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The birth of genetics

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In 1854, a monk with remarkable intellectual curiosity named Gregor Mendel began growing peas in the greenhouse of St. Thomas Monastery in what is now the Czech Republic. In 1865, Mendel had made a series of observations that ultimately changed the fundamental understanding of how traits are inherited by living organisms. He presented his main findings on what he called “certain laws of inheritance” in 1865, and thereafter his studies were largely ignored by the scientific community for the next 35 years. Why?

Steve Caplan

Steve Caplan’s new book, “Today’s Curiosity Is Tomorrow’s Cure: The Case for Basic Biomedical Research,” was published in November by CRC Press.

Consistent with the idea that the timing of scientific discoveries must be appropriate for those discoveries to be properly appreciated, Mendel’s laws of heredity preceded the wide-scale acceptance of Darwin’s theory of evolution. Indeed, it was not until the early 1900s that widespread knowledge of Mendel’s laws became widespread, and it took even longer for his ideas to gain full acceptance.

At the turn of the 20th century, three new papers were published, each rediscovering Mendel’s laws of inheritance. In the prologue to his 2000 book “The Monk in the Garden”, Robin Marantz Henig noted: “The explanation generally given for this curious turn of events is that the world was not ready for Mendel’s laws in 1865 , and that in 1900 it was.”

Whether Mendel was a genius or simply a seasoned plant breeder who happened to be in the right place at the right time – a question debated by some scholars and historians – is, for the most part, irrelevant. Importantly, he was clearly among the first researchers to observe and publish findings that showed traits are inherited, and he described predictable and strict mathematical rules that govern trait passing. individuals from parent to offspring as discrete particulate units that exist in pairs in all individuals.

A fierce battle

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In addition to his work in genetics, Gregor Mendel (1822-1884) was a meteorologist,
a mathematician and an Augustinian friar.

Perhaps one of the main reasons Mendel’s work remained relatively obscure until the early 1900s was that the entity known as a gene remained so nebulous in the absence of a molecular understanding of what this entity entailed. The lack of a solid understanding of how genetic material is passed from generation to generation – or more specifically, what makes up that genetic material – has made it extremely difficult for contemporary scientists to accept Mendelian genetics.

Indeed, great debate erupted following the publication of Cambridge scholar William Bateson’s 1894 book “Materials for the Study of Variation: Treated with Especial Regard to Discontinuity in the Origin of Species”, in which he described 886 examples of discontinuous variation in heredity. Bateson, who after reading Mendel’s studies of pea genetics years earlier would have felt he had been co-opted by Mendel, became one of the biggest advocates of Mendelian genetics. The idea that genetic traits could skip generations because they were recessive – meaning the trait is only passed on to offspring if it is inherited from both parents – was somewhat revolutionary in that he apparently opposed some of the new ideas that had emerged. modern statistics.

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William Bateson (1961-1926) was the first to use the term genetics to describe the study of heredity.

During Mendel’s time in the late 1800s, Francis Galton discovered the statistical concept of regression to the mean; Simply put, if a sample point is extreme when observing random variables, additional points observed in the future will be more likely to be closer to the mean and less likely to be outliers. Galton calculated that Darwin’s evolution must occur in larger, discontinuous steps rather than small incremental steps to prevent regression to the mean.

Scientists who favored the idea that evolution was a regular, continuous event were known as biometricians. Bateson and those who supported the Mendelian model were convinced that only discontinuity could explain the inheritance of many traits, and so an uphill battle was fought in a series of letters and counter-letters published in the journal Nature.

A chromosome theory

In the early 1900s, however, more evidence in support of Mendel’s ideas came from a different direction. In particular, two scientists, Walter Sutton and Theodor Boveri, contributed greatly to this endeavor.

Sutton did important research under the tutelage of the famous Edmund B. Wilson at Columbia University in New York, publishing “The Chromosomes in Heredity” in 1903 with the conclusion that the chromosomes (which were now visible under the microscope by new cytological techniques) carry Mendel’s disease. hereditary material.

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Walter Sutton (1862-1915) and Theodor Boveri (1877-1916) developed the theory
that Mendelian laws of inheritance could be applied to chromosomes.

The German cytologist, cell biologist and zoologist Boveri had a remarkable career in which he made great discoveries, often drawing on his zoological experience to exploit interesting systems to study. For example, he took advantage of fertilized sea urchin eggs and later of the nematode Ascaris megalocephala, a horse gut parasite that later infected him in life and possibly caused his death. Boveri studied the centrosome or what he called the “centrosome” and documented its importance for cell division. He also discovered that the centrosome itself divides and organizes the surrounding cytoplasm in such a way that spindle fibers radiate from it and come into contact with the chromosomes. Premonitory Boveri also published a lesser known study in which he proposed that aberrant chromosomes might even be responsible for the generation of cancers in his 1914 book “Concerning the Origin of Malignant Tumors”.

The work of these two scientists greatly advanced the idea of ​​Mendelian genetics and led to what is known as the Boveri-Sutton chromosome theory.

A hereditary twist

Further support for Mendelian genetics came from the work of Nettie Stevens, who was to be an extraordinarily brilliant scientist to overcome the endemic misogyny of her time. Stevens was a geneticist trained in the laboratory of Thomas Hunt Morgan, famous fly geneticist and Nobel laureate for his contributions to chromosomes and genetics at Bryn Mawr, where Edmund Wilson was also a faculty member. During his doctoral studies, Stevens received a scholarship to travel to Germany and train with Boveri before completing his doctorate.

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In her studies of the sperm produced by male mealworms, Nettie Stevens (1861-1912) discovered the sex chromosomes.

Back in the United States, Stevens worked on mealworms (Tenebrio molitor) and observed that the somatic cells of female mealworms contained 20 large chromosomes, while those of the male mealworm had 19 large chromosomes and one. small. She also discovered that exactly half of the male sperm contained nine large chromosomes and one small chromosome, while the other half had 10 large chromosomes. His conclusion was that sperm-fertilized eggs with the 10 large chromosomes gave rise to female mealworms and therefore the small chromosome dictated the generation of male mealworms. His findings were later validated by Wilson when he examined the number of chromosomes in many species of hemipteran insects, thus supporting Mendelian genetics and bringing a new twist to the mechanisms of heredity.

Supported by the discoveries of great cytologists and cell biologists, Mendelian genetics prevailed. However, Mendelian genetics was eventually combined with the mathematical advances of Galton to produce new statistical methods and give rise to the modern field of genetics. All that was missing was an understanding of what constituted the particulate hereditary element in cells and chromosomes that allowed traits to pass from one generation to the next. Thus, the field of genetics was born out of careful observation and experimentation, but without understanding the role of DNA in this process, scientists were still looking at the tip of the iceberg.

This article is an excerpt from Steve Caplan’s new book, “Today’s Curiosity Is Tomorrow’s Medicine: The Case for Basic Biomedical Research,” published by CRC Press. It was edited for ASBMB Today.

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Gregor Mendel studied these monogenic traits in his pea experiments.