It’s hard to believe that a Catholic priest in Austria with a fascination for pea plants could change the world we live in, but Gregor Mendel, who happened to be such a priest, did exactly that in 1865. Before Mendel’s work with pea plants (and for nearly thirty years after), Hippocrates’ pangenesis and Darwin’s natural selection handily explained why offspring had a tendency to resemble their parents. In 1900 Mendel’s work was rediscovered, and practically over night the field of genetics, whose practitioners came to call themselves “Mendelians,” was born. This sudden change in the way a phenomenon is explained is known to the scientific community as a paradigm shift. This paper, and its accompanying website, will explain the nature of Mendel’s research and the resultant paradigm shift in the context of Thomas Kuhn’s The Structure of Scientific Revolutions.
To some degree man has always had at least a basic knowledge of the existence of heredity. For centuries farmers have chosen the most high yielding animals for breeding, noticing that the traits of mothers and fathers seem to be passed to their offspring. The question has always been why. Why sometimes does one child resemble the father and another the mother? Why do some children look like their grandparents? The quizzical creatures called humans came up with answers which, for their time, did a wonderful job of explaining the phenomenon. Hippocrates, hailed as the founder of modern medical science, concocted a theory called ‘pangenesis,’ which accredited an offspring’s resemblance to its parents to tiny particles of every part of the body in the parents’ semen, which fused together to create said offspring as an amalgamation of the two (Orel ¶3). Aristotle’s theory differed from Hippocrates’ that in his model, the female’s menstrual blood latently contained every part of her body, and that the male’s semen would create qualitative changes in the resultant embryo (Orel ¶4). These theories served as the framework for which later ones would be defined and refined by great thinkers of their respective times.
After Descartes’ research on the principles of mechanics, a paradigm shift of sorts occurred in the natural science world. By the seventeenth century, microscopes became integral in the struggle to answer the “enigma of generations.” The answer would not come easily. Leeuwenhoek believed he saw Samentierchen(microscopic animals) upon inspection of sperm, while Malphigi was convinced he’d seen a preformed embryo in the egg. These findings created a conflict of two rival schools of preformation called the ovistic and the spermistic respectively (Orel ¶8).
The next great stride came with the work of French mathematician and astronomer, Pierre de Maupertuis. His studies of trait sharing between parents and offspring came in direct conflict with the accepted school of preformation, and would later be expounded upon by Charles Darwin in The Origin of Species. A problem common to all of these theories was their reliance on divine intervention. If one assumes the existence of pure science, god invariably must be left out of the equation. Darwin was the first to do this. He explained heredity as the random modification of traits from the parents tempered by natural selection. This didn’t explain the laws of inheritance, but it was a good explanation in a time when evolution and natural selection were more intriguing to the scientific community. The research was there in the 1860s, but it wouldn’t be rediscovered until 1900.
Though it didn’t immediately set the scientific world ablaze when it was written, Mendel’s Experiments with Plant Hybrids would get it’s day in the sun almost 35 years after it’s publication, and dramatically change the way people think about heredity. While strolling through the monastery one day, Mendel happened upon an unusual plant. In an effort to illustrate Lamarck’s idea on the influence of environment on plants, he placed it next to a normal plant of the same variety and observed the results. He found that no variation took place, and thus came to the conclusion that plants were in fact not influenced by their environment. Bested by curiosity, Mendel decided to experiment with plants some more. For the experiments he would become famous for, Mendel chose the pisum sativum, known to laymen as the garden pea, because it’s flowers are often different colors. He would breed the peas through self-pollination for several generations until pure breeding plants were created. He called these plants the P generation, with P standing for parental. He would then cross pollinate plants from the P generation with white flowers with plants with purple flowers. Their offspring would be the F1, or filial generation. These plants would be allowed to self pollinate, giving way to the F2 generation. After hundreds of trials, Mendel observed the following results: the F1 generation’s offspring displayed only one of the P generation’s traits, while the F2 generation displayed both parental traits in a three to one ratio. Mendel presented his research to the scientific community in 1865, but it wouldn’t be until 1900 that the relevance of his findings would be realized. From Mendel’s experiments, two laws called Mendel’s Laws were discovered. The law of segregation states that members of each pair of alleles of a gene separate when gametes are produced in meiosis. The law of independent assortment states that pairs of alleles separate independently of each other during gamete formation. For now, these laws more aptly explain the enigma of generations than divine intervention or simple natural selection.
Though Mendel’s experiments didn’t exactly revolutionize the way people think about pea plants, their importance to, nay, their creation of the fields of genetics and molecular biology is undeniable. We now know that many diseases are inherited, and by harnessing that knowledge we can calculate the probability that it will be passed on. Plants can be designed and manipulated in laboratories to exhibit desired traits, making farming more efficient and food more abundant. His work, while decidedly small in scope, truly did spark a revolution in the scientific community; one in which we all reap the benefits. Everything we know today about chromosomes, genes, alleles, meiosis, and DNA can be credited to the work of an Austrian monk who was curious about plants. Perhaps divine right did have something to do with it after all.