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| Gregor Mendel |
Gregor
Mendel is recognized as the father of genetics. He:
•
Founded the science of genetics.
•
Showed that people’s ideas about how living organisms passed traits on to their
offspring were wrong.
•
Identified many of the rules of heredity. These rules determine how traits are
passed through generations of living things.
• Saw
that living things pass traits to the next generation by something which
remains unchanged in successive generations of an organism – we now call this
‘something’ genes.
•
Realized that traits could skip a generation – seemingly lost traits could
appear again in another generation – he called these recessive traits.
•
Identified recessive and dominant traits which pass from
parents to offspring.
•
Established, momentously, that traits pass from parents to their offspring in a
mathematically predictable way.
Mendel’s
work only made a big impact in 1900, 16 years after his death, and 34 years
after he first published it.
Mendel’s Education and
the Abbey of St. Thomas
Johann Mendel (he wasn’t called Gregor until later) was
born July 20, 1822, in Heinzendorf bei Odrau. This small village was in the
Austrian Empire, but is now in the Czech Republic.
Mendel’s parents were small farmers who made financial
sacrifices to pay for his education.
He did well enough at high school to make it, aged 18, to
the University of Olomouc in 1840. The university was about 40 miles (60 km)
from his home village. He took courses in physics, mathematics and philosophy.
You want to keep doing
science? You need to be a monk!
In 1843, aged 21, and in financial difficulty, one of his
teachers, Professor Friedrich Franz, a physicist, advised Mendel to join the
Abbey of St. Thomas in Brünn as a monk.
The Abbey actually had a good reputation for its teaching
of sciences, and its director, Abbot Franz Cyril Napp, was particularly
interested in heredity of traits in plants and animals on farms.
If he could join the Abbey, he could continue studying
science, while ensuring he could get by financially. And so Mendel, who was
more interested in science than religion, became a monk.
The move to Brünn carried him much farther away from his
home village. On joining the Abbey, he took the name Gregor. From then on he
ceased to be Johann Mendel and became Gregor Mendel.
Learning and Teaching
Science
In 1846, aged 24, Mendel took fruit-growing classes given
by Professor Franz Diebl at the Brünn Philosophical Institute. Diebl was an
authority on plant breeding.
Mendel became a priest in 1847 and got his own parish in
1848. He did not enjoy working as a parish priest and got a job as a high
school teacher in 1849.
In 1850, aged 28, he failed exams which would have
qualified him as a high school teacher.
A year later, he went to the University of Vienna where he
studied chemistry, biology and physics. The idea was that by strengthening his
knowledge in these subjects, he could qualify as a high school teacher.
Two years later, after completing his studies, he returned
to the monastery in 1854 and took a position as a physics teacher at a school
at Brünn, where he taught for the next 16 years.
Research and Admin
In 1856, aged 34, he again failed to qualify formally as a
high school teacher. This time, illness prevented him completing the exams.
In the same year, he began his major, groundbreaking study
of heredity in plants.
In 1865, still interested in physical science, he founded
the Austrian Meteorological Society. In fact, during his life, Mendel
published more about meteorology than he did biology!
In 1866, he published his heredity work. Unfortunately,
most people who read it did not recognize the intellectual gold that his paper
contained.
In 1867, aged 45, he became Abbot of his monastery and
devoted himself to its smooth running as its administrator.
At the monastery in Brünn in the early 1860s. Mendel is
pictured back right, looking at something in his left hand. Abbot Franz Cyril
Napp sits in the front row, wearing a large cross. Abbot Napp encouraged
Mendel’s science and heredity studies.
Mendel and Genetics:
Experiments with Peas: 1856 to 1863
During his time in Olomouc, Mendel had made friends with
two university professors: Friedrich Franz, a physicist, and Johann Karl
Nestler, an agricultural biologist, who was interested in heredity.
Nestler passed his interest in heredity to Mendel, who was
intrigued by the subject.
Mendel’s monastery had a 5 acre (2 hectare) garden, and his
two former professors encouraged Mendel to pursue his interest in heredity by
using the garden for experiments.
Abbot Franz Cyril Napp and Professor Franz Diebl also
encouraged him to follow this path.
Mendel was unhappy
with how inheritance of traits was being explained
People had known for millennia about selective breeding.
They knew that by breeding from those individuals that showed the most
desirable traits, future generations were more likely to show these desirable
traits.
- Guard dogs might be bred from parents that were loyal and friendly to their owners, but were suspicious or even aggressive with strangers.
- Cattle might be bred from cows that yielded most milk and bulls that yielded most meat.
- Wheat might be kept and sown the following year from those plants which had produced the most abundant crop.
The main theory of heredity in Mendel’s time was that
offspring were a smooth blend of their two parents’ traits.
Mendel set himself the very ambitious task of discovering
the laws of heredity.
To achieve this, he embarked on a mammoth sized, highly
systematic, eight year study of edible peas, individually and carefully
recording the traits shown by every plant in successive generations.
His work involved growing and recording the traits in about
30,000 plants.
One of the keys to his success was breeding from closely
related pea varieties which would differ in only a small number of traits.
The seven traits of pea plants that Mendel chose to study:
seed wrinkles; seed color; seed-coat color, which leads to flower color; pod
shape; pod color; flower location; and plant height. Image by Mariana Ruiz.
Mendel’s Results for
Flower Color
Mendel found the same results for all traits, but we’ll
look at flower color as an example.
When Mendel bred purple-flowered peas (BB) with
white-flowered peas (bb), every plant in the the next generation had only
purple flowers (Bb).
When these purple-flowered plants (Bb) were bred with
one-another to create a second-generation of plants, some white flowered plants
appeared again (bb).
Mendel realized that his purple-flowered plants still held
instructions for making white flowers somewhere inside them.
He also found that the number of purple to white was
predictable.
75 percent of the second-generation of plants had purple
flowers, while 25 percent had white flowers. He called the purple trait dominant
and the white trait recessive.
A Punnett Square. Both of the starting plants have purple
flowers but they contain the genes for purple (B) and white (b). The pollen
from the male plant fertilizes the egg in the female flower. In this variety of
plant, purple flowers are caused by a dominant gene (B). Dominance is indicated
by a capital letter. White flowers are caused by recessive genes, indicated by
the small letter (b). Both the male and female parent plants in the diagram
above carry the dominant gene B for purple and the recessive gene b for white
flowers. The ratio of purple flowers to white flowers in their offspring will
be 3:1 as shown in this diagram. For a white flower to appear, the offspring
must inherit the recessive gene from both parents. Purple appears with any
other combination of genes inherited from the parent plants. Image by Madeleine
Price Ball
Mendel’s Conclusions
Mendel’s most important conclusions were:
- The inheritance of each trait is determined by something (which we now call genes) passed from parent to offspring unchanged. In other words, genes from parents do not ‘blend’ in the offspring.
- For each trait, an organism inherits one gene from each parent.
- Although a trait may not appear in an individual, the gene that can cause the trait is still there, so the trait can be passed to and appear again in a future generation.
Scientists who did research later found that Mendel’s
results do not only apply to pea plants. Trait inheritance in most plants and
animals, including humans, follows the patterns Mendel recorded.
In Mendel’s honor, these very common patterns of heredity
are now called Mendelian Inheritance.
Fast Forward to 1900:
The Sleeping Giant Awakes
In 1900, three scientists independently carrying out heredity
research got exciting results.
However, when they searched the literature, they realized
their results were not really new. Their results actually verified the
forgotten results Mendel had published 34 years earlier.
Mendel’s results gave the scientists of 1900 greater
confidence in their own results and the new science of genetics was truly born.
The scientists were Carl Correns, Hugo de Vries, and Erich
von Tschermak.
“I
thought that I had found something new. But then I convinced myself that the Abbot
Gregor Mendel in Brünn, had, during the sixties, not only obtained the same
result through extensive experiments with peas, which lasted for many years, as
did de Vries and I, but had also given exactly the same explanation, as far as
that was possible in 1866.”
Carl Erich Correns, 1864 to 1933
Geneticist and Botanist
Mendel’s Results Were
“Too Good”
Mendel’s published work was rather vague about detailed
experimental procedures, including dates.
Enter Ronald Fisher, a very eminent geneticist and statistician.
It was Fisher who first used the term ‘null hypothesis’ in statistical testing.
In 1936, Fisher tried to reconstruct on paper the way
Mendel carried out his experiments.
He also wanted to discover why Mendel’s work had been
overlooked for so long until it was rediscovered in 1900.
He found that, although some people in a position to see
the importance of Mendel’s work had actually read it, they did not realize its
importance. Their minds were unreceptive to Mendel’s words and ideas. They may
have believed he was repeating plant hybridization work others had already
carried out.
Controversially, Fisher said that his statistical analysis
of Mendel’s results showed too few random errors to have come from real
experiments. Nearly all of Mendel’s data showed an unnatural bias.
Fischer wrote:
“Although
no explanation can be expected to be satisfactory, it remains a possibility
among others that Mendel was deceived by some assistant who knew too well what
was expected. This possibility is supported by independent evidence that the
data of most, if not all, of the experiments have been falsified so as to agree
closely with Mendel’s expectations.”
Ronald Fisher, 1890 to 1962
Statistician, Geneticist, Evolutionary Biologist
Fisher’s analysis said there was only a 1 in 2000 chance
that Mendel’s results were the fully reported results of real experiments.
The controversy begun by Fisher continues to this day, with
a steady stream of publications seeking to give reasons for Mendel’s results.
One possibility is that results from ‘bad’ experiments were discarded to leave
only the results of ‘good’ experiments. Another is that the results arose from
an unconscious bias on the part of the experimenters.
The End
Gregor Mendel was unaware of the new science of genetics
which he had founded, and unaware of any future controversies. He died of a
kidney disease, aged 61, on January 6, 1884.
