The history of genetics dates from the classical era with contributions by Pythagoras,
Hippocrates, Aristotle, Epicurus, and others.
Modern genetics began with the work of the Augustinian friar Gregor Johann Mendel.
His work on pea plants, published in 1866, established the theory of Mendelian inheritance.
The year 1900 marked the "rediscovery of Mendel" by Hugo de Vries, Carl Correns and Erich von
Tschermak, and by 1915 the basic principles of Mendelian genetics had been studied in a wide
variety of organisms — most notably the fruit fly Drosophila melanogaster. Led by Thomas Hunt
Morgan and his fellow "drosophilists", geneticists developed the Mendelian model, which was widely
accepted by 1925. Alongside experimental work, mathematicians developed the statistical framework
of population genetics, bringing genetic explanations into the study of evolution.
With the basic patterns of genetic inheritance established,
many biologists turned to investigations of the physical nature of the gene. In the 1940s and
early 1950s, experiments pointed to DNA as the portion of chromosomes that held genes.
A focus on new model organisms such as viruses and bacteria, along with the discovery of the
double helical structure of DNA in 1953, marked the transition to the era of molecular genetics.
In the following years, chemists developed techniques for sequencing both nucleic acids
and proteins, while many others worked out the relationship between these two forms of
biological molecules and discovered the genetic code. The regulation of gene expression became
a central issue in the 1960s; by the 1970s gene expression could be controlled and
manipulated through genetic engineering. In the last decades of the 20th century,
many biologists focused on large-scale genetics projects, such as sequencing entire genomes.
Pre-Mendelian ideas on heredity Ancient theories
The most influential early theories of heredity were that of Hippocrates and Aristotle.
Hippocrates' theory was similar to Darwin's later ideas on pangenesis,
involving heredity material that collects from throughout the body. Aristotle suggested instead
that the form-giving principle of an organism was transmitted through semen and the mother's
menstrual blood, which interacted in the womb to direct an organism's early development.
For both Hippocrates and Aristotle—and nearly all Western scholars through to
the late 19th century—the inheritance of acquired characters was a supposedly well-established fact
that any adequate theory of heredity had to explain. At the same time, individual species
were taken to have a fixed essence; such inherited changes were merely superficial.
The Athenian philosopher Epicurus observed families and proposed the contribution of both
males and females of hereditary characters , noticed dominant and recessive types of
inheritance and described segregation and independent assortment of "sperm atoms".
In the Charaka Samhita of 300CE, ancient Indian medical writers saw
the characteristics of the child as determined by four factors:
1) those from the mother's reproductive material, those from the father's sperm, those from the diet
of the pregnant mother and those accompanying the soul which enters into the fetus. Each of
these four factors had four parts creating sixteen factors of which the karma of the parents and the
soul determined which attributes predominated and thereby gave the child its characteristics.
In the 9th century CE, the Afro-Arab writer Al-Jahiz considered the effects of the environment
on the likelihood of an animal to survive. In 1000 CE, the Arab physician, Abu al-Qasim al-Zahrawi
was the first physician to describe clearly the hereditary nature of haemophilia in his Al-Tasrif.
In 1140 CE, Judah HaLevi described dominant and recessive genetic traits in The Kuzari.
Preformation theory The preformation theory is a developmental
biological theory, which was represented in antiquity by the Greek philosopher Anaxagoras.
It reappeared in modern times in the 17th century and then prevailed until the 19th century.
Another common term at that time was the theory of evolution,
although "evolution" had a completely different meaning than today. The preformists assumed that
the entire organism was preformed in the sperm or in the egg and only had to unfold and grow.
This was contrasted by the theory of epigenesis, according to which the structures and organs of an
organism only develop in the course of individual development . Epigenesis had been the dominant
opinion since antiquity and into the 17th century, but was then replaced by preformist ideas.
Since the 19th century epigenesis was again able to establish itself as a view valid to this day.
Plant systematics and hybridization In the 18th century, with increased
knowledge of plant and animal diversity and the accompanying increased focus on taxonomy,
new ideas about heredity began to appear. Linnaeus and others conducted extensive experiments with
hybridisation, especially hybrids between species. Species hybridizers described a wide variety of
inheritance phenomena, include hybrid sterility and the high variability of back-crosses.
Plant breeders were also developing an array of stable varieties in many important plant
species. In the early 19th century, Augustin Sageret established the concept of dominance,
recognizing that when some plant varieties are crossed, certain characteristics usually
appear in the offspring; he also found that some ancestral characteristics found in neither parent
may appear in offspring. However, plant breeders made little attempt to establish a theoretical
foundation for their work or to share their knowledge with current work of physiology,
although Gartons Agricultural Plant Breeders in England explained their system.
Mendel Between 1856 and 1865,
Gregor Mendel conducted breeding experiments using the pea plant Pisum sativum and traced the
inheritance patterns of certain traits. Through these experiments, Mendel saw that the genotypes
and phenotypes of the progeny were predictable and that some traits were dominant over others.
These patterns of Mendelian inheritance demonstrated the usefulness of applying
statistics to inheritance. They also contradicted 19th-century theories of blending inheritance,
showing, rather, that genes remain discrete through multiple generations of hybridization.
From his statistical analysis, Mendel defined a concept that he described as
a character . In only one sentence of his historical paper, he used the term
"factors" to designate the "material creating" the character: " So far as experience goes,
we find it in every case confirmed that constant progeny can only be formed when the egg cells and
the fertilizing pollen are off like the character so that both are provided with the material for
creating quite similar individuals, as is the case with the normal fertilization of pure species.
We must, therefore, regard it as certain that exactly similar factors must be
at work also in the production of the constant forms in the hybrid plants.".
Mendel's work was published in 1866 as "Versuche über Pflanzen-Hybriden" in the Verhandlungen des
Naturforschenden Vereins zu Brünn , following two lectures he gave on the work in early 1865.
Post-Mendel, pre-rediscovery Pangenesis
Mendel's work was published in a relatively obscure scientific journal,
and it was not given any attention in the scientific community. Instead,
discussions about modes of heredity were galvanized by Darwin's theory of evolution
by natural selection, in which mechanisms of non-Lamarckian heredity seemed to be required.
Darwin's own theory of heredity, pangenesis, did not meet with any large degree of acceptance. A
more mathematical version of pangenesis, one which dropped much of Darwin's Lamarckian holdovers,
was developed as the "biometrical" school of heredity by Darwin's cousin, Francis Galton.
Germ plasm In 1883 August Weismann conducted
experiments involving breeding mice whose tails had been surgically removed. His results — that
surgically removing a mouse's tail had no effect on the tail of its offspring — challenged the
theories of pangenesis and Lamarckism, which held that changes to an organism during its lifetime
could be inherited by its descendants. Weismann proposed the germ plasm theory of inheritance,
which held that hereditary information was carried only in sperm and egg cells.
Rediscovery of Mendel Hugo de Vries wondered what the
nature of germ plasm might be, and in particular he wondered whether or not germ plasm was mixed
like paint or whether the information was carried in discrete packets that remained unbroken.
In the 1890s he was conducting breeding experiments with a variety of plant species
and in 1897 he published a paper on his results that stated that each inherited trait was governed
by two discrete particles of information, one from each parent, and that these particles were passed
along intact to the next generation. In 1900 he was preparing another paper on his further results
when he was shown a copy of Mendel's 1866 paper by a friend who thought it might be relevant to
de Vries's work. He went ahead and published his 1900 paper without mentioning Mendel's priority.
Later that same year another botanist, Carl Correns, who had been conducting hybridization
experiments with maize and peas, was searching the literature for related experiments prior
to publishing his own results when he came across Mendel's paper, which had results
similar to his own. Correns accused de Vries of appropriating terminology from Mendel's
paper without crediting him or recognizing his priority. At the same time another botanist,
Erich von Tschermak was experimenting with pea breeding and producing results like Mendel's.
He too discovered Mendel's paper while searching the literature for relevant work. In a subsequent
paper de Vries praised Mendel and acknowledged that he had only extended his earlier work.
Emergence of molecular genetics After the rediscovery of Mendel's work there was
a feud between William Bateson and Pearson over the hereditary mechanism, solved by Ronald Fisher
in his work "The Correlation Between Relatives on the Supposition of Mendelian Inheritance".
In 1910, Thomas Hunt Morgan showed that genes reside on specific chromosomes.
He later showed that genes occupy specific locations on the chromosome. With this knowledge,
Alfred Sturtevant, a member of Morgan's famous fly room, using Drosophila melanogaster,
provided the first chromosomal map of any biological organism. In 1928,
Frederick Griffith showed that genes could be transferred. In what is now known as Griffith's
experiment, injections into a mouse of a deadly strain of bacteria that had been heat-killed
transferred genetic information to a safe strain of the same bacteria, killing the mouse.
A series of subsequent discoveries led to the realization decades later that the
genetic material is made of DNA and not, as was widely believed until then, of proteins.
In 1941, George Wells Beadle and Edward Lawrie Tatum showed that mutations in genes caused
errors in specific steps of metabolic pathways. This showed that specific genes code for specific
proteins, leading to the "one gene, one enzyme" hypothesis. Oswald Avery, Colin Munro MacLeod, and
Maclyn McCarty showed in 1944 that DNA holds the gene's information. In 1952, Rosalind Franklin and
Raymond Gosling produced a strikingly clear x-ray diffraction pattern indicating a helical form.
Using these x-rays and information already known about the chemistry of DNA,
James D. Watson and Francis Crick demonstrated the molecular structure of DNA in 1953.
Together, these discoveries established the central dogma of molecular biology,
which states that proteins are translated from RNA which is transcribed by DNA. This
dogma has since been shown to have exceptions, such as reverse transcription in retroviruses.
In 1972, Walter Fiers and his team at the University of Ghent were the first
to determine the sequence of a gene: the gene for bacteriophage MS2 coat protein.
Richard J. Roberts and Phillip Sharp discovered in 1977 that genes can be split into segments.
This led to the idea that one gene can make several proteins. The successful sequencing
of many organisms' genomes has complicated the molecular definition of the gene.
In particular, genes do not always sit side by side on DNA like discrete beads.
Instead, regions of the DNA producing distinct proteins may overlap, so that the idea emerges
that "genes are one long continuum". It was first hypothesized in 1986 by Walter Gilbert that
neither DNA nor protein would be required in such a primitive system as that of a very early stage
of the earth if RNA could serve both as a catalyst and as genetic information storage processor.
The modern study of genetics at the level of DNA
is known as molecular genetics and the synthesis of molecular
genetics with traditional Darwinian evolution is known as the modern evolutionary synthesis.