phenomena such as epigenetics
Epigenetics
In biology, and specifically genetics, epigenetics is the study of heritable changes in gene expression or cellular phenotype caused by mechanisms other than changes in the underlying DNA sequence – hence the name epi- -genetics...
), it is clear that instances where a single gene "codes for" a single, discernible phenotypic effect are rare, and that media presentations of "a gene for X" grossly oversimplify the vast majority of situations.
Genes as a blueprint
It is widely believed that genes provide a "blueprint" for the body in much the same way that architecturalArchitecture
Architecture is both the process and product of planning, designing and construction. Architectural works, in the material form of buildings, are often perceived as cultural and political symbols and as works of art...
or mechanical engineering
Mechanical engineering
Mechanical engineering is a discipline of engineering that applies the principles of physics and materials science for analysis, design, manufacturing, and maintenance of mechanical systems. It is the branch of engineering that involves the production and usage of heat and mechanical power for the...
blueprints describe buildings or machines. At a superficial level, genes and conventional blueprints share the common property of being low dimension
Dimension
In physics and mathematics, the dimension of a space or object is informally defined as the minimum number of coordinates needed to specify any point within it. Thus a line has a dimension of one because only one coordinate is needed to specify a point on it...
al (genes are organised as a one-dimensional string of nucleotide
Nucleotide
Nucleotides are molecules that, when joined together, make up the structural units of RNA and DNA. In addition, nucleotides participate in cellular signaling , and are incorporated into important cofactors of enzymatic reactions...
s; blueprints are typically two-dimensional drawings on paper) but containing information about fully three-dimensional structures. However, this view ignores the fundamental differences between genes and blueprints in the nature of the mapping from low order information to the high order object.
In the case of biological systems, a long and complicated chain of interactions separates genetic information from macroscopic
Macroscopic
The macroscopic scale is the length scale on which objects or processes are of a size which is measurable and observable by the naked eye.When applied to phenomena and abstract objects, the macroscopic scale describes existence in the world as we perceive it, often in contrast to experiences or...
structures and functions. The following simplified diagram of causality illustrates this:
- Genes → Gene expression → Proteins → Metabolic pathways → Sub-cellular structures → Cells → Tissues → Organs → Organisms
Even at the small scale, the relationship between genes and proteins (once thought of as "one gene, one polypeptide") is known to be complicated, with approximately 5 proteins in the human body for each gene. More significantly, the causal chains from genes to functionality are not separate or isolated but are entangled together, most obviously in metabolic pathway
Metabolic pathway
In biochemistry, metabolic pathways are series of chemical reactions occurring within a cell. In each pathway, a principal chemical is modified by a series of chemical reactions. Enzymes catalyze these reactions, and often require dietary minerals, vitamins, and other cofactors in order to function...
s (such as the Calvin
Calvin cycle
The Calvin cycle or Calvin–Benson-Bassham cycle or reductive pentose phosphate cycle or C3 cycle or CBB cycle is a series of biochemical redox reactions that take place in the stroma of chloroplasts in photosynthetic organisms...
and citric acid
Citric acid cycle
The citric acid cycle — also known as the tricarboxylic acid cycle , the Krebs cycle, or the Szent-Györgyi-Krebs cycle — is a series of chemical reactions which is used by all aerobic living organisms to generate energy through the oxidization of acetate derived from carbohydrates, fats and...
cycles) which link a succession of enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
s (and, thus, gene products) to form a coherent biochemical
Biochemistry
Biochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes...
system. Furthermore, information flow in the chain is not exclusively one-way. While the central dogma of molecular biology
Central dogma of molecular biology
The central dogma of molecular biology was first articulated by Francis Crick in 1958 and re-stated in a Nature paper published in 1970:In other words, the process of producing proteins is irreversible: a protein cannot be used to create DNA....
describes how information cannot be passed back to inheritable genetic information, the other causal arrows in this chain can be bidirectional
Bidirectional
The term "bidirectional" may refer to:*Anything that can move in two directions*A roadway that carries traffic moving in opposite directions*A tram or train or any other vehicle that can be controlled from either end and can move forward or backward with equal ease without any need to be turned...
, with complex feedbacks ultimately regulating gene expression
Gene expression
Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as ribosomal RNA , transfer RNA or small nuclear RNA genes, the product is a functional RNA...
.
Instead of being a simple, linear mapping, this complex relationship between genotype and phenotype is not straightforward to deconvolute
Deconvolution
In mathematics, deconvolution is an algorithm-based process used to reverse the effects of convolution on recorded data. The concept of deconvolution is widely used in the techniques of signal processing and image processing...
. Rather than describing genetic information as a blueprint, some have suggested that a more appropriate analogy
Analogy
Analogy is a cognitive process of transferring information or meaning from a particular subject to another particular subject , and a linguistic expression corresponding to such a process...
is that of a recipe
Recipe
A recipe is a set of instructions that describe how to prepare or make something, especially a culinary dish.-Components:Modern culinary recipes normally consist of several components*The name of the dish...
for cooking
Cooking
Cooking is the process of preparing food by use of heat. Cooking techniques and ingredients vary widely across the world, reflecting unique environmental, economic, and cultural traditions. Cooks themselves also vary widely in skill and training...
, where a collection of ingredient
Ingredient
An ingredient is a substance that forms part of a mixture . For example, in cooking, recipes specify which ingredients are used to prepare a specific dish. Many commercial products contain a secret ingredient that is purported to make them better than competing products...
s is combined via a set of instructions to form an emergent structure, such as a cake, that is not described explicitly in the recipe itself.
Genes as words
 |
It is popularly supposed that a gene is "a linear sequence of nucleotides along a segment of DNA that provides the coded instructions for synthesis of RNA" and even some current medical dictionaries define a gene as "a hereditary unit that occupies a specific location on a chromosome, determines a particular characteristic in an organism by directing the formation of a specific protein, and is capable of replicating itself at each cell division."
In fact, as the diagram illustrates schematically, genes are much more complicated and elusive concepts. A reasonable modern definition of a gene is "a locatable region of genomic sequence, corresponding to a unit of inheritance, which is associated with regulatory regions, transcribed regions and/or other functional sequence regions." One of the major complicating factors is that the exons which code
Genetic code
The genetic code is the set of rules by which information encoded in genetic material is translated into proteins by living cells....
for the proteins are often separated by many introns, which used to be called "junk DNA" but appear to have various as-yet-ill-understood purposes. The exons can be combined in different orders (splice variants) to produce different proteins. For example the gene called Dscam
DSCAM
DSCAM and Dscam are both abbreviations for Down Syndrome Cell Adhesion Molecule. The case difference is significant: DSCAM refers to the human protein, and Dscam refers to its analog in Drosophila....
in Drosophila
Drosophila
Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...
has 110 introns and therefore tens of thousands of possible splice variants.
This kind of misperception is perpetuated when mainstream media report that an organism's genome has been "decyphered" when they mean that it has simply been sequenced
Sequencing
In genetics and biochemistry, sequencing means to determine the primary structure of an unbranched biopolymer...
.
A related misconception is that the sole function of genes is to code for proteins, with the non-coding remainder being "junk DNA". However, it now appears that, although protein-coding DNA makes up barely 2% of the human genome
Human genome
The human genome is the genome of Homo sapiens, which is stored on 23 chromosome pairs plus the small mitochondrial DNA. 22 of the 23 chromosomes are autosomal chromosome pairs, while the remaining pair is sex-determining...
, about 80% of the bases in the genome may be being expressed, so the term "junk DNA" may be a misnomer.