What are Mendelian genetics?

Early Life

Born Johann Mendel on July 22, 1822, the future teacher, monk, abbot, botanist, and meteorologist grew up in a village in Moravia, a province of the Austrian Empire that later became part of Czechoslovakia (1918) and the Czech Republic (1993). His parents were peasant farmers and belonged to the large, German-speaking minority in this predominantly Czech province. Like most places in Moravia, Mendel’s hometown had two names: Hynčice in Czech and Heinzendorf in German.

Johann Mendel was an exceptional pupil, but no local schooling was available for him beyond the age of ten. In 1833, he persuaded his parents to send him to town to continue his education. They were reluctant to let him go because they could ill afford to dispense with his help on the farm or finance his studies. In 1838, Mendel’s father was partially disabled in a logging accident, and Johann, then sixteen and still at school, had to support himself. He earned just enough from tutoring to get by. At times, however, the pressure became too much for him. He suffered a breakdown in 1839 and returned home for several months to recuperate. He was to have several more of these stress-related illnesses, but no precise information is available about their causes and symptoms.

In 1840, Mendel completed Gymnasium, as the elite secondary schools were called, and entered the University of Olomouc for the two-year program in philosophy that preceded higher university studies. He had trouble supporting himself in Olomouc, perhaps because there was less demand for German-speaking tutors, and his Czech was not good enough for teaching. He suffered another breakdown in 1841 and retreated to Hynčice during spring exams.

That summer, Mendel decided once more against staying and taking over the farm. Since his father could not work, the farm was sold to his elder sister’s husband. Johann’s share of the proceeds was not enough to see him through the Olomouc program, especially since he had to repeat a year because of the missed exams. However, his twelve-year-old sister sacrificed part of her future dowry so that he could continue. (He repaid her years later by putting her three sons through Gymnasium and university.)

Upon finishing at Olomouc in 1843, Mendel decided to enter the clergy. The priesthood filled his need for a secure position and held out possibilities for further learning and teaching, but Mendel did not seem to be called to it. Aided by a professor’s recommendation, Mendel was accepted into the Augustinian monastery in Brno, the capital of Moravia, where he took the name Gregor. In 1847, after four years of preparation at the monastery, he was ordained a priest.

Priesthood and Teaching

The Brno monastery was active in the community and provided highly qualified instructors for Gymnasia and technical schools throughout Moravia. Several monks, including the abbot, were interested in science, and they had experimental gardens, a herbarium, a mineralogical collection, and an extensive library. Mendel found himself in learned company with opportunities for research in his spare time.

Unfortunately, Mendel’s nerves failed him when he had to minister to the sick and dying. Assigned to a local hospital in 1848, he was so upset by it that he was bedridden himself within five months. However, his abbot was sympathetic and let him switch to teaching. A letter survives in which the abbot explains this decision to the bishop: “[Mendel] leads a retiring, modest and virtuous religious life . . . and he devotes himself diligently to scholarly pursuits. For pastoral duties, however, he is less suited, because at the sick-bed or at the sight of the sick or suffering he is seized by an insurmountable dread, from which he has even fallen dangerously ill.”

Mendel taught Latin and Greek, German literature, math, and science as a substitute at the Gymnasium and was found to be very good at teaching. Therefore, he was sent to Vienna in 1850 to take the licensing examinations so that he could be promoted to a regular position. These exams were very demanding and normally required more preparation than Mendel’s two years at Olomouc. Mendel failed, but one examiner advised the abbot to let him try again after further study. The abbot took this advice and sent Mendel to study in Vienna for two years (1851–53). There he took courses in biology, physics, and meteorology with some of the best-known scientists of his day, including physicist Christian Doppler and botanist Franz Unger.

For unknown reasons, Mendel returned to Moravia to resume substitute teaching and did not go to Vienna for the exams until 1856. This time he was too nervous to finish. After writing one essay, he fell ill and returned to Brno. Despite this failure, he was allowed to teach regular classes until 1868 even though he was technically only a substitute.

Scientific Work

During his teaching career, Mendel performed his famous experiments on peas in a garden at the monastery. He published the results in an 1866 article, which introduced fundamental concepts and methods of genetics. The first set of experiments involved fourteen varieties of pea plant, each with a single distinguishing trait. These traits made up seven contrasting pairs, such as seeds that were either round or wrinkled in outline or seed colors that were green or yellow. Upon crossing each pair, Mendel obtained hybrids identical to one parent variety. For example, the cross of round with wrinkled peas yielded only round peas; the cross of green with yellow peas yielded only yellow peas. He referred to traits that asserted themselves in the hybrids as “dominant.” The others were “recessive” because they receded from view. The effect was the same regardless of whether he fertilized the wrinkled variety with pollen from the round or the round variety with pollen from the wrinkled. This indicated to Mendel that both pollen cells and egg cells contributed equally to heredity; this was a significant finding because the details of plant reproduction were still unclear.

Mendel next allowed the seven hybrids to pollinate themselves, and the recessive traits reappeared in the second generation. For instance, the round peas, which were hybrids of round and wrinkled peas, yielded not only more round peas but also some wrinkled ones. Moreover, the dominant forms outnumbered the recessives three to one. Mendel explained the 3:1 ratio as follows. He used the symbols A for the dominant form, a for the recessive, and Aa for the hybrid. A hybrid, he argued, could produce two types of pollen cell, one containing some sort of hereditary element corresponding to trait A and the other an element corresponding to trait a. Likewise, it could produce eggs containing either A or a elements. This process of dividing up the hereditary factors among the gametes became known as segregation.

The gametes from the Aa hybrids could come together in any of four combinations: pollen A with egg A, pollen A with egg a, pollen a with egg A, and pollen a with egg a. The first three of these combinations all grew into plants with the dominant trait A; only the fourth produced the recessive a. Therefore, if all four combinations were equally common, one could expect an average of three plants exhibiting A for every one exhibiting a.

Allowing self-pollination to continue, Mendel found that the recessives always bred true. In other words, they only produced more plants with that same recessive trait; no dominant forms reappeared, not even in subsequent generations. Mendel’s explanation was that the recessives could only have arisen from the pollen a and egg a combination, which excludes the A element. For similar reasons, plants with the dominant trait bred true one-third of the time, depending on whether they were the pure forms from the pollen A and egg A combination or the hybrids from the pollen A and egg a or pollen a and egg A combinations.

Mendel’s hereditary elements sound like the modern geneticist’s genes or alleles, and Mendel usually receives credit for introducing the gene concept. Like genes, Mendel’s elements were material entities inherited from both parents and transmitted to the gametes. They also retained their integrity even when recessive in a hybrid. However, it is not clear whether he pictured two copies of each element in every cell, one copy from each parent, and he certainly did not associate them with chromosomes.

In a second set of experiments, Mendel tested combinations of traits to see whether they would segregate freely or tend to be inherited together. For example, he crossed round, yellow peas with wrinkled, green ones. That cross first yielded only round, yellow peas, as could be expected from the dominance relationships. Then, in the second generation, all four possible combinations of traits segregated out: not only the parental round yellow and wrinkled green peas but also new round green and wrinkled yellow ones. Mendel was able to explain the ratios as before, based on equally likely combinations of hereditary elements coming together at fertilization. The free regrouping of hereditary traits became known as independent assortment. In the twentieth century, it was found not to occur universally because some genes are linked together on the same chromosome.

Mendel’s paper did not reach many readers. As a Gymnasium teacher and a monk in Moravia without even a doctoral degree, Mendel could not command the same attention as a university professor in a major city. Also, it was not obvious that the behavior of these seven pea traits illustrated fundamental principles of heredity. Mendel wrote to several leading botanists in Germany and Austria about his findings, but only Carl von Nägeli at the University of Munich is known to have responded, and even he was skeptical of Mendel’s conclusions. Mendel published only one more paper on heredity (in 1869) and did little else to follow up his experiments or gain wider attention from scientists.

Mendel pursued other scientific interests as well. He was active in local scientific societies and was an avid meteorologist. He set up a weather station at the monastery and sent reports to the Central Meteorological Institute in Vienna. He also helped organize a network of weather stations in Moravia. He envisioned telegraph connections among the stations and with Vienna that would make weather forecasting feasible. In his later years, Mendel studied sunspots and tested the idea that they affected the weather. He also monitored the water level in the monastery well in order to test a theory that changes in the water table were related to epidemics. A common thread that ran through these diverse research interests was that they all involved counting or measuring, with the goal of discovering scientific laws behind the numerical patterns. His one great success was in explaining the pea data with his concepts of dominance, segregation, and independent assortment.

Mendel felt pleased and honored to be elected abbot in 1868, even though he had to give up teaching and most of his research. He did not have the heart to say good-bye to his pupils. Instead, he asked the school director to announce his departure and give his last month’s salary to the three neediest boys in the class. As abbot, Mendel had a reputation for generosity to the poor and to scientific and cultural institutions. He was also an efficient manager of the monastery and its extensive land holdings and a fierce defender of the monastery’s interests. From 1874 on, he feuded with imperial authorities over a new tax on the monastery, which he refused to pay as long as he lived. Mendel’s health failed gradually in the last years of his life. He had kidney problems and an abnormally fast heartbeat, the latter probably from nerves and nicotine. (A doctor recommended smoking to control his weight, and he developed a twenty-cigar-a-day habit.) He died January 6, 1884, of heart and kidney failure.

Impact and Applications

Years after Mendel’s death, a scientific colleague remembered him saying, prophetically, “my time will come.” It came in 1900, when papers by three different botanists reported experimental results that were similar to Mendel’s and endorsed Mendel’s long-overlooked explanations. This event became known as the rediscovery of Mendelism. By 1910, Mendel’s theory had given rise to a whole new field of research, which was given the name “genetics.” Mendel’s hereditary elements were described more precisely as “genes” and were presumed to be located on the chromosomes. By the 1920s, the sex chromosomes were identified, the determination of sex was explained in Mendelian terms, and the arrangements of genes on chromosomes could be mapped.

The study of evolution was also transformed by Mendelian genetics, as Darwinians and anti-Darwinians alike had to take the new information about heredity into account. By 1930, it had been shown that natural selection could cause evolutionary change in a population by shifting the proportions of individuals with different genes. This principle of population genetics became a cornerstone of modern Darwinism.

Investigations of the material basis of heredity led to the discovery of the gene’s DNA structure in 1953. This breakthrough marked the beginning of molecular genetics, which studies how genes are copied, how mutations occur, and how genes exert their influence on cells. In short, all genetics can trace its heritage back to the ideas and experiments of Gregor Mendel.

Key terms



gametes

:

reproductive cells that unite during fertilization to form an embryo; in plants, the pollen cells and egg cells are gametes




hybrid

:

a plant form resulting from a cross between two distinct varieties




independent assortment

:

the segregation of two or more pairs of genes without any tendency for certain genes to stay together




segregation

:

the process of separating a pair of Mendelian hereditary elements (genes), one from each parent, and distributing them at random into the gametes





Bibliography

Carlson, Elof Axel. Mendel’s Legacy: The Origin of Classical Genetics. Cold Spring Harbor: Cold Spring Harbor Laboratory, 2004. Print.

Corcos, A., and F. Monaghan. Mendel’s Experiments on Plant Hybrids: A Guided Study. New Brunswick: Rutgers UP, 1993. Print.

Edelson, Edward. Gregor Mendel and the Roots of Genetics. New York: Oxford UP, 1999. Print.

Henig, Robin Marantz. The Monk in the Garden: The Lost and Found Genius of Gregor Mendel, the Father of Genetics. Boston: Houghton, 2000. Print.

Iltis, Hugo. Life of Mendel. Translated by Eden Paul and Cedar Paul. 1932. Reprint. New York: Hafner, 1966. Print.

Klug, William S., et al. Essentials of Genetics. Boston: Pearson, 2013. Print.

Korf, Bruce R., and Mira B. Irons. Human Genetics and Genomics. Chicester: Wiler-Blackwell, 2013. Print.

Krimsky, Sheldon, and Jeremy Gruber. Genetic Explanations: Sense and Nonsense. Cambridge: Harvard UP, 2013. Print.

Mawer, Simon. Gregor Mendel: Planting the Seeds of Genetics. New York: Abrams, in association with the Field Museum, Chicago, 2006. Print.

Olby, Robert. The Origins of Mendelism. 2d ed. Chicago: U of Chicago P, 1985. Print.

Orel, Vítezslav. Gregor Mendel: The First Geneticist. Trans. Stephen Finn. New York: Oxford UP, 1996. Print.

Tudge, Colin. In Mendel’s Footnotes: An Introduction to the Science and Technologies of Genes and Genetics from the Nineteenth Century to the Twenty-second. London: Cape, 2000. Print.

Wood, Roger J., and Vitezslav Orel. Genetic Prehistory in Selective Breeding: A Prelude to Mendel. New York: Oxford UP, 2001. Print.