How Gregor Mendel discovered the existence of genes

If you have ever noticed that children resemble their parents, then you have made a fundamental observation about genetics. Genetics concerns itself largely with heritability: the passage of traits from parents to offspring. Early geneticists wondered how characteristics are passed from one generation to the next and ultimately discovered a unifying thread linking all biological life here.

The first scientist to make serious inroads into these big questions was Gregor Mendel, a monk living in Brno in the present-day Czech Republic in the mid-19th century. The word "gene" hadn't been invented and it was little more than a hazy concept with no physical basis.

The prevailing view at the time was that biological inheritance was a kind of blending process, a bit like mixing paint. This didn't seem too bad a hypothesis at first. Each of us obviously has characteristics from both parents.

However, when you mix paint colours you end up with a muddy brown with no way to ever return to the individual colours. Biological inheritance through blending leaves no way for brown-haired parents to sometimes have blond-haired children, and no way to get faster, taller, better or fitter. In fact, evolution just wouldn’t work.

Mendel asked a beautifully simple question: what do you get if you cross a true-bred tall plant with a true-bred short plant? And then what happens in the next generation? And so on.

If blending inheritance is fact, then the offspring of one tall and one short plant should have a middle height. Instead, Mendel found that all the resulting plants were tall. Height was not blended. Mendel repeated this on thousands of pea plants and the results were always the same. The short trait didn’t blend in; it appeared to be completely lost, at least at first glance.

Mendel used this new generation of tall plants as the parents for a new cross. When these plants were crossed, the offspring were mostly tall plants like their parents, but around a quarter of them were short.

The “short” trait, which seemed to have disappeared in the first cross, reappeared again. How could this be possible? The explanation he proposed was that the traits were carried as some sort of particle: a discrete, unmixing, physical entity. Mendel had discovered genes.

Dominant gene

In Mendel’s experiment the first generation plants had all inherited the gene to make a short plant, but its effect was masked by the dominant tall gene. In the subsequent generations made from this mixed stock, the plants might inherit only tall genes, or both tall and short genes, or only short genes. It is only in the last case that the plant is actually short.

Mendel had started to understand the relationship between the combination of genes (known as the genotype) and the physical manifestation of those genes (known as the phenotype).

Had Mendel chosen a different study organism, or different traits to measure, then it is unlikely he would have been so successful. Human height is highly heritable (in that children strongly resemble their parents) but rather than a simple system of one gene with either tall or short variants, our height is controlled by a multitude of genes. So far, more than 400 genes have been discovered that act in combination to influence human height, but they explain only about 20 per cent of the heritable differences; many more genes remain undiscovered.

The traits that Mendel chose to measure in plants had relatively simple genetics in terms of the one-to-one relationship of genes to outward physical characters.

It was this simplicity that made it possible for Mendel to interpret his experiments in a meaningful way and discover genes.

These cases of straightforward genetics are known today as "Mendelian traits". Examples include the gene for Huntington's disease and the gene for cystic fibrosis, but not eye colour or hair colour, which are more complicated.

However, eye colour and hair colour are often erroneously used as “textbook examples” of Mendelian traits, because to a first approximation they seem to be inherited simply.

Mendels’s fortune (or wisdom) in choosing simple traits for analysis meant he was able to employ relatively simple logic to “see” genes for the first time, earning him the moniker of “father of genetics”.

Aoife McLysaght is a professor in genetics in Trinity College Dublin where she leads a research group focusing on identifying and interpreting the evolutionary patterns in animal genomes