TRADITIONAL BREEDING METHODS AND GENETIC
ENGINEERING: THE FUNDAMENTALS
Dr Michael Antoniou, Senior Lecturer in
Molecular Pathology, London, UK
The greatest claim of those who endorse the
use of genetic engineering or modification (GM) in agriculture,
is that it is not only a natural extension of traditional breeding
methods for the production of new varieties of crops and farm
animals but it is also more precise and safer. It is said that
(i) simply gives nature a "nudge"
speeding it along a pathway that it would take anyway;
(ii) by moving a single gene between organisms,
the outcomes of GM are more predictable (and therefore more precise
and safer) than what occurs in traditional methods;
(iii) in molecular chemical terms, DNA is
the same in all organisms and therefore poses no great danger
when genes are moved between unrelated organisms (e.g., animals
However, since technically speaking traditional
breeding and GM bear no resemblance to each other, how valid are
these claims? The following is a discussion which tries to address
this question from a fundamental genetics viewpoint.
1. GENES AND
Genes are discrete units of DNA which each individual
inherits from its parents. Genes are the blueprints which carry
the information for the tens of thousands of proteins which constitute
the structures and carry out the biochemical functions of the
body of any organism from bacteria to humans. Therefore, what
gives each gene its own unique identity is its information content.
The fact that all DNA is made of the same chemical units no matter
what its source is, from a functional point of view, irrelevant.
Life forms are different from one another due to differences in
the information content of their genes. Variations between individuals
within the population of a given species are also due to subtle
differences in the genes which they all have in common.
Genetics, the study of genes, has two basic
components. Firstly, there is the information content of each
gene; that is, what gene carries the blueprint for which protein.
Secondly, gene function or expression is extremely tightly controlled
or regulated. Gene function needs to be controlled because the
totality of the genetic information or DNA which is inherited,
is retained in all the cells of the body. In other words, the
information for the whole organism is present in every part. So,
for example, the knowledge for making a kidney is present in the
cells of the muscles and vice versa. This basic fact of life was
perhaps most dramatically demonstrated with the recent creation
of Dolly the cloned sheep. The genetic material for making Dolly
was apparently derived from an adult cell from the udder of a
ewe clearly showing that the genetic information for making a
whole sheep was present in this cell derived from the ewe's mammary
2. HIERARCHY OF
Each gene occupies its own special place along
the DNA molecule which is vital for its correct function. It is
vital that the correct families of genes are switched on at the
right time and within appropriate cells to ensure that the correct
protein and therefore appropriate structure and function, is present
in the right place, time and quantity in the body. In order to
achieve this, life has evolved sets of sophisticated on-off switches
to regulate the expression of genes. It would not only be wasteful
but potentially disastrous for genes carrying the information
for proteins needed in the liver to be switched on in the cells
of the brain.
In addition, genes are now also known to be
organised in distinct groups or families within the DNA in structures
called "chromatin domains". It is now clear that the
expression and function of genes within a given chromatin domain
are closely interdependent and that the function in one domain
can influence gene functions within another distant domain. The
function of one chromatin domain being influenced by another distantly
located gene family can occur by at least two different mechanisms.
Firstly, one domain may contain genes that possess
the information for a special class of proteins (called "transcription
factors") that are directly involved in regulating expression
of other genes; i.e., genes control the expression of other genes.
If the function of these transcription factors is disturbed by
a disruption of their genes, then the knock-on effect will be
a disturbance in other, perhaps distantly located groups of genes
whose activity is dependent on these particular transcription
Secondly, the still poorly understood phenomenon
known as "co-suppression" first described in plants
and now also shown to occur in insect and mammalian systems, also
demonstrates "action at a distance" between groups of
genes that are on separate DNA molecules (chromosomes). When extra
copies of a gene that is already present in a plant are introduced
by GM, the expected result is that you should get an additive
effect; i.e., the more copies of a given gene that you have the
more of that protein you should make. However, quite unexpectedly
it was discovered that in some cases this GM manipulation can
cause a switching off or silencing of genes. Although originally
described in plants, co-suppression type activity has now been
demonstrated in GM flies and mice.
Generally, these chromatin domains are turned
on and off as needed to provide an intricate, finely balanced
state of gene control the complexities of which we are only just
beginning to unravel. Nevertheless, tight gene control means,
for example, that you will never find liver proteins and functions
in your brain or leaf specific processes in the fruit and vice
Nature has also evolved mechanisms whereby cross
breeding can only take place between very closely related species.
You can cross a cow with a cow and a sheep with a sheep but you
will not have much luck crossing a cow with a sheep. The same
principles apply to plants.
Clearly any technology which aims to manipulate
the genetic makeup of a given organism must preserve the natural
order and groupings of genes that have evolved to work together
over many millions of years. This is indeed the case with traditional
breeding methods where different variations of the same genes
in their natural context (chromatin domains) are exchanged. This
preserves tight control and complex interelationships between
genetic functions and their protein products that are vital for
integrity of life.
GM: A NATURAL
In order to asses the validity of the claim
that GM represents a natural extension of traditional breeding
methods, it is important to know how GM ("transgenic")
plants and animals are produced.
As an example, let us see how the herbicide
resistant, GM soya was generated. The objective here was to introduce
into the soya plants a gene from a common soil bacterium which
would allow it to survive when sprayed with the herbicide Roundup.
Clearly you cannot "cross" a bacterium with a plant.
Therefore, the first step was to grow cells from soya bean plants
on plastic dishes in the laboratory. Now, in order to allow the
bacterial gene to be able to work once introduced into its new
plant host, it had to be linked to a genetic switch combining
parts from a cauliflower virus and petunias. (As we discussed
above, the bacterial gene's own switch will only work in the bacteria
from which it came). This combination of cauliflower virus, petunia
and bacterial DNA was then introduced into the soya bean cells
growing on the dishes in the laboratory using a procedure known
as "biolistics" which employs a device called a "gene
gun". In this technique, tiny spheres of gold or tungsten
are coated with the DNA one wishes to introduce into the plant
cells. These DNA-coated metal articles are then shot at the plant
cells using the gene gun at high speed. As a result some of these
metal beads enter inside the plant cells carrying the new DNA
with them Unfortunately from the point of view of the plant biotechnologist,
the efficiency with which the new DNA is taken up by the soya
bean cells on the dish is very low. Most of the cells don't take
it up at all. So the key is to find those few cells among the
many millions on the dish which have taken up the DNA. This is
done by using another genetic trick. The introduction of the bacterial
gene into the soya bean cells for herbicide resistance, was accompanied
by a second gene which confers resistance to an antibiotic (called
kanamycin). The soya bean cells were then treated with the antibiotic.
The few cells which had taken up the herbicide resistance antibiotic:resistance
"marker" gene combination survived and flourished whereas
the majority of the cells which had not taken up these genes were
simply killed by the antibiotic. Finally, by changing the conditions
under which the soya bean cells are grown, the cells clump together
to form what is called a callus which in turn starts to put down
roots and sprout green shoots. These little "seedlings"
are then potted so as to grow into fully mature plants which will
carry in all their cells (including those for reproduction; i.e.,
pollen, etc.) the new bacterial gene. The plant which then displays
the best agronomic performance, in this case resistance to herbicide,
is then selected for further development (crossing to form new
The generation of transgenic animals is a no
less artificial procedure. Fertilised eggs are first removed from
the animal of choice. These eggs are then injected with the genes
one wishes to engineer into the animal. The DNA-injected eggs
are then returned to the womb of a surrogate mother where they
complete their development and are born in due course.
Therefore, in marked contrast to traditional
breeding methods, all transgenic plants and animals start life
as individual or groups of cells growing on a plastic dish in
GM: A NO
It is evident from the procedure we just described
that with GM there are no holds barred. GM allows the isolation,
cutting, joining and transfer of single or multiple genes between
totally unrelated organisms circumventing natural species barriers.
As a result combinations of genes are produced that would never
occur naturally. Transgenic crops containing genes from viruses,
bacteria, animals as well as from unrelated plants have been generated.
In the case of the herbicide resistant soya beans, the final outcome
was the combination of genetic material from four totally unrelated
organisms; a cauliflower virus, petunia, bacteria and soya. Furthermore,
again as we saw in the case of the GM soya beans, the newly introduced
gene units are composed of artificial combinations of genetic
material. Another example which illustrates the extreme combinations
of genetic material that can be produced, is the introduction
of the "anti-freeze" gene from an arctic fish (the sea
flounder) into tomatoes, strawberries and potatoes in the hope
of producing resistance to frost. As with the bacterial gene in
the soya beans, the fish anti-freeze gene is joined to the cauliflower
virus genetic switch to allow it to turn on and work in its new
host. (The fish genetic switch naturally only works in the fish).
All this is in turn coupled to an antibiotic resistance marker
gene to allow selection of the newly transformed plants.
GM DISRUPTS HOST
This is clearly a great technological advance.
However, the manipulation and transfer of DNA from one organism
to another by GM can only be carried out with any degree of precision
in lower forms of life such as bacteria and yeast although complications
may arise even in these cases resulting from biochemical disturbances.
The generation of transgenic plants and animals is currently an
imperfect technique. Once injected into the cells of the organism,
the introduced gene is randomly incorporated or spliced into the
DNA of its new plant or animal host. As a result the normal order
of genes within the chromatin domains is disrupted.
There is a further complication in the case
of plants. As discussed already, the genetic engineering of plant
cells is a very inefficient process. We saw how in order to identify
the few plant cells in the laboratory culture that have permanently
assimilated the new genes, the plant biotechnologist has to rely
on the presence of an antibiotic marker gene. This approach is
used in the production of all GM plants. As one can see this method
totally depends on the function of the antibiotic resistance gene.
This gene must be assimilated in a manner that will allow it to
be switched on, otherwise the cells will die once treated with
As we discussed above, regions of DNA (chromatin
domains) can be switched off ("inactive") or expressing
genes ("active") as part of vital, normal genetic control
mechanisms. Since the incorporation of the new genes into the
host DNA in GM technology is a random affair totally beyond the
control of the genetic engineer, the antibiotic resistance gene
can be incorporated into either silent or active DNA. If the antibiotic
resistance gene is incorporated into silent DNA it will not be
switched on and therefore the cell will die in the presence of
the antibiotic. If on the other hand the antibiotic resistance
gene is assimilated in active DNA, it will be switched on and
the cells will survive antibiotic treatment. However, by definition,
active DNA is a region where other genes are already switched
on and trying to function. The random incorporation of a foreign
gene into the already active domain will therefore always risk
disrupting the balanced functioning of the host genes. It was
previously thought that host gene functions would only be disturbed
if the foreign gene spliced into the middle of another gene or
into the genetic switch region which controls its expression.
However, it is now known that the functions of genes within a
given chromatin domain are interdependent and in many cases genes
within a family grouping compete for common "master"
control switches called "locus control regions"). This
latest model of gene organisation and function predicts that the
mere presence of another gene introduced by GM into a given chromatin
domain, will compete with the host genes and disrupt their balanced
function. Therefore, by relying on the selection of the transformed
plant cells by the function of an antibiotic resistance gene,
the biotechnologist in turn selects for events where the new genes
have been spliced into regions of DNA where other genes are trying
to function, therefore maximising the degree of disruption to
normal host gene function. This in turn maximises the degree of
biochemical disturbance resulting from the disrupted gene function.
Therefore, GM of animals and especially plants, always results
in a loss, to a lesser or greater degree, of the tight genetic
control and balanced functioning which is retained through conventional
cross breeding. With GM, host genes can be silenced (rendered
inactive) or inappropriately activated resulting in either a deficiency
in a given protein(s) or the presence of the wrong protein(s)
in the wrong place or in the wrong quantity or all these combined.
In addition, it is assumed that the introduced
gene will behave in exactly the same way in its new host as it
does in its native environment which frequently will not be the
case. Gene and protein functions have evolved over millions of
years to work together in any given organism. the anti-freeze
gene/protein in the arctic sea flounder has evolved to work together
with the other genes/proteins in this fish. It is purely an assumption
that it will work in exactly the same way with no unwanted side
effects in its new hosts where it will now be surrounded by plant
These effects combine to always produce a totally
unpredictable disturbance in host genetic function as well as
in that of the introduced gene. These phenomena which are technically
called "position effects", complicate the production
of every GM crop or animal. Of the several tens of individual
plants or animals that will be produced with the same genes, only
a few will meet the agricultural performance criteria that are
being sought. This is because in each individual the foreign gene
is spliced into a different location in the host DNA. Plants or
animals with gross defects can always be spotted and discarded.
However, subtle changes in host biochemistry that will always
accompany the desired effects and which in addition to producing
variable agronomic performance under different soil and climatic
conditions can result in the production of novel toxins, allergens
as well as adversely affecting nutritional value, are on the whole
ignored by the producers of these GM organisms.
GM and Traditional Breeding Methods Are Worlds
The proponents of the use of GM in agriculture
argue that mankind has been selecting and manipulating plant and
animal food stocks for millennia and that this new technology
is simply the next stage in this process. However, we have seen:
Technically speaking, GM and traditional
breeding methods bear no resemblance to each other.
GM plants and animals start out life
in a laboratory culture dish.
GM employs totally artificial units
of genetic material which are introduced into plant and animal
cells using chemical, mechanical or bacterial methods.
GM always results in disruptions
to the natural order of genes within the host DNA.
GM also brings about combinations
of genes that would never occur naturally.
Clearly these procedures are worlds apart when
compared to cross fertilisation between closely related species.
The totally artificial nature of GM does not
automatically make it dangerous. It is the imprecision in the
manner by which genes are combined and the unpredictability in
how the introduced gene will interact within its new environment
which results in uncertainty. The balanced gene functions that
have evolved together and which are preserved with traditional
methods, are lost with GM.
GM VIOLATES THE
Genes have evolved to exist and work in families
within the context of a given species. With traditional breeding
which can take place only between closely related organisms, these
natural groupings of genes are preserved. Given these basic principles
of life, the claim that the reductionist approach of GM, which
moves one or a few genes between unrelated organisms, is a precise
technology is highly questionable. What makes these assertions
even more disputable, is that by selecting for the function of
the foreign gene and looking only at the desired agronomic performance
as an end point, GM always results in a disruption in the natural
genetic order of the host. Therefore, from the standpoint of the
fundamental principles of genetics and the limitations in the
technology, GM is neither more precise nor a natural extension
of traditional cross breeding methods. If anything the opposite
would appear to be true. GM violates the laws of genetics while
traditional methods work within and make the best use of the well
established laws of genetics that have been laid down over millions
of years of evolution.
Therefore GM foods possess new and unique safety
considerations both in terms of health and to the environment.
It would appear to be quite erroneous to view GM technology from
purely an agriculture performance perspective upon which the current
claims of precision and safety are based.
The availability of safe, sustainable, natural
methods of breeding and husbandry utilising the many thousands
of different varieties of any given food crop, makes the risks
associated with GM foods simply not worth taking. These risks
are even less acceptable when one takes into account the fact
that once released into the environment, genetic mistakes/pollution
cannot be contained, cleaned up or recalled like a chemical spill
or a BSE epidemic but will be passed on to all future generations
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