“Like father, like son”. We have all heard this saying and agreed with it, since children usually resemble their parents in several ways. But if there is continuity between generations, biological processes must account for it. How does heredity work? Why do parents not produce children identical to them? What happens to the hereditary information in the process? Where is it located and how is it organized? Can we make sense of how its transmission occurs? In this section, we follow the path that has led to the formulation of what we now call the gene, the unit of hereditary information.
1 | Pangenesis: Darwin’s attempt to explain heredity In the mid-1860s, Charles Darwin was one of the most controversial naturalists in Britain. Critics often pointed out that the theory did not explain how individuals of the same species were able to vary randomly, nor how could this variation be transmitted between generations, and Darwin proposed a theory of heredity he termed pangenesis to explain these phenomena. Contrary to his wishes, however, it did not have a good reception. His cousin Francis Galton was one of the few people with scientific interests who took pangenesis seriously, but the experiments he performed to test it only yielded negative results.
2 | Hugo de Vries and a new twist to pangenesis The Dutch botanist Hugo de Vries was another of the few researchers who took Darwin’s theory of pangenesis seriously. De Vries further elaborated Darwin’s theory of pangenesis and published his own version of it in 1889, Intracellular Pangenesis. The theory was significantly different from Darwin’s own because de Vries had incorporated new discoveries on cytology. Like Darwin's theory of heredity, de Vries’s also had a poor reception. During the 1890s, de Vries made crosses between closely related plants with evident contrasting characters and statistically analyzed the results to find evidence for his theory.
3 | The inner lives of cells In the nineteenth century, important improvements in microscopy revolutionized the study of cells. The combined use of new technological advances and methods of preparing biological material led to several discoveries in the 1870s and 1880s, such as the identification of chromosomes and the phases of cell division (mitosis). The German researcher August Weismann shaped his own ideas on the transmission of hereditary material taking into account these new findings, and he influenced de Vries's views.
4 | The forgotten work of Gregor Mendel Early in 1900, one of de Vries's colleagues sent him an old paper he thought relevant to his research. Its author was Gregor Mendel, an Austrian friar essentially unknown in the scientific community. Upon reading this paper, de Vries noticed that Mendel had reported more than thirty years ago the same findings he had verified! In spite of his efforts to publicize his results, no one understood the significance of Mendel's discovery because he used a mathematical approach that was quite unusual for his time and presented his findings in a way that was not easy to follow. When he died in 1884, Mendel was only regarded as a good teacher and an amateur naturalist.
5 | Mendel’s experiments From around 1854 to 1863, Mendel crossed dozens of varieties of pea plants and discovered important regularities in the transmission of hereditary characters. He was not interested in heredity, but rather in understanding whether species changed or not. He was influenced by his interests in physics and mathematics, and analyzed his results using statistics. Mendel proposed that some forms of these hereditary units (he termed them "factors") dominated over others (yellow vs. green color in peas, for example), and that each factor (pea color vs. pea shape, for example) was independently transmitted to the offspring.
6 | The rediscovery of Mendel’s findings When de Vries read Mendel’s paper in 1900, he understood its importance and similarities with his own work. He then rapidly prepared an article to assert his priority on the rediscovery of Mendel’s findings. De Vries sent one of the versions of his paper to some colleagues who would likely be interested, such as the German botanist Carl Correns. When Correns received de Vries’s letter on April 1900, he could not believe – it contained the research he himself had been conducting in the last years! Determined to be recognized as co-rediscoverer of Mendel’s laws, Correns put rapidly his results together, wrote an article, and published it. The Austrian plant breeder Erich von Tschermak also received another copy of de Vries’s paper in 1900, and noticed that he too had obtained similar results. In the same year, Tschermak published his studies so that he could also obtain recognition for his work. The rediscovery of Mendel’s laws was an important turning point for the scientific study of heredity.
7 | A material basis for Mendelian factors Mendel had vaguely talked about “factors”, but were they mere abstractions with no physical counterpart or had they a material existence like water molecules? The opinions diverged, but a few years after Mendel’s rediscovery some cytologists, such as the German Theodor Boveri and the American Walter Sutton, argued they had found the probable source of Mendel’s factors. These researchers were influenced by experimental embryology, and performed cytological analyses that provided valuable insights into the mechanics of heredity, linking Mendel's abstract “factors” to concrete physical entities inside the cell nucleus, the chromosomes.
8 | A material basis for sex determination The search for the causes that determined whether an embryo would become male or female was one of the quests that interested various researchers from the 1880s onward. By the early 1900s, discussions on this topic were starting to change, due to new discoveries in cytology. Clarence McClung, an American cytologist who mentored Sutton, proposed in 1902 that a strange nuclear bodythat behaved like a chromosome determined the sex of the insects. The discoveries of Boveri and Sutton lent further support to McClung’s hypothesis, but it was the work of Nettie Stevens and E. B. Wilson, Sutton’s new supervisor at Columbia University, that presented more decisive evidence.
9 | The fruit fly as a new research organism Like other embryologists, the American researcher T. H. Morgan did not agree with the emphasis that some of his colleagues were ascribing to either the nucleus or the chromosomes in development, stressing instead the importance of the cytoplasm in the process. He tried to disprove his colleagues' ideas by experimenting with insects in the 1900s. He was also interested in understanding evolution, and around 1907 he turned to the fruit fly Drosophila melanogaster, to put to test some of de Vries's ideas on evolution.
10 | Morgan’s sex-linked characters At first, Morgan's studies with the fruit fly yielded no interesting results. But from January 1910 on, various types of mutant flies started to appear, with different wing shapes, body colors or wing sizes. By May 1910, Morgan found a remarkable mutant that had its eyes colored white, instead of the usual red. Although all these mutants did not seem to entirely follow Mendelian laws, he noticed the white mutants seemed to alwaysbe male. Further experiments showed that although the white mutation was not sex-specific, the eye color factor was in some way coupled to the X chromosome.
11 | From Mendelian crosses to chromosome mapping By the first months of 1911, Morgan had already too many mutant flies to analyze, and the various other lines of work he kept were progressively pushed to the sidelines. He analyzed mutants with a physiological model, but as the approach became impractical he was forced to find a different one. Following a suggestion from Alfred Sturtevant, one of his Ph.D. students, he decided they should try mapping Drosophila’s chromosomes 2 and 3 by calculating the crossover rate between pairs of factors.
12 | The first modern concept of the gene In 1915, Morgan and his most prominent students published a book that provided an updated picture of the features of Mendelian factors, later termed genes. Genes had at least one specific function, formed a hereditary unit, and were linearly arranged in chromosomes like beads on a necklace.
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