People have always understood that characteristics of parents can be passed to offspring. But how this was actually done was a mystery. One theory — the now defunct Blending Inheritance theory — held that progeny inherited an average, or blending, of both parents’ features. Yet this didn’t adequately explain many observable results. For example, why does red hair or blue eyes skip generations? Why are most haemophiliacs male, and why is the disease as debilitating in them as in their ancestors, rather than lessening with each generation? Where did a solitary black spot on an otherwise pure white Merino sheep, with generations of pure white ancestors behind it, come from?
The breakthrough in understanding began in 1858, when Austrian monk Gregor Mendel began experiments breeding garden (edible) peas (Pisum sativum). He published his findings in 1866, but the significance of his work went unrecognised until rediscovered in 1900, sixteen years after his death in 1884. His paper described what came to be known as Mendelian Inheritance, and today he is universally acknowledged as the father of modern genetics.
What were his insights that had eluded everyone else? Mendel, with his (unusual for the time) background in both biology and mathematics, had deduced from his well-planned meticulous work and careful observations that inheritable characteristics aren’t blended, but are instead comprised of heredity units. He called these units “factors”, but you’d know them as genes.
Mendel set out to study seven characteristics of peas: seed shape, seed colour, seed coat colour, pod shape, pod colour, flower position, and stem length.
He began his experiments by first creating pure-breeding plants for each of those seven traits: plants that bred true generation after generation. Any plant that didn’t breed true was removed. This first step alone took a few years, as pea plants are seasonal and produce one generation a year.
At the end of this first step he had the following purebred strains:
purebred round and purebred wrinkled
purebred yellow and purebred green
Seed coat colour:
purebred coloured and purebred white
purebred inflated and purebred constricted
purebred green and purebred yellow
purebred axial (along the stem) and purebred terminal (at end of stem)
purebred long and purebred short
With these lines established, step two was to cross the purebred strains for each characteristic. His approach was very methodical, in that he not only focused on only one or two characteristics at a time, but he also undertook many identical matings so as to produce a large, statistically more meaningful data set.
With, for example, purebred round seed lines and purebred wrinkled seed lines established, only then did he cross round seed strains with wrinkled seed strains. Or green pod strains with yellow pod strains. If characteristics were indeed “blended”, the resulting offspring should have exhibited a seed shape partly round and partly wrinkled, or some indeterminate yellow-green pod colour. But they did not: every single seed was as round as the purebred round seed lines, and pods were as green their green pod parents. Mendel observed similar behaviour for the other characteristics studied (eg yellow seed strains crossed with green seed strains produced all yellow seeds), and called a character that was the only one to appear in a first cross generation dominant.
Step three was to cross the first cross generation with each other. From these results he noted that, for example, the round seed (dominant) phenotype appeared in roughly three quarters of the second cross generation, with the remaining quarter producing wrinkled seed. Mendel called the wrinkled phenotype recessive, as it had receded from a generation, or skipped a generation as we’d say today.
Mendel went further still, and crossed the second cross generation with each other to produce a third cross generation. He had found from his first crosses that, for example, short-stemmed plants were recessive to long-stemmed plants, and that yellow pods were recessive to green pods. In the third cross generations, plants with recessive phenotypes produced only plants with those same recessive phenotypes. The offspring of short-stemmed plants were always short-stemmed, and plants with yellow pods only ever produced yellow pods.
Mendel wrote his results in a lengthy paper called Experiments in Plant Hybridization, which was published in 1865 in Verhandlungen des naturforschenden Vereines in Brünn, the official journal of the Natural History Society in Brno (Brünn). You can read it here, and see where he drew on his mathematical background, if of interest. I will step through the results of Mendel’s crossings in more detail in the next post, hopefully in a way more clear and understandable!