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Dr. Manuel Varela- The Lasting Legacy of Gregor Mendel

Aug 13, 2017 by

An Interview with Dr. Manuel Varela- The Lasting Legacy of Gregor Mendel

Michael F. Shaughnessy –

1) Dr. Varela–if there is one name that stands out from high school biology, it is Gregor Mendel, and his work with dominant and recessive genes.  Let’s get some background. Where and when was he born and what did he first start out as?

The Father of Genetics, Johann Gregor Mendel, was born on the 22nd day in the month of June in 1822 in Heinzendorf, a village now called Hyncice, which was located within the province of Silesia, in the Hapsburg Empire of Austria, and now currently an enclave of the Czech Republic (Czechoslovakia).

The name given to Mendel by his parents on the day of his birth was Johann, and Gregor was the name he took when he became ordained as a monk in 1851. Mendel’s father was Anton, a peasant farmer, and his mother was Rosine Schwirtlich.  Only two of Mendel’s siblings survived early childhood, an older sister, Veronica (b. 1820), and a younger sister, Theresa (b. 1829). The Mendel family lived in poverty during his childhood.

In 1840, Mendel graduated from gymnasium in Opava (also referred to as Troppau) and entered the Olomouc Philosophical Institute, which was part of the University of Olomouc (Olmütz), and in 1843 he graduated from this institution.  Shortly thereafter, in 1843, Mendel was admitted to the St. Thomas monastery housed in Brünn (now called Brno) and became a monk.

2) Now, the big question in science is often WHY-? Why did he get involved in science, WHY did he get involved with genetics, and WHY peas?

There is an interesting story regarding Mendel’s choice to study the sciences while in the monastery. Apparently, his choice of studying science was, at first, dictated by a failure.

According to the story, in 1851 Mendel failed the natural science portion of his qualifying examinations for his teaching license; therefore, Mendel was sent to the University of Vienna in order to become proficient in the natural sciences.

While at university, Mendel had enrolled in courses pertaining to systematics of botany, physiology, microscopy, morphology, and mathematics, becoming quite proficient in this particular area. I believe that genius of Mendel was his unprecedented ability to combine the field of mathematics to that of heredity, in order to become the father of genetics.

Additionally, Mendel became accustomed with plant and animal breeding knowledge and practices. Thus, Mendel had now acquired an expertise in agriculture and botany, alongside his foundation in theology. Mendel had been aware of the studies by Joseph Gottlieb Kölreuter in 1761 showing the formation of so-called plant hybrids. It is this work that had so interested Mendel.

Upon his graduation from university in 1853, Mendel returned to his monastery in order to teach, and he began gardening in order to pursue his interests in studying the hereditary nature of plants and their hybrid offspring.

In particular, Mendel studied Pisum sativum, a variety of sweet pea plants, for several important reasons. First, the pea plants generated rather large numbers of seeds, in which he could uniquely apply his knowledge and expertise in mathematics towards analysis of these seed numbers.  Further, Mendel noticed that his pea plants had relatively shorter generation times, compared to those of, let’s say, animals and other plants. Thirdly, Mendel found that his pea plants had easily observable and distinctive characteristics, like the shapes of the seeds (round peas versus wrinkled peas) or of the seed pods (swollen versus thin), the colors of the seeds (green or yellow), the colors of the flowers (white or red), the locations of the flowers along the stem (proximal versus distal), and the lengths of the stems (tall or short).

Mendel used these easily observable plant traits and his horticultural pollination techniques to produce so-called pure-bred pea plants, i.e., plants which repeatedly produced offspring harboring the same traits during each subsequent generation (e.g., offspring always produced round peas or always wrinkled peas).

These efforts took Mendel two years to accomplish—it was a tremendous effort on his part.  But he finally acquired his needed and pure lines of pea plants, each line with their consistently generated individual pure traits. These pure plants have been referred to as the so-called parental (P) generation.

He was, thus, ready to conduct his hereditary experiments on painstakingly generated pure parental pea plants, generation P.

To begin with, Mendel cross-pollinated his various generation P plants with each other. For example he crossed-bred the tall plants with the short plants, or his green pea plants with his yellow pea plants, or his round pea plants with his wrinkled pea plants, or his white flowered plants with his red flowered pea plants, and so on.

In order to do his cross pollination experiments using his pure-bred pea plants, Mendel performed the following procedures to do so.

First, from one pure plant (e.g. a tall one) Mendel removed the stamens, which contained the pollen and brushed the stigmas of the other pure plant (i.e., a short pea plant) and allowed the cross-pollinated plants to produce flowers, which then produced pea pods, containing the seeds, i.e., peas. Mendel then planted the newly emerged pea seeds and made observations of the character traits of his new progeny plants, which are referred to as the F1 generation, for the “first filial” generation.

Initially, Mendel reasoned that the F1 generation would be a mixture of the two pure traits, e.g., medium height, if the cross-bred P generation individuals were tall and short plants. To his great surprise, Mendel found that certain traits disappeared completely! For instance, his F1 generation plants were all tall; the short trait failed to show itself in any of the F1 generation progeny.

The main question that was burning in Mendel’s mind was:  what had happened to these “disappeared” traits?  Did they go away permanently—never to be seen again in later generations?  Or, were they dominated by the traits that had manifested themselves in all of the progeny of the F1 generation?

In order to provide answers to these perplexing “disappearing traits” results, Mendel conducted another set of experiments. Here, he cross-bred individual plants from the F1 generation to other F1 generation plants, to make a third generation, denoted as an F2 generation; that is to say, he performed an F1 x F1 cross, thus, producing an F2 generation of pea plants. In these F2 generation pea plants the repressed traits re-emerged, but only in a certain subset of them, certainly not all of them.

In other words, the ratio of the dominant traits to the suppressed traits in the F2 generation was roughly about 3:1 for each of the various traits.  Since the traits that were repressed in the F1 generation had not disappeared forever and had actually reappeared in the F2 generation, Mendel concluded that the traits had been prevented from manifesting themselves by the dominant traits. The traits that had been suppressed were termed recessive.

Thus, Mendel had discovered the principles of dominance versus recessiveness. The first principle in the field of genetics, dominance, had now been discovered, and it was Mendel who was the very first person to discover it.

3) Back in his day, there was no internet or web- how did he communicate his scientific results to others?

Although Mendel was ordained as a monk, with its required education in theology, he was also trained, nonetheless, as a bona fide scientific investigator, complete with practical knowledge in the methods he needed to conduct his experiments and in the general scientific method for expanding the frontiers of new scientific knowledge.

Thus, like any scientific experimentalist of his day, Mendel shared his original data in a public presentation venue and in a permanent publishing of his work in print.

There is an accounting, as told by biographers, that Mendel, in an effort to spread his knowledge about, had sent a reprint of his 1866 paper to other scientific investigators, whom Mendel thought might perhaps be interested in the novel nature of his work, including, it is said, a rather famous investigator, Carl von Nägeli (or Karl von Näegeli), an eminent botanist.

Dr. von Nägeli had previously conducted studies of cellular division, pollination, gamete production in plants, and especially in hawkweed plant taxonomy. In his accompanying letter to von Nägeli, Mendel also included some of his pea seeds, requested some hawkweed seeds, a favorite plant of von Nägeli’s, and then offered to test his inheritance theories in the hawkweed plants.

At first, von Nägeli ignored Mendel’s correspondence for what seemed like to Mendel an unusually long period of time of silence. When von Nägeli finally replied by letter, he criticized Mendel’s findings as too preliminary to make any definitive conclusions regarding heredity—keep in mind that Mendel had cultivated an estimated 10,000 pea plants in order to come up with his correct inheritance concepts.

It has been reported that von Nägeli was leery of Darwin’s theory of evolution. Consequently, von Nägeli had trouble in believing Mendel’s inheritance ideas.  In his same correspondence to Mendel, von Nägeli included the hawkweed seeds that Mendel had requested. Incidentally, there is no evidence that von Nägeli did anything with Mendel’s pea seeds.

In contrast, Mendel duly cultivated von Nägeli’s hawkweed plants, and the data became obfuscated. Unbeknownst to both Mendel and von Nägeli, or to anyone else at the time for that matter, individual hawkweed plants each have inseparable allelic variants and unusual gene clusters; plus they mate both sexually and non-sexually. Thus, the Mendelian approach to the study of hawkweed inheritance did not apply. The work went nowhere.

One historian has opined that von Nägeli’s advice to Mendel (“study the hawkweeds”) was the worst advice any investigator ever gave to another investigator. Indeed, had Mendel studied the hawkweed first, it might have set back the founding of genetics as a bona fide scientific discipline for generations of scientists to come.

4) What was the immediate feedback about his discoveries or theories?

Mendel’s very first historical release of his heredity work came about in February of 1865 during a scientific conference in which he presented his data to a nearby local group, called the Brünn Society for the Study of Natural Sciences.  During this very first presentation of his work, Mendel took his audience, who were primarily botanists, through much of his work, describing his methods, his mathematical data, and his conclusions about dominance, independent assortment, and independent segregation.

To Mendel’s disappointment, no one asked questions afterwards, even though it was (and still is) customary to ask questions at scientific seminars, conferences, colloquia, and symposia. Biographers of Mendel speculate that no one in the audience of early botanists understood his mathematics-based presentation, presumably due to Mendel’s emphasis on the use of numbers to sort out inheritance patterns of pea plant traits.

A second presentation was given by Mendel to the same Society only a month later, this time expanding on how he had used his mathematical methods to arrive at his genotypic and phenotypic ratios and to arrive at his notions of inheritance. Once again, his audience sat in stunned silence—apparently in a complete incomprehension of the novel and unprecedented genetic principles just presented by Mendel.

It was, unfortunately, virtually incomprehensible to the members of the Society how inheritance patterns of clearly visible traits in plants could be arrived at simply by using numerical data and analysis. At this point, it is recorded that several members of the Society even felt sorry for poor old Mendel, for having wasted so many years (about 7 years) of his life conducting painstaking cross-breeding work and “counting peas” only to be met with virtual incomprehension, if not outright disbelief.

Nevertheless, in 1866, the Society published his presentations in its dedicated scientific journal, called the Proceedings of the Natural Society of Brünn.  It was written in German. The seminal paper was duly bound and sent by the society publisher to the main libraries worldwide, where it was summarily ignored and collected dust in the libraries’ journal stacks for more than 35 years.

5) What else did he investigate and what else did he discover?

As I mentioned above, Mendel formulated the genetic principles of dominance versus recessiveness with respect to characteristic traits in living beings like his pea plants. A dominant trait would somehow mask a corresponding recessive trait.  From his findings, Mendel formulated early on other important principles of genetics.

He reasoned that because the various recessive pea plant traits disappeared for a generation and then actually came back in the next generation but at certain mathematical ratios, the traits must therefore be determined by so-called discrete factors that he called “unit characters.” We now know that these so-called unit characters are what we modern investigators call genes.

Mendel further reasoned that these unit character genes occurred in pairs: one unit character (or gene) originating from one of the parents and the second unit character gene arising from the other parent.

Another concept arising out of Mendel’s studies is referred to as the so-called segregation of alleles.  An allele is a term given to an alternative form of a particular unit character, or gene. For example, one allele of a gene for flower color may dictate red flowers and the other allele of that gene for flower color may specify white flowers, in the pea plants.

In any case, Mendel’s principle of segregation states that an individual’s set of two versions (i.e., alleles) of a gene will separate from each other (i.e., segregate) during their transmission to the next generation, like from a parental generation to an offspring F1 generation. That is to say, when gametes or sex cells from an individual form, like an egg or a sperm, the alleles separate into each of these types of formed gametes.

Since the gametes have only one of the gene’s two allelic versions, they are termed haploid. Whereas, on the other hand, the individual organisms with their full complement of gene versions, i.e., both alleles of the gene are present, are referred to as diploid.

The resulting offspring may inherit one allele from one parent and the other allele from the other parent. In 1905, Reginald Punnett invoked the use of the so-called Punnett Square (See Figure 1) to distinguish between the genetic natures of the parents versus their gametes versus the genetic nature of their offspring.

 

First parent’s gametes:

 

 

 

Second parent’s gametes:

 

R

haploid

(red)

 

r

haploid

(white)

 

R

haploid

(red)

 

RR

diploid

(red)

 

 

Rr

diploid

(red)

 

 

r

haploid

(white)

 

Rr

diploid

(red)

 

rr

diploid

(white)

 

Figure 1. An F1 x F1 hybrid cross between parental individuals with Rr genotypes.

In Figure 1, in which, for example, the flower color trait is considered, R is the allele for the gene that specifies flower color, red, which is dominant, and r represents the allele of the flower color gene that specifies a white flower color and is recessive.

Each parent produces two gamete types: R (dominant, red flower color) and r (recessive, white flower color). There are three types of outcomes in the offspring: homozygous dominant (RR), homozygous recessive (rr), and heterozygous dominant (Rr).

The genotypes of the offspring produced in Mendel’s work occurred as a ratio of 1: 2: 1 for the RR: Rr: rr, and a flower color trait as a phenotype (i.e., an observable trait) in a ratio of 3: 1 for red: white.

As an aside, in Mendel’s publication he referred to these red flower colors as being red-purple.

In other work, Mendel then developed the methodological concept of the so-called “test-cross” analysis, in order to evaluate the genotype status of an individual plant. That is, Mendel wanted to determine whether an offspring, let’s say with a red flowered pea plant, was either genotype RR or genotype Rr.

The test-cross procedure involved crossing the yet unknown RR or the Rr individual with a known rr, a homozygous recessive, individual.

If the genotype of the individual was RR, then the resulting new offspring in a test-cross with an rr individual (Figure 2) would be in a 4: 0 ratio of red versus white—all offspring would be Rr—hence, all offspring will have red flowers.

There are no RR or rr offspring in the test-cross of an RR individual with an rr individual, and these types of progeny result are, therefore, not observed in the Punnett square; and, as can be seen in Figure 2, only Rr individuals are observed in the progeny.

 

Homozygous recessive’s gametes, rr, for test-cross:

 

 

 

Individual’s gametes:

 

r

haploid

(white)

 

r

haploid

(white)

 

R

haploid

(red)

 

Rr

diploid

(red)

 

 

Rr

diploid

(red)

 

 

R

haploid

(red)

 

Rr

diploid

(red)

 

Rr

diploid

(red)

 

Figure 2. Test-cross in which an individual has an RR homozygous dominant genotype.

If, on the other hand, the genotype of an individual was Rr, then the resulting new offspring in a test-cross with rr pea plants, see Figure 3, would be expressed in phenotype ratio of 1: 1 of red flower color versus a white flower color—half of the offspring would be red (Rr) and the other half would be white (rr). There would be no RR in this latter test-cross progeny.

 

Homozygous recessive’s gametes, rr, for test-cross:

 

 

 

Individual’s gametes:

 

r

haploid

(white)

 

r

haploid

(white)

 

R

haploid

(red)

 

Rr

diploid

(red)

 

 

Rr

diploid

(red)

 

 

r

haploid

(white)

 

rr

diploid

(white)

 

rr

diploid

(white)

 

Figure 3. Test-cross in which an individual has an Rr heterozygous dominant genotype.

Another concept that was developed by Mendel was the notion of independent assortment. During this new concept formulation, Mendel speculated whether, on the one hand, any two given unit character genes would always be linked to each other—this meant that during gamete formation, the two linked genes always stayed in association with each other.  On the other hand, the two unit character genes could very well assort independently and randomly of each other.

Mendel put these two distinct hypotheses, i.e., linked versus independent assortments, to the test, using a so-called two-factor cross breeding experiment in which his parental pea plants each had two pure traits, and cross-pollinated these parental individuals, producing new F1 generation plants.

Next, he crossed individuals from the F1 generation plants with each other to produce new F2 generation plants. These F2 generation progeny had phenotypes that were mathematically consistent with Mendel’s notion of independent allelic assortment during the formation of gametes.

I think it is an interesting fact that Mendel not only studied pea plants, for which he is now quite famous, he studied string beans (scientific name is Phaseolus vulgaris) and corn (scientific name Zea mays), as well. Many of his inheritance principles held up to his own scrutiny in these plants just as well as they had for his pea plants. In this regard, Mendel is said to have been quite lucky in choice of organisms for study, in addition to being an astute scientific research investigator.

6) How was he received at the time, was his work acknowledged?

Aside from Mendel’s 1866 paper having been cited only 3 known times in the intervening years between its initial publication and its later re-discovery by the established scientific world in the early 1900s, Mendel died in virtual obscurity, never knowing what a great contribution of such historical and scientific magnitude it was to become. It is said that after he died, on the 6th of January, in 1884, at 61 years of age, all of his papers were burned, sadly, in the very same yard where his pea plant garden had been located.

Mendel’s scientific work had not been uncovered from its obscurity until 16 years after his death. The number of investigators with claims to having independently re-discovered Mendel’s work, increases with time. I will mention a few notable examples here.

One of the best known of these so-called re-discoverers of Mendelian genetics is Dr. Hugo de Vries, a noted plant physiologist and professor of botany at the University of Amsterdam. Between 1892 and 1896, years after the death of Mendel, in 1884, de Vries had studied the breeding characteristics of the evening primrose plants and actually obtained the famous 3: 1 offspring ratios that Mendel had beforehand obtained. While waiting for an opportunity to publish his works in a monograph, de Vries had serendipitously stumbled upon Mendel’s published works, and he (de Vries) then consequently published his own findings, providing a statement of confirmation for Mendel’s work alongside his own, starting in 1901. Prof. de Vries was also one of the first to propose the mutational nature of the gene.

Another independent confirmation of Mendel’s work was brought about by Karl Erich Correns, a German botanist, who also studied pea plants and who also acquired quite similar phenotypic ratios as Mendel and de Vries had obtained.  When Correns sat down to write up his manuscript detailing his findings, he also encountered Mendel’s seminal paper; and Correns published his work in 1900, providing evidence in support of Mendel’s results pertaining to botanical inheritance.

Later, Correns was to follow-up on the fledgling genetics field and build upon the Mendelian principles of inheritance. For instance, Correns had discovered that genes could be physically linked to each other along their chromosomal structures, a finding that was later to be beneficial towards mapping the actual positions of genes located within their genomes. Interestingly, he found that other plants passed along their genetic complement onto succeeding generations via non-Mendelian inheritance patterns.

Lastly, Correns was the first investigator to correlate Mendel’s so-called law of segregation to the process of meiosis, during which paired genes undergo separation of their alleles. This was an important discovery. Without Mendel’s earlier supporting work to provide a context, later advances in our understanding of genetics might have very well been even further delayed.

Another line of supporting evidence for Mendel’s work arose in 1900 from the bacteriological studies by Martinus Beijerinck, who found that the bright red color, produced by the bacterium named Bacillus prodigiosus, also called Serratia marcescens, could be altered to produce no color or color variations. Prof. Beijerinck interpreted these microbial phenotypic findings in terms of genetics and gene mutation. This work started a new line of research delving into microbial genetics.

In 1902, Prof. Rollins Adams Emerson, an American and plant geneticist, had confirmed Mendel’s work with beans with his own similar type of work with bean plants, incorporating Mendel’s concepts with his own and publishing this work. Emerson later follow-up on the bean work with genetic studies of maize (corn), a plant which was later to prove decisive in the later discovery of the so-called “jumping genes” by Nobel Laureate Barbara McClintock, a phenomenon known as DNA transposition.

7) His later years- what did he do in the twilight of his career?

Unfortunately, in 1868 Mendel discontinued his ground-breaking inheritance studies in order to become an administrator, the abbot of the Brno monastery, and his twilight years were spent partaking in unending managerial tasks and, unfortunately, internal and external politics. Along with his monastic administrative duties, Mendel became a curator and chairperson to a variety of local committees; he also stood firm against newly imposed taxes, which Mendel had believed were unfair, especially to the poor.

Unfortunately, an opposing faction within his own monastery had tried unsuccessfully to imply, if not outright claim, that he had gone mad.  Nevertheless, the tax fight took out his remaining energy, and he was removed as head of the monastery.  Mendel had become overweight and his health had suffered during these twilight years.

8) Why is his work still important today?

It seems that on a daily basis, news reports announce important new genetic discoveries and advances. In short, Mendel’s work started a scientific revolution of such epic proportions that the life of everyone on Earth shall never be quite the same. I am convinced that Mendel’s work is very likely even more important today than ever before.

Many distinctive lines of scientific study emerged from Mendel’s work. First, the field of genetics, per se, demonstrated for the first time how genes which confer phenotypic traits are inherited and passed on to subsequent generations. Genetic inheritance is a never-ending process that maintains the continuity of all life.

Out of the curiosity held by certain early investigators about what constitutes life, there had arisen an intense interest in the nature of the genes, which then caused an even more intense interest the physical and chemical nature of the material that make up the genes, namely, the chemistry and the structural nature of DNA.

Although Mendel was to never know it, his so-called unit characters, the genes, were harbored within DNA, the great molecule of life. With the elucidation of the structure of DNA in the early 1950s, the fields of molecular biology and biotechnology exploded in the 1960s and 1970s.

The study of the complete nucleotide compositions of entire genomes, e.g., genomics, became possible during the various genome projects that materialized in the 1970s and 1980s. It has even become possible to determine the total genome sequences for each and every individual living being.  Construction of a genome map and DNA sequence for each person may be relatively easy to acquire and analyze.

Moreover, it is possible in modern times to compare DNA sequences of entire genomes with those of potentially all other genomes. Interestingly, mathematics, a discipline that was near and dear to Mendel, plays an important role in genomics and other genetics-based technologies.

As a direct consequence of the studies of genetics and genomes arose the possibilities of being able to fundamentally understand at the cellular and molecular levels the basic processes of life.  We now have a clearer picture of how genes, Mendel’s unit characters, direct the biological activities inherent in every single living cell.

We now have a basic and rudimentary understanding of how living organisms evolve with time. Investigators have developed an appreciation of the inner workings that occur as individual living biological cells manage to produce increasingly complex living tissues, organs, and organisms.

Importantly, the field of genetics has made it possible to know which genes may be altered in a defective manner such that the normal functioning is reduced or even lost, causing genetic diseases, many of which are severe in their effects on suffering patients.

Lastly, knowledge of genetics made possible by Mendel’s powerful scientific breakthrough led ultimately to the advancement in various technologies.

In agriculture, for instance, improvements in molecular, gene, and animal cloning has led to better quality plants and animals with less predispositions to disease and to better yields of food.

Diagnosis of many types of diseases has become possible, such as infectious diseases, as well as in genetic diseases. In the early 1990s, the biotechnological invention called gene therapy was put to use, in an attempt to correct defective genes in genetic disease patients.

Very recently, modern investigators were able to use a new gene editing invention, called CRISPR—Cas9 (the acronym denotes a clustered regularly interspaced short palindromic repeats—caspase 9 enzyme), in order to correct a genetic disease gene for a severe heart condition in the cells of an actual human embryo.

Thus, improvements in modern healthcare therapies has become possible. Thus, genetics will no doubt continue to make new advances in our basic knowledge of life and of new technologies, many of which will have the potential to improve the quality of life for all living beings well into the foreseeable future.

Genetics has become an extremely large discipline of study. For instance, college and university courses in genetics are often required courses for students majoring in biology, biochemistry, molecular biology, bioinformatics, cell biology, wildlife biology, to name only a few. Furthermore, students can major in genetics. In fact, genetics may be represented as full academic departments housed within higher learning institutions. Entire institutes devoted to the study of genetics have been established worldwide. It was Gregor Mendel who made all of these possible.

9) What have I neglected to ask?

In his studies, Mendel had made the assumption that one gene determined one particular trait, an impression supported by the so-called “one gene, one protein” perception propounded by George Beadle and Edward Tatum, who shared a Nobel Prize in 1958 for their work regarding the genetics of metabolism. Later, the notion emerged that one gene may specify more than a single trait, i.e., one gene specifies two or more traits, a notion called pleiotropy.

Along these lines, several genes may work together to specify a range of variations for a particular trait, like, for instance, height, or color, etc. Such instances are referred to as polygenic determinism of inheritance. Multiple genes will be involved in dictating one particular phenotype.

Lastly, once the field of Mendelian genetics was firmly established in scientific circles, the question arose amongst population geneticists about how genes with dominant traits didn’t just simply take over, leaving the recessive traits to eventually disappear with time. The answer came about from the population studies of Wilhelm Weinberg and Godfrey Hardy.

They proposed the so-called Hardy-Weinberg principle to account for the persistence of this genetic variation. The principle maintained that in a rather large population of individuals with no selection pressures imposed upon it, and in which mating was randomly occurring, the allelic frequencies do not significantly change, thus, maintaining the persistence of the recessive traits during the course of evolution and in the passing along of the genetic material to later generations.

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