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How Well Do You Know Mendel?

History often has a strong sense of irony. A person praised in one generation can be the contempt of the following generation. History, conversely, can turn an obscure figure into a celebrity, an inspiration for the future generations. Johann Mendel’s life is an example of the latter, but with some twists. His work, for example, only became well-known about four decades after his death, something perhaps even he couldn’t have anticipated.  At the time of his death, Mendel had written more about meteorology than biology; yet, his fame came from his work in genetics. Every biologist knows him, though perhaps not as Johann Mendel, but as Gregor Mendel—a name he took up when he joined the Augustinian Friars. This essay is about his ingenuity and dedication to science, and to some extent, his life.


Thought Experiment With Cards
To fully appreciate his brilliance, let’s do a thought experiment.  Let’s suppose we have two decks of cards: pink and white. We take one card from each deck and put them on top of each other to make a stack, as shown in the image above. If we were doing this with our eyes closed, 50% of the time pink card would be on top and 50% of the times it would be the white card (same as flipping a coin). However, we do that many times (8 times shown in the image), and pink cards always come up on top! To investigate this weird occurrence further, we now take one card each from two adjacent stacks to make a second set of stacks (round II). After doing this many times, we find that in this second round, three times out of four, the top card is pink and one time it's white. In other words, the ratio of pink to white is 3:1. 

Now, how can we explain what is happening? And can we make any sense of what we observed if we didn’t know that each stack had two cards and didn’t know there was another card underneath the pink one? The scenario is somewhat similar to what Mendel found from his experiments, and the explanations he came up to explain his findings with gave rise to the field of genetics. 

Let’s take a look at one of the experiments here. Mendel wanted to find out how heritable traits passed onto the offspring from the parents and chose to work on pea plants to study this. Why did he choose pea plants? 

One of the reasons was that the bishop at his church did not like the idea that Mendel would study animal sex. Since Mendel had enough space at the church to grow his plants, it didn’t matter much. Moreover, the pea plants actually offered him some great benefits that helped his research. For example, the plants reproduced quickly, and this allowed Mendel to collect data from some 29,000 plants in less than a decade! With extreme care, he could protect these plants from randomly pollinating (by insects or air, for example), an important consideration when one is studying inheritance. Depending on the experimental setup, he could either allow the plants to self-pollinate or selectively cross-pollinate with other plants. Finally, those plants had seven distinct observable features like flower color, seed shape, and color that Mendel could use to understand inheritance. 

For now, I will focus on the flower color (the results he got from his experiments on the other six traits were similar). Mendel selected two purebred plants that produced two different colored flowers: pink and white. When he cross-pollinated these two plants, all the plants in the next generation—referred to as F1 or first filial generation—produced only pink flowers. When he let these plants self-pollinate or cross-pollinate with other plants from the F1 generation, the ratio of pink to white in the F2 generation was close to 3:1, similar to our thought experiment. 

From this data, Mendel correctly deduced that even though there were no white flowers in the F1 generation, the white trait still there, but it was just masked by the pink trait. He called the pink trait 'dominant' and the white trait 'recessive'. This led to the Law of Dominance, and the law simply states that a dominant trait will always be visible in the offspring because a recessive trait will be hidden or masked. 

When Mendel was doing these experiments in the 1850s, the general consensus among the biologists was that traits from both parents would blend in the offspring to give rise to mixed or in-between traits. If that were the case, one would predict that in the F1 generation, all the flowers would be less pinkish. But that was not what Mendel had observed. 

Even though Mendel didn’t know anything about DNA or genes, he reasoned that parents must pass down some heritable particles and called his substance ‘elementen”. But how many of these elements are there for a trait in a person? Since in sexual reproduction, two parents are involved, we can start with the assumption that we are dealing with two elements here. But does that explain Mendel’s data? 

Since we started with the thought experiment, let’s start with the cards here. As we saw, card stacks in round I comprised both pink and white cards. Things got more interesting in our second round though because we got a 3:1 pink to white ratio. There, we took one card each from round I stacks. Since all round I stacks have a pink and a white card, there is a 50-50 chance that a randomly picked card would be white.

Hence, the probability of getting a white card is 1/2. 

The probability of getting a pink card is also 1/2.

Since these events are independent, we multiply the probabilities. This is similar to the probability of only heads coming up when we toss two coins ( 1/2  * 1/2   or 1/4). Thus, in round II:

The probability of both cards being white is 1/2 * 1/2 or 1/4.

The probability of both cards being pink cards is 1/4.

However, the probability of having one card pink and one white would be 1/4 + 1/4 or 1/2 (we add the probabilities for mutually exclusive events, go here to read more on this). This calculation would be the same if we want to determine the probability of getting a head and a tail when we toss two coins. 

Since we assumed the pink card is dominant, the last three options would all give us a pink card on top—that is 3/4th of stacks in round II would have a pink card on top. This is exactly what Mendel saw in his experiments. This allowed him to come up with the Law of Segregation which states that an organism has two elements for a particular trait and that those two elements get separated in sperm or egg production. We now call those elements alleles, alternative copies of a gene (more on this shortly).

What is particularly interesting is that the notation scheme Mendel used is still used by the geneticists. For example, if we say Pink gene controls flower color, we would use upper case P for the dominant allele and lower case p for the recessive allele. So the purebred plants with pink flower would have PP genotype or allele composition and the plants with white flowers would have pp genotype. In the F1 generation, all the plants would have Pp genotype (they would all have pink flowers because of the dominant allele P). In the F2 generation, 1/4th of the plants would have pp genotype, and these would have white flowers. What about the plants with pink flowers? Here, half of the plants would have Pp genotype, and 1/4th of the plants would have PP genotype. Since Pp genotype or PP genotype both give us pink flowers, combined, 3/4th of the plants in the F2 generation would have pink flowers. 

As we can see, using the letter W to designate white flowers would be confusing and misleading because we are talking about the same gene here, the Pink gene.
Gregor Mendel. Courtesy: Wikipedia
Mendel also came up with another law based on a different set of experiments where he bred plants for two traits instead of one trait (or ‘dihybrid’ traits instead of ‘monohybrid’ traits that we saw before). In one experiment, he pollinated purebred plants with yellow and round peas with plants with green and wrinkled peas. Here, all the peas from the F1 generation plants were yellow and round (yellow and round, thus, were the dominant traits). When he allowed these plants from F1 generation to self-pollinate, he got round yellow, yellow wrinkled, green round and green wrinkled peas with a ratio close to 9:3:3:1, respectively, in the F2 generation. 

While the numbers look very different from what we saw before, they are actually very similar. If we worked on one trait, say pea shape, we would get a ratio of 3:1 (round being dominant) in the F2 generation. Similarly with yellow being the dominant color, yellow to green ratio in the F2 generation would also be 3:1. Another way of stating the above would be, in the F2 generation: 

The probability of getting yellow colored peas is 3/4 …(A)

The probability of getting green colored peas is 1/4 …(B)

The probability of getting round peas is 3/4 …(C)

The probability of getting wrinkled peas is 1/4 …(D)

If the color and shape are independent traits, we can determine what we can expect in the F2 generation with the same math:

A * C would give us the probability of getting yellow, round peas or 3/4 * 3/4 or 9/16.

A * D would give us the probability of getting yellow, wrinkled peas or 1/4 * 3/4 or 3/16.

B * C would give us the probability of getting round, green peas or 1/4 * 3/4 or 3/16.

Finally B * D would give us the probability of getting wrinkled, green peas or 1/4 * 1/4 or 1/16

Since that’s what Mendel observed, pea shape and pea color traits must be inherited independently. This led to the Law of Independent Assortment. According to this law, traits inherited through one gene (seed color, for example) will be inherited independently of traits inherited through another gene (seed shape, in this case). 

Mendel published first published his findings in 1865, but because it contradicted the prevailing dogma, his research failed to generate any interest. However, almost four decades later—and about two decades after his death (Mendel passed away in 1884)—three scientists came up with the same conclusions. They also discovered Mendel’s long-forgotten papers, and the rest of the world then came to know about his research. For his contribution, Mendel is now considered the founder of modern genetics.  Even though there are several exceptions to his laws, geneticists still use his laws to explain inheritance through sexual reproduction (I will write about one of these exceptions in another essay). 

One unfortunate aspect of his legacy—besides the fact that he didn’t live long enough to see his work appreciated—was that after his death, his successor burnt most of his writings and letters (apparently, it was done to solve some tax dispute). We can, however, piece together some aspects of his life from the surviving documents. He was, for example, born in a relatedly poor family in the Austrian Empire (now part of the Czech Republic) in 1922. To some extent, his parents’ economic situation influenced his choices in life: one of the reasons he became a friar was to receive a free college education. In college, he studied philosophy and physics.

Going over his life, I get a sense that even though Mendel didn’t receive the recognition he deserved in his lifetime, he was perhaps not that discontent with his life. As a friar, he had managed to escape poverty, had enough time to pursue his hobbies, and more importantly, he was a founding member of Austrian meteorological society.  Since he wrote more papers on meteorology than biology, I think at the very least, Mendel would have been content with his contribution to science because of the meteorological society he founded.

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