I have deeply regretted that I did not proceed far enough at least to understand something of the great leading principles of mathematics, for men thus endowed seem to have an extra sense.Charles Darwin
Darwin’s theory of evolution by natural selection depends heavily on the notion of inheritance, i.e., traits being passed on from generation to generation. Birds lay eggs from which little birds hatch, and humans give birth to little humans. Even individual traits like eye or skin color are inherited from your parents. However, the molecular mechanisms of DNA and genes underlying those traits, and their inheritance, was not yet discovered in Darwin’s time.
Interestingly, though, the foundation of what is now known as the theory of genetics was actually already laid down in Darwin’s time, but completely unbeknown to him and other contemporary biologists. The single person responsible for this work was a humble Augustinian monk by the name of Mendel.
A humble monk
Johann Mendel was born on 20 July 1822 in Heinzendorf, Silesia, then part of the Austrian Empire (currently Hynčice, Czech Republic). His parents were German peasants, and Johann was expected to follow his father’s footsteps. However, already at a young age he showed an interest and talent in learning, and was able to attend several schools during his youth.
In 1843, at the age of 21, Johann was admitted to the Augustinian Abbey in Old Brno, under Abbot Napp. On that occasion he adopted the name Gregor, and was subsequently able to continue his studies. Thanks to financial support and a personal recommendation from Abbot Napp, Mendel was even allowed to attend the University of Vienna during 1851-1853, where he studied mathematics, physics, and natural sciences. These years of university studies heavily influenced his own later scientific work.
After finishing his studies, Mendel returned to the abbey and also became a teacher at a secondary school in Brno. It was during this time, between 1854 and 1864, that Mendel performed his now-famous experiments on crossbreeding pea plants, which led him to formulate his “laws of inheritance”.
The laws of inheritance
Mendel observed that the flowers of his pea plants were always either white or purple, but never any color in between. We now know that there is such a thing as a gene (a piece of DNA) that determines the color of these flowers. In fact, as in most sexually reproducing organisms (including humans), there actually is a pair of duplicate genes, one inherited from the mother and the other one from the father.
Each of these genes can exist in several variants, called alleles. For example, one allele might result in purple flowers, while another allele (of the same gene) might result in white flowers. What is now also known is that often one of these alleles is “dominant” over the other. For example, if the pair of genes in a particular pea plant contains one “purple” allele and one “white” allele, then its flowers will always be purple. In other words, the purple allele is dominant over the white allele, and only if both genes happen to be in the white allele will the plant’s flowers be white.
So, back to Mendel and his pea plants. Mendel started out with some plants from a lineage that had only purple flowers for several generations, and some other plants from a lineage that had only white flowers for several generations. This insured that all plants from the first (purple) group had both genes in the purple allele, while all plants from the second (white) group had both genes in the white allele (although, of course, Mendel could not have known this). This situation is illustrated in the image below, in the first row indicated with P (for “parent generation”). The two genes and their respective alleles are represented by the horizontal bars within each flower.
Next, Mendel crossbred between these two groups. In other words, one parent always came from the purple group while the other parent always came from the white group. Clearly this means that the plants in the first offspring generation all have one purple and one white allele (which, again, Mendel could not have known). As a consequence, all flowers of all these offspring plants were purple. This is illustrated in the second row in the above image, indicated with F1.
But now comes the interesting part. Since Mendel could not distinguish the plants in the offspring generation anymore (after all, they all had purple flowers), he randomly crossbred between plants from this offspring generation (F1) to create yet another generation (i.e., “grandchildren” of the parent generation). To his surprise, pea plants with white flowers reappeared, but in a ratio of one white-flower plant to three purple-flower plants. This is illustrated in the third row of the image above, indicated with F2.
Obviously Mendel (like Darwin) did not know anything about DNA or genes. He simply used the term “units of inheritance”. But he correctly deduced that the only way to explain his experimental observations statistically, was to assume that: (1) there is a pair of genes responsible for a given trait, with one gene inherited from one parent and the other one from the other parent, (2) genes can have different alleles that result in different trait variants (such as purple vs. white flowers), and (3) one allele can be dominant over others (e.g., purple over white).
Mendel repeated his experiments many times, also examining other traits such as pod shape or plant height. In early 1865, Mendel presented two lectures about his experiments and mathematical results to the Natural History Society in Brno. A year later, his classic paper Versuche über Pflanzen-Hybriden (Experiments in Plant Hybridization) was published in the proceedings of that society. But despite these lectures and publication, his results remained largely uncomprehended and ignored.
On 30 March 1868, Mendel was elected Abbot of the Augustinian abbey, after his predecessor and mentor Napp had died the year before. Next to this demanding position, he was (or later became) also a member of the Meteorology Society, Pomology Society, Natural History Society, and Zoology and Botanic Society, among others. Besides his well-known experiments in plant crossbreeding, he also conducted experiments in (and published about) beekeeping, fruit tree cultivation, and meteorology.
Gregor Johann Mendel died on 6 January 1884, at the age of 62, from a kidney infection. He was buried in the central cemetery of Brno.
It was not until the spring of 1900 that Mendel’s work on crossbreeding and his “laws of inheritance” were rediscovered, when three scientists (Hugo de Vries, Carl Correns, and Erich von Tschermak) independently published about them. But this time Mendel’s results were correctly understood and recognized, and eventually became the basis of the modern (and mathematical) theory of genetics.
The Mendel museum in Brno presents an overview of Mendel’s life and work, in the original abbey where he spent most of his time and where he conducted his experiments. The museum was founded in 2007 in an effort to promote the legacy of this “humble genius”.
As it turns out, Mendel was actually somewhat lucky that he chose pea plants to experiment with. Several of the traits he studied (such as flower color, pod shape, and plant height) are indeed largely determined by a single pair of duplicate genes in these plants. However, in general the situation is much more complicated, and the “single gene for a single trait” picture is too simplistic. In fact, most genes influence multiple traits, and most traits are influenced by multiple genes.
Moreover, genes can even influence each other. One gene could give rise to a molecule that can bind to another gene, thereby either promoting or supressing this other gene’s expression. And finally, also the idea that genes are the sole entities responsible for inheritance is being challenged. Even so, Mendel’s work is still recognized today as a major achievement, especially given the status of scientific knowledge at the time, and he is rightfully remembered as the father of genetics.
All photographs and flower image © Wim Hordijk
This article is part of the Beauty and Science of Nature series.