A Century of Genetics
Daniel J. Fairbanks
Abstract—In 1866, Gregor Mendel published his experiments on
heredity in the garden pea (Pisum sativum). The fundamental
principles of inheritance derived from his work apply to nearly all
eukaryotic species and are now known as Mendelian principles.
Since 1900, Mendel has been recognized as the founder of genetics.
In 1900, three botanists, Carl Correns, Hugo De Vries, and Erich
Tschermack von Seysenegg, had independently completed experiments that were similar to Mendel’s, although much less extensive.
When searching the literature, all three encountered Mendel’s
paper and realized that he had described the principles of inheritance and the experimental data to confirm them 34 years earlier.
With their rediscovery, the science of genetics was born in 1900
along with the 20th century. William Bateson coined the term
“genetics” and was the person most responsible for establishing the
th
new science in the first decade of the 20 century. Thomas Hunt
Morgan and his students established the chromosomal basis of
heredity beginning in 1910. During the 1920s and 1930s, classical
genetics was established, and eugenics became very popular among
the more educated and wealthy members of society. Laws mandating sterilization of perceived unfit people were passed and carried
out. By the late 1930s, Nicolai Vavilov had published his theory of
centers of origin for cultivated plants and established a gene bank
for his collections in Leningrad. During the siege of Leningrad in
1941–1942, nine of his coworkers chose to die of starvation rather
than sacrifice the seeds and tubers that Vavilov had collected. In the
meantime, Vavilov was imprisoned for his opposition to Lysenkoism. He soon died in prison of maltreatment. In the mid-1940s,
George Beadle and Edward Tatum discovered the relationship
between genes and enzymes, and Oswald Avery and his coworkers
discovered that DNA is the genetic material of a bacterial species.
In the early 1950s, Alfred Hershey and Martha Chase discovered
that DNA is the genetic material of a bacteriophage. Shortly after
this time, in 1953, James Watson and Francis Crick determined the
structure of DNA based on evidence collected in several laboratories. Cracking the genetic code became one of the next priorities, a
task that was completed in the 1960s. During the 1970s, recombinant DNA was made, an event that led to molecular applications in
genetics and genetic engineering. The first genetically engineered
pharmaceuticals soon followed. The 1970s and 1980s saw the
development of efficient methods for DNA sequencing, which ultimately led to whole genome sequencing. The first bacterial genome
was sequenced in 1995, the Brewer’s yeast genome in 1996, and the
Drosophila, Arabidopsis, and human genomes in 2000. Fittingly,
the sequencing of the human genome, one of the greatest accomth
plishments in genetics, came at the 100 birthday of the science of
genetics.
In the year 2000, the science of genetics celebrated its
100th anniversary. In the spring and early summer of 1900,
three botanists, Hugo de Vries, Carl Correns, and Erich von
Tschermack, reported their simultaneous and independent
rediscovery of Mendel’s principle of segregation, an event
that sparked the rapid establishment of genetics as a new
and important science. This paper highlights just a few of
the major events that have made genetics one of the most
powerful and rapidly progressing sciences.
Although genetics entered mainstream science rather
suddenly in 1900, it traces its origin to the mid-1800s with
the experiments of Gregor Mendel. In 1843, Mendel entered
the St. Thomas monastery in the city of Brno, now in the
Czech Republic, and was soon appointed as a seventh-grade
science teacher. He failed his teacher certification examination, an event that prompted the abbot of the monastery to
send him to the University of Vienna. While at the University, Mendel was enamored with the teachings of his botany
professor, Dr. Franz Unger, who, even though Darwin’s
Origin of Species had not yet been published, focused his
teachings on the evolution of species over long periods of
geological time. During the time that Mendel was a student,
Unger wrote an article in which he wrote this remarkable
passage that foreshadowed Mendel’s work: “Who can deny
that new combinations arise out of this permutation of
vegetation, always reducible to certain law-combinations,
which emancipate themselves from the preceding characteristics of the species and appear as a new species” (Orel 1996).
Unger’s teachings infuriated the local clergy who attempted
to have him dismissed. Among the most vocal was Dr. Sebastian
Brunner who wrote of Unger as “a man who openly denied the
Creation and the Creator” and as one of the “professors at socalled Catholic Universities [who] deliver lectures on really
beastly theories for years on end” (Olby 1985). Ironically
Mendel was a devout member of the clergy, but he appears to
have sided with Unger. Mendel wrote of his own work as “the
only right way by which we can finally reach the solution of a
question the importance of which cannot be overestimated in
connection with the history of the evolution of organic forms”
(Stern and Sherwood 1966).
In 1852, Mendel began experiments with pea hybrids that
would last for 8 years. From the scientific literature that he
had read extensively he already knew of the concept of
dominance in peas for four of the seven traits he studied. His
contribution was his development of a mathematical model
to explain heredity. He discovered regular 3:1 ratios in the
F2 generations in all seven of his monohybrid experiments,
and developed the now familiar mathematical model to
explain the 3:1 ratio, which he expressed in the following
equation:
In: McArthur, E. Durant; Fairbanks, Daniel J., comps. 2001. Shrubland
ecosystem genetics and biodiversity: proceedings; 2000 June 13–15; Provo,
UT. Proc. RMRS-P-21. Ogden, UT: U.S. Department of Agriculture, Forest
Service, Rocky Mountain Research Station.
Daniel J. Fairbanks is a Professor in the Department of Botany and Range
Science, Brigham Young University, Provo, UT 84602.
A A a a
+ + +
A a A a
42
where the letters in the numerator represent the alleles
contributed by the male parent and those in the denominator
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A Century of Genetics
as the alleles contributed by the female parent. Because the
large A is dominant, three of the four combinations produce
the dominant phenotype and one the recessive. He tested
this model by allowing the F2 plants to self fertilize to
produce F3 offspring. He found that 1⁄3 of the F2 plants with
the dominant phenotype produced offspring with only the
dominant phenotype, and that 2⁄3 produced offspring with
both dominant and recessive phenotypes, precisely as his
model predicted.
He then turned his attention to dihybrid and trihybrid
experiments to ascertain whether the inheritance of one
trait influences the inheritance of another. He discovered
that all seven traits in various combinations of twos and
threes were inherited independently of one another. On
these two discoveries, the mathematical segregation of differing elements and the independent inheritance of traits,
are based the two laws of inheritance attributed to Mendel:
the law of segregation and the law of independent assortment.
Mendel presented his paper in 1865 and had it published
the following year (Mendel 1866). For the next 34 years, no
one, including Mendel, recognized that the laws he discovered applied almost universally to plants, animals, and
humans. His paper was not completely forgotten; it was cited
at least 15 times before 1900. However, it is clear from these
citations that no one recognized its most important points.
Mendel sent reprints with cover letters to several botanists,
including Franz Unger, his former professor, but the only
one to respond was Karl von Nägeli, who carried on correspondence with Mendel over a period of 7 years. Mendel had
initiated studies with the hawkweed, and Nägeli encouraged Mendel to continue with these species. Neither Mendel
nor Nägeli knew that hawkweed was an apomict, and thus
it did not display the patterns of inheritance that Mendel
had observed in peas, and by now in several other species.
Mendel died in 1884, unaware that he would become
known as the founder of genetics. Sixteen years later, in
1900, DeVries working with several plant species, Correns
with peas and maize, and Tschermak with peas, rediscovered Mendel’s principle of segregation and the science of
genetics was born. Two of the three rediscoverers, Correns
and Tschermak, became strong advocates of Mendelism. De
Vries, on the other hand, dismissed Mendelian inheritance
within less than a year of his paper on it. The person most
responsible for the establishment of Mendelism was the
British naturalist William Bateson. Bateson read Mendel’s
paper while on a train in 1901 and soon thereafter embraced
Mendelism with the passion of religious zealot. Bateson was
so passionately supportive of Mendelism that De Vries
warned him in a 1902 letter, “I prayed you last time, please
don’t stop at Mendel. I am now writing the second part of my
book which treats crossing, and it becomes more and more
clear to me that Mendelism is an exception to the general
rule of crossing. In no way is it the rule!” (Olby 1985). The
opposition to Mendelism only encouraged Bateson, who,
together with Reginald Punnett and Edith Saunders, found
Mendelian inheritance in many different species. It is to
Bateson that we owe much of our current genetic terminology (including the word “genetics”) and the establishment of
Mendelian genetics as a credible science in the first decade
th
of the 20 century.
Thomas Hunt Morgan was an American zoologist at
Columbia University who at first rejected both the Mendelian
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Fairbanks
theory of inheritance espoused by Bateson and the chromosomal theory of inheritance promoted by Edmund Wilson,
also at Columbia. He opted instead for de Vries’ mutation
theory (which differs substantially from our current understanding of mutation). Complaining of the intellectual climate at Columbia, Morgan wrote in 1905 that it was “an
atmosphere saturated with chromosomic acid” (Allen 1978).
He completely reversed his views, however, within 5 years
as he brought Mendelian and chromosomal theories of
inheritance together as a single theory. In 1910 he observed
a white-eyed fruit fly that was to change the direction of his
career and the science of genetics. He discovered that the
white-eye phenotype was associated with inheritance of the
X chromosome.
At first, he was reluctant to conclude that the genes were
actually a part of the chromosome. However, two discoveries
by his students, Alfred Sturtevant and Calvin Bridges, made
it clear that their so-called sex-linked genes must be a
physical part of the X chromosome. Sturtevant, as an undergraduate student in 1911, had a flash of genius when he
realized that genes might be located in a linear fashion on
the chromosome. He gathered up the notebooks with the
data from several of their experiments, and, in his words, “I
went home and spent most of the night (to the neglect of my
undergraduate homework) in producing the first chromosome map” (Sturtevant 1965). He placed five genes on a
linear map and calculated the distances between them
based on the frequencies of crossing over. Bridges discovered
in 1913 that unusual cases of inheritance of mutant sexlinked alleles were associated with nondisjunction of chromosomes. The collective data strongly indicated that genes
were organized in a linear fashion as part of the chromosome. Morgan and three of his students, Sturtevant, Muller,
and Bridges, published in 1915 a landmark book entitled the
Mechanism of Mendelian Heredity (Morgan and others 1915)
in which they summarized all of their evidence that Mendelian and chromosomal theories of inheritance were one and
the same. Bateson rejected the idea, but respected their
work so much that he aptly wrote “not even the most
skeptical of readers can go through the Drosophila work
unmoved by a sense of admiration for the zeal and penetration with which it has been conducted, and for the great
extension of genetic knowledge to which it has led—greater
far than has been made in any one line of work since
Mendel’s own experiments” (Sturtevant 1965).
The students who coauthored this work with Morgan,
Sturtevant, Bridges, and Muller, were to become three of the
most influential geneticists in later years. Another laboratory
was also soon to produce a similar group of prominent geneticists. At Cornell University, Rollins Emerson had taken on
George Beadle, Marcus Rhoads, Charles Burnham, and Barbara McClintock as students to work with him on maize.
McClintock had hoped to study plant breeding, but the Department of Plant Breeding at Cornell did not admit women
at the time, so Emerson invited her to work with him in maize
genetics. McClintock’s enthusiasm for her knowledge is well
illustrated in the following story, given in her words:
I couldn’t wait to take [the final exam for geology]. I loved
the subject so much that I knew they wouldn’t ask me
anything I couldn’t answer. I just knew the course. So I
couldn’t wait to get into the final exam. They gave out these
blue books, to write in and on the front page you put down your
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Fairbanks
own name. Well, I couldn’t be bothered with putting my name
down; I wanted to see those questions. I started writing right
away—I was delighted, I just enjoyed it immensely. Everything was fine, but when I got to write my name down, I
couldn’t remember it. I couldn’t remember to save me, and I
waited there. I was much too embarrassed to ask anybody
what my name was, because I knew they would think I was
a screwball. I got more and more nervous, until finally (it took
about twenty minutes) my name came to me.
—Keller (1983)
Although McClintock is best known for her later discovery
of transposable elements in maize, among her most significant publications was her demonstration that chromosomes
were the physical carriers of genes. The work of Morgan and
his students had shown the association of genes and chromosomes, which led them to conclude that genes were on
chromosomes. However, McClintock and her coworker
Harriet Creighton had observed cytological evidence of crossing over that was clearly associated with the recombination
of linked genes in maize. Morgan gave a lecture at Cornell in
1931, and then took a tour of the labs. Creighton and
McClintock showed Morgan their data, which they felt were
rather meager. They had intended to grow their maize
plants another year to collect more data before publishing
the results. Morgan knew of similar work in Drosophila
being conducted by Curt Stern, but did not tell them of it. He
insisted that Creighton and McClintock publish their results immediately. He asked for a pen and paper and in their
presence wrote a letter to the editor of the Proceedings of the
National Academy of Sciences telling him that he would
receive within 2 weeks a significant article from Creighton
and McClintock that should be immediately published. The
article beat Stern’s by several months. Explaining his actions, Morgan simply said, “I thought it was about time that
corn got a chance to beat Drosophila” (Keller 1983).
Like most other sciences, genetics was not immune to
political influences. The eugenic movement gained momentum in the period from turn of the century to the 1930s.
Based on incorrect assumptions about the inheritance of
such traits as feeblemindedness, imbecility, and criminality, antimiscegenation and mandatory sterilization laws
were passed in many states and in several European countries. Before such laws were rescinded, over 60,000 people
suffered involuntary sterilization in the United States. Although many geneticists favored eugenics to some degree,
some of them pointed out the theoretical flaws implicit in
these laws. Morgan in particular renounced his membership
in a society that promoted eugenics, and later in his Nobel
acceptance speech, given in 1935, he stated, “The claims of
a few enthusiasts that the human race can be entirely
purified or renovated at this later date, by proper breeding,
have I think been greatly exaggerated. Rather must we look
to medical research to discover remedial measures to insure
better health and more happiness for mankind” (Morgan
1935). Eugenics reached its most tragic point with the
genocide of millions by the Nazi regime before and during
World War II. Unfortunately, eugenic measures are still
with us. Reports of genocide and ethnic cleansing are still an
atrocious part of our modern world.
Some of the most tragic effects of politics and war on
genetics were in Russia in the 1930s and 1940s. Nicolai
Vavilov was a brilliant Russian plant geneticist who recognized the need to preserve genetic diversity in plants. His
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A Century of Genetics
worldwide studies identified the centers of origin and diversity for the major food crops of the world. Vavilov and his
associates collected seeds and tubers of those crops and
created one of the world’s first gene banks in Leningrad. In
the winter of 1941–1942, Hitler’s army laid siege to Leningrad
and food soon ran out. Tens of thousands of the city’s
residents died of starvation. The scientists in the institute
Vavilov had directed were surrounded by stores of rice,
wheat, corn, peanuts, potatoes, and peas that contained the
genetic diversity collected by Vavilov and his associates.
Recognizing the need to preserve that diversity, they made
a pact among themselves that none of them would eat the
seeds and tubers. Nine of the scientists died of starvation
while at their posts in the Institute rather than sacrifice the
genetic diversity that was stored there.
A few years earlier, Vavilov was the most prominent
geneticist in Russia. However, in the 1930s, Trofim Lysenko
began to promote his Lamarckian ideas that the environment could direct specific changes in hereditary elements,
and referred in distaste to Vavilov and his colleagues as “the
dogmatic followers of Mendel and Morgan” (Medvedev 1969).
Vavilov publicly resisted Lysenko, but in the end, Stalin
declared Lysenko’s views as state policy. In 1939, at one of
his last attempts to challenge Lysenko, Vavilov stated his
resolve, “We shall go to the pyre, we shall burn, but we shall
not retreat from our convictions” (Medvedev 1969). Shortly
thereafter, in 1940, Vavilov and several other Soviet geneticists were arrested and imprisoned for their refusal to follow
Lysenko. As Hitler’s army advanced during 1941, Vavilov as
a prisoner was evacuated from St. Petersburg to Saratov
prison and placed in a windowless underground cell called a
death cell. There he died in January 1943, a year after the
deaths of his colleagues in the institute. Numerous Soviet
geneticists were imprisoned and killed because of their
refusal to accept Lysenkoism. Lysenko reigned over Soviet
genetics for nearly 3 decades until 1964 when Khrushchev,
who had firmly supported Lysenko, was forced to resign.
We now return to the United States. In the early 1940s,
George Beadle, one of Emerson’s former students, and Edward Tatum discovered that genes were related to enzymes.
In a sense, they had simply clarified the same concept that
Archibald Garrod and William Bateson had proposed in
1902, but with much more evidence and in much more detail.
The association between genes and proteins was evident,
but their real relationship was yet to be revealed.
Although DNA was discovered by Friedrich Miescher
shortly after Mendel did his work, the connection between
DNA and heredity began to surface about the same time that
Beadle and Tatum demonstrated the association of genes
and proteins. In 1944, Oswald Avery, Colin McLeod, and
Maclyn McCarty found that the hereditary substance of the
bacterium Streptococcus pneumoniae was DNA. In 1952,
Alfred Hershey and Martha Chase demonstrated that the
hereditary material of the bacteriophage T2 was also DNA.
However, even in light of these results, most scientists still
refused to accept DNA as the hereditary substance. According to James Watson, “Al Hershey had sent me a long
letter...summarizing the recently completed experiments by
which he and Martha Chase established that a key feature
of the infection of a bacterium by a phage was injection of
DNA.…Their experiment was…a powerful new proof that
DNA is the primary genetic material.…Nonetheless, almost
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A Century of Genetics
no one in the audience of over four hundred microbiologists
seemed interested as I read long sections of Hershey’s letter”
(Watson 1969).
The disinterest didn’t last long. In 1953, Watson and
Francis Crick, using data gathered entirely from the experiments of others, deduced the structure of DNA. One of the
chief requirements of the hereditary material was the ability
to self-replicate. At the conclusion of their classic paper, they
penned this now-classic line, “It has not escaped our notice
that the specific pairing we have postulated immediately
suggests a possible copying mechanism for the genetic material” (Watson and Crick 1953).
About this same time, it was clear that DNA somehow
encoded the composition of proteins, and that RNA was an
intermediate between DNA and protein. The relationship
between nucleotide sequence and amino acid sequence was
yet to be established. Through brilliant mathematical analysis by Syndey Brenner, and through analysis of the effect of
mutations on amino acid sequence, geneticists realized that
the genetic code must be nonoverlapping and that three
nucleotides must encode each amino acid. These realizations led Marshall Nirenberg, Heinrich Matthaei, Severo
Ochoa, Philip Leder, Francis Crick, and others to decipher
the genetic code. By 1965, they had identified the amino
acids for 50 of the 64 codons. Then in 1966, the final gaps
were bridged, and the genetic code was revealed.
The genetic code turned out to be nearly universal, an
observation that had profound implications. Because of a
universal code, a gene transferred from one species to another
should encode the same protein in the recipient species as in
the original species. This opened up great possibilities of
genetic engineering, especially with bacteria. In the late
1970s, Goeddel and others (1979) successfully introduced a
cDNA from the human growth hormone gene into Escherichia
coli, and with a little genetic engineering of the promoter and
initiation codon regions, achieved expression of pure human
growth hormone in bacteria. This initiated the age of genetic
pharmacology, which now produces such important human
proteins as insulin, interferon, and clotting factors in bacteria
for medicinal use. So valuable is this industry that the
University of California and Eli Lilly & Co. spent over 30
million dollars in legal fees fighting each other over the patent
for genetically engineered insulin. Lilly won the dispute.
For the most part, the molecular revolution and traditional genetics and breeding remained almost separate
fields. Then in the 1980s came the convergence of the two
with the use of DNA markers to address questions of inheritance and to improve the efficiency of both medical and
agricultural genetics. The 1980s marked the beginning of
genomics. Genetic maps, which before had taken years to
develop, now could be completed sometimes in a period of
weeks. DNA markers overcame the obstacles that had prevented efficient genetic analysis in humans, and the first
saturated human genetic map was completed in 1994.
The ultimate genetic map, however, was the entire nucleotide sequence of a genome. Frederick Sanger, who developed the basic process that is now used in automated
sequencers, reported with colleagues in 1977 the first entire
nucleotide sequence of a genome, that of the phage phi X-174
which consists of 5,386 nucleotide pairs (Sanger and others
1977). It was a huge step from those days to the sequence of
the genome from a cellular organism. Long before the first
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Fairbanks
bacterial genome was sequenced, the human genome project,
one of the most ambitious undertakings in the history of
science, was begun. To sequence the 3 billion nucleotides of
the human genome, scientists required substantial improvements in laboratory automation and vast cooperation among
many different laboratories. The projected cost of the project
was enormous and it drew significant criticism because
many perceived it as drawing research funding away from
other projects. However, the benefits that came from it,
automated DNA sequencing in particular, have benefited
genetic research in many areas, including much of the
research presented in these proceedings.
In 1995, Craig Venter and his colleagues at the Institute
for Genomic Research published the first genomic sequence
of a cellular organism, that of the bacterium Haemophilus
influenzae with 1,830,137 nucleotide pairs in its genome
(Fleischmann and others 1995). Goffeau and others (1996)
published the first sequence of a eukaryotic genome, that of
Saccharomyces cerevisiae, Brewers yeast, whose genome
contains about 13 million nucleotide pairs and 6,000 genes.
The sequences of numerous bacterial species and that of the
nematode worm Caenorhabditis elegans were also completed. In March 2000, the genomic sequence of Drosophila
melanogaster was published, with 120 million nucleotide
pairs in the euchromatic regions and about 13,600 genes
(Adams and others 2000). Arabidopsis thaliana was the first
plant species to be sequenced (The Arabidopsis Genome
Initiative 2000). Fittingly, at the conclusion of one century of
genetics, the first draft of the human genome was announced, 5 years ahead of schedule.
We have come a long way from Mendel’s garden to the
sequence of the human genome. I’ll resist the temptation to
speculate about what the future holds. One of the delights of
studying the history of genetics is to see how wrong many
authors were when they wrote decades ago about genetics in
our day. This much I feel confident in predicting, however,
that the future of genetics is very bright.
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