A Century of Genetics Daniel J. Fairbanks Abstract—In 1866, Gregor Mendel published his experiments on In the year 2000, the science of genetics celebrated its th heredity in the garden pea (Pisum sativum). The fundamental 100 anniversary. In the spring and early summer of 1900, principles of inheritance derived from his work apply to nearly all three botanists, Hugo de Vries, Carl Correns, and Erich von eukaryotic species and are now known as Mendelian principles. Tschermack, reported their simultaneous and independent Since 1900, Mendel has been recognized as the founder of genetics. rediscovery of Mendel’s principle of segregation, an event In 1900, three botanists, Carl Correns, Hugo De Vries, and Erich that sparked the rapid establishment of genetics as a new Tschermack von Seysenegg, had independently completed experi- and important science. This paper highlights just a few of ments that were similar to Mendel’s, although much less extensive. the major events that have made genetics one of the most When searching the literature, all three encountered Mendel’s powerful and rapidly progressing sciences. paper and realized that he had described the principles of inherit- Although genetics entered mainstream science rather ance and the experimental data to confirm them 34 years earlier. suddenly in 1900, it traces its origin to the mid-1800s with With their rediscovery, the science of genetics was born in 1900 the experiments of Gregor Mendel. In 1843, Mendel entered along with the 20th century. William Bateson coined the term the St. Thomas monastery in the city of Brno, now in the “genetics” and was the person most responsible for establishing the Czech Republic, and was soon appointed as a seventh-grade new science in the first decade of the 20th century. Thomas Hunt science teacher. He failed his teacher certification examina- Morgan and his students established the chromosomal basis of tion, an event that prompted the abbot of the monastery to heredity beginning in 1910. During the 1920s and 1930s, classical send him to the University of Vienna. While at the Univer- genetics was established, and eugenics became very popular among sity, Mendel was enamored with the teachings of his botany the more educated and wealthy members of society. Laws mandat- professor, Dr. Franz Unger, who, even though Darwin’s ing sterilization of perceived unfit people were passed and carried Origin of Species had not yet been published, focused his out. By the late 1930s, Nicolai Vavilov had published his theory of teachings on the evolution of species over long periods of centers of origin for cultivated plants and established a gene bank geological time. During the time that Mendel was a student, for his collections in Leningrad. During the siege of Leningrad in Unger wrote an article in which he wrote this remarkable 1941–1942, nine of his coworkers chose to die of starvation rather passage that foreshadowed Mendel’s work: “Who can deny than sacrifice the seeds and tubers that Vavilov had collected. In the that new combinations arise out of this permutation of meantime, Vavilov was imprisoned for his opposition to Lysenko- vegetation, always reducible to certain law-combinations, ism. He soon died in prison of maltreatment. In the mid-1940s, which emancipate themselves from the preceding character- George Beadle and Edward Tatum discovered the relationship istics of the species and appear as a new species” (Orel 1996). between genes and enzymes, and Oswald Avery and his coworkers Unger’s teachings infuriated the local clergy who attempted discovered that DNA is the genetic material of a bacterial species. to have him dismissed. Among the most vocal was Dr. Sebastian In the early 1950s, Alfred Hershey and Martha Chase discovered Brunner who wrote of Unger as “a man who openly denied the that DNA is the genetic material of a bacteriophage. Shortly after Creation and the Creator” and as one of the “professors at so- this time, in 1953, James Watson and Francis Crick determined the called Catholic Universities [who] deliver lectures on really structure of DNA based on evidence collected in several laborato- beastly theories for years on end” (Olby 1985). Ironically ries. Cracking the genetic code became one of the next priorities, a Mendel was a devout member of the clergy, but he appears to task that was completed in the 1960s. During the 1970s, recombi- have sided with Unger. Mendel wrote of his own work as “the nant DNA was made, an event that led to molecular applications in only right way by which we can finally reach the solution of a genetics and genetic engineering. The first genetically engineered question the importance of which cannot be overestimated in pharmaceuticals soon followed. The 1970s and 1980s saw the connection with the history of the evolution of organic forms” development of efficient methods for DNA sequencing, which ulti- (Stern and Sherwood 1966). mately led to whole genome sequencing. The first bacterial genome In 1852, Mendel began experiments with pea hybrids that was sequenced in 1995, the Brewer’s yeast genome in 1996, and the would last for 8 years. From the scientific literature that he Drosophila, Arabidopsis, and human genomes in 2000. Fittingly, had read extensively he already knew of the concept of the sequencing of the human genome, one of the greatest accom- dominance in peas for four of the seven traits he studied. His plishments in genetics, came at the 100th birthday of the science of contribution was his development of a mathematical model genetics. 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 A A a a ecosystem genetics and biodiversity: proceedings; 2000 June 13–15; Provo, +++ UT. Proc. RMRS-P-21. Ogden, UT: U.S. Department of Agriculture, Forest A a A a Service, Rocky Mountain Research Station. Daniel J. Fairbanks is a Professor in the Department of Botany and Range where the letters in the numerator represent the alleles Science, Brigham Young University, Provo, UT 84602. contributed by the male parent and those in the denominator 42 USDA Forest Service Proceedings RMRS-P-21. 2001 A Century of Genetics Fairbanks as the alleles contributed by the female parent. Because the theory of inheritance espoused by Bateson and the chromo- large A is dominant, three of the four combinations produce somal theory of inheritance promoted by Edmund Wilson, the dominant phenotype and one the recessive. He tested also at Columbia. He opted instead for de Vries’ mutation this model by allowing the F2 plants to self fertilize to theory (which differs substantially from our current under- 1 produce F3 offspring. He found that ⁄3 of the F2 plants with standing of mutation). Complaining of the intellectual cli- the dominant phenotype produced offspring with only the mate at Columbia, Morgan wrote in 1905 that it was “an 2 dominant phenotype, and that ⁄3 produced offspring with atmosphere saturated with chromosomic acid” (Allen 1978). both dominant and recessive phenotypes, precisely as his He completely reversed his views, however, within 5 years model predicted. as he brought Mendelian and chromosomal theories of He then turned his attention to dihybrid and trihybrid inheritance together as a single theory. In 1910 he observed experiments to ascertain whether the inheritance of one a white-eyed fruit fly that was to change the direction of his trait influences the inheritance of another. He discovered career and the science of genetics. He discovered that the that all seven traits in various combinations of twos and white-eye phenotype was associated with inheritance of the threes were inherited independently of one another. On X chromosome. these two discoveries, the mathematical segregation of dif- At first, he was reluctant to conclude that the genes were fering elements and the independent inheritance of traits, actually a part of the chromosome. However, two discoveries are based the two laws of inheritance attributed to Mendel: by his students, Alfred Sturtevant and Calvin Bridges, made the law of segregation and the law of independent assortment. it clear that their so-called sex-linked genes must be a Mendel presented his paper in 1865 and had it published physical part of the X chromosome. Sturtevant, as an under- the following year (Mendel 1866). For the next 34 years, no graduate student in 1911, had a flash of genius when he one, including Mendel, recognized that the laws he discov- realized that genes might be located in a linear fashion on ered applied almost universally to plants, animals, and the chromosome. He gathered up the notebooks with the humans. His paper was not completely forgotten; it was cited data from several of their experiments, and, in his words, “I at least 15 times before 1900. However, it is clear from these went home and spent most of the night (to the neglect of my citations that no one recognized its most important points. undergraduate homework) in producing the first chromo- Mendel sent reprints with cover letters to several botanists, some map” (Sturtevant 1965). He placed five genes on a including Franz Unger, his former professor, but the only linear map and calculated the distances between them one to respond was Karl von Nägeli, who carried on corre- based on the frequencies of crossing over. Bridges discovered spondence with Mendel over a period of 7 years. Mendel had in 1913 that unusual cases of inheritance of mutant sex- initiated studies with the hawkweed, and Nägeli encour- linked alleles were associated with nondisjunction of chro- aged Mendel to continue with these species.