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05 January 2016

Mendel, a Paradigmatic Scientist

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2016 marks the 150th anniversary of the publication of Mendel‘s work. As the date draws near, some of the criticisms Mendel received throughout the 20th century have been revived. For example, he is accused of having “embellished” his experimental results, and there have even been suggestions that he falsified them. However, what is noteworthy about Mendel is that he was one of the first biologists –if he can be called that– to use the modern experimental scientific method in a paradigmatic form.

Phase 1: The experiments

To resolve the complex problem of inheritance in hybrid plants, Mendel followed the first “recipe” for the scientific method: simplify to the maximum. Specifically, he conducted seven hybridization experiments between pure varieties of peas that only differed in one aspect of their morphology. That is, he made what are known as monohybrid crosses. He crossed one variety with smooth seeds with another with rough seeds; a second experiment involved one variety with yellow seeds and another with green seeds; and five more experiments related to plant size, flower color and so on. He continued these experiments for at least two generations and did not merely describe the results qualitatively (as had generally been done before), but made a quantitative count of the variants appearing in each generation.

He was thus able to observe that in the first generation the hybrids resembled one of the parental lines and in the second generation (obtained from the hybrids of the first), approximately 75% of the plants had the same traits as the first generation (smooth or yellow seeds for example), while the remaining 25% had the standard traits of the variation that had apparently been lost (rough or green seeds).

Example of a dihybrid cross with two varieties of peas distinguished by seed color and shape /Wikimedia

He also crossed pure lines of peas that differed in two aspects of their morphology (dihybrid crosses). For example, he crossed a pure line with smooth yellow seeds with another with rough green seeds, and observed in the first generation that –in agreement with his previous experiments– all the hybrid plants had smooth yellow seeds, and that in the second generation obtained from them, the four possible combinations appeared in the following proportions: 9 smooth and yellow, 3 smooth and green, 3 rough and yellow and 1 rough and green.

Phase 2: The hypothesis (of traits-genes)

Armed with these results, Mendel continued to the second phase of the experimental scientific method: he issued a hypothesis to explain these results. Specifically, he proposed that the variations presented by the different lines of peas are caused by the different “traits” or “factors” (which would subsequently become known as genes) that exist in plants. For each of these traits there would be two alternatives (alleles). So within the trait for the seed type there would be one alternative for smooth seed and another alternative for rough seed.

Therefore when one pure line is hybridized with another –for example, one with smooth seeds with another with rough seeds– this would produce plants with one trait for smooth seeds (inherited through the gametes of that pure line), and another for rough seeds (inherited from the pure rough line). These first-generation hybrid plants only resemble one of the pure lines. In this case, they have only smooth seeds, as within the two alternatives of each trait or gene, one –in this case the smooth alternative– would be dominant over the other (the rough one), which would be recessive.

But the recessive alternative would not be in any way lost or “contaminated” in the first-generation hybrids. In a cross between a pure line with smooth seeds and another with rough seeds, the hybrid plants would produce some gametes with the smooth seed alternative and another with the trait for the rough alternative. When they are randomly crossed, these gametes produce some second-generation plants with smooth seeds and others with rough seeds.

Phase 3: Falsification

In the third place, Mendel set out to prove his hypothesis of the traits underlying the variation in plants. Specifically, he thought that if the first generation hybrids of the crosses between pure lines had both alternatives –one dominant and another recessive– but without the recessive alternative being lost or mixed, when these hybrids were crossed with plants from the pure line carrying a recessive alternative, it would give rise to plants that would be half the dominant type and half the recessive type.

This was in fact what he observed when he crossed –for example– first-generation hybrids from the cross between the pure smooth and rough lines with the pure line of rough seeds (he conducted this experiment with hybrid plants from dihybrid crosses).

Phase 4: Generalizing the hypothesis

Finally, and within a totally modern scientific line of thought that pursues the exploration of the universality of scientific theories, Mendel attempted to prove that his hypothesis was also true in other plants, and even in animals. Indeed he obtained similar results to peas with other legumes such as Phaseolus. But his next steps were unsuccessful. He chose plants (Hieracium, a plant in the daisy family commonly known as hawkweed) and animals (bees) which we now know do not have normal systems of sexual reproduction.

This may be why his brilliant hypothesis on traits or genes was rejected when it was first published in 1856. Another 34 years had to elapse before –in a more favorable scientific context, once the chromosome theory of inheritance had been established –his experiments were rediscovered and his hypothesis was confirmed and extended, thus laying the foundations for the birth of genetics.

All this points to the figure of Mendel as a great scientist –perhaps the first modern scientist in the field of biology–, quite aside from any possible “embellishment” he may have made (and which has never been proved) to the specific numbers in his experiments.

Manuel Ruiz Rejón

Granada University, Autonomous University of Madrid

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