GREGOR JOHANN MENDEL and His Law/Principle of Dominance

Image Source: Krithika nagaraj, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

1. Introduction

Since ancient times, people have noticed that offspring of humans resemble humans, and those of dogs and cats resemble their respective species. These observations are captured in well-known sayings like ‘like begets like.’ When a baby is born, people examine its likeness to parents and close relatives. This suggests that a baby’s traits are somehow connected to those of its parents and ancestors. People have wondered about the nature and basis of this connection for centuries. However, systematic efforts to find answers began in the 18th century when scientists started studying plant hybridization. These studies paved the way for Gregor J. Mendel’s (1822-1884) investigations, which are still the most remarkable and conclusive experiments in genetic analysis that support Mendel’s Laws.

2. Genetics Before Mendel

Before Mendel, several scientists conducted studies on plant hybridization during the 18th and 19th centuries. Some of the notable researchers include Joseph Koelreuter, John Goss, Sargaret Gaertner, Darwin, Herbert, Lecoq, Vichura, and Naudin. For example, Koelreuter examined hybridization among different types of Nicotiana (tobacco) species from 1761 to 1767. He observed that in the first generation (F1), hybrids exhibited both uniformity and heterosis (increased variation) compared to their parents. Additionally, he noticed that hybrids resulting from reciprocal crosses were indistinguishable, indicating that both parent plants contributed equally to the characteristics of the hybrids. Other scientists like Gaertner, Naudin, and Darwin supported Koelreuter’s findings.

Josef Gottlieb Koelreuter (Image Source: https://garystockbridge617.getarchive.net/amp/ media/josef-gottlieb-koelreuter-original-upload-0a24f0)

Gaertner even conducted experiments where he transferred the nucleus of one species into the cytoplasm of another species, transforming one species into another. He performed these transfers in various species combinations, demonstrating that certain traits in hybrids were similar to one parent, others were similar to the other parent, and some were intermediate between the two. These observations are foundational to the concept of dominance.

Key Findings From These Studies Were

• Hybrids from different varieties or species displayed traits resembling one or both parents, with some traits being intermediate. This concept relates to dominance in inheritance.
  
  
• Progeny from reciprocal crosses had identical characteristics, confirming that male and female parent contributions were the same.
  
  
• F1 progeny from a single cross had consistent traits, while F2 generation displayed greater variation. The increased variation in F2 was later understood to result from segregation and recombination.
  
  
• F2 generation plants could resemble one parent, exhibit a mix of parental traits, or even display entirely new traits.

Despite these significant observations, these early scientists couldn’t provide a satisfactory explanation for their findings. Mendel later brilliantly explained these data through his laws of inheritance.

Factors Contributing to the Unsuccessfulness of Previous Scientists

Mendel’s 1865 publication showcased a comprehensive assessment of the limitations in the experimental methodologies employed by his forerunners. It is reasonable to deduce that inadequacies in material selection and experimental procedures were the primary factors leading to their lack of success. The following are key points summarizing these issues:

• The earlier scientists examined the entire plant as a whole, encompassing a multitude of characteristics.
  
  
• Consequently, the plants couldn’t be distinctly categorized into a limited number of well-defined classes. These researchers did not attempt a thorough classification of the diverse character forms present in the offspring.
  
  
• The primary focus of these scientists was on describing the various character forms emerging in the first filial (F1) and second filial (F2) generations. They did not make an effort to quantify the frequencies of different character forms in the offspring.
  
  
• Many instances lacked meticulous and separate record-keeping of data from different generations.
  
  
• Often, precise control over pollination in the F1 generation was absent. The potential for cross-pollination existed, introducing potential confusion in the results of the F2 generation.
  
  
• Several studies involved F1 interspecific hybrids that displayed varying degrees of sterility. This sterility hindered the complete recovery of all character forms and disrupted the ratio of different character forms in the F2 generation.
  
  
• The number of plants examined in the F2 generation was comparatively limited. Conclusions drawn from a small sample size are prone to being inconclusive.
  
  
• Additionally, most of the traits studied by earlier researchers were quantitative in nature. The understanding of the inheritance of such traits only came about in 1909.

3. Gregor Johann Mendel (The Father of Genetics): Unveiling the Foundations of Genetics

Gregor Mendel, a visionary scientist whose groundbreaking work laid the cornerstone of modern genetics, emerged from humble beginnings. Born in 1822 in the vicinity of Brunn, Austria (now Brno, Czechoslovakia), Mendel grew up in a family of modest means, navigating the challenges posed by financial constraints and health issues that often impeded his pursuit of education. Despite these obstacles, Mendel’s journey would eventually take him on a transformative path that reshaped our understanding of the natural world.

Image Source: Krithika nagaraj, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

Mendel’s educational aspirations were stymied by his family’s financial struggles, forcing him to join the St. Augustinian monastery at Brunn in 1843. His dedication to the monastery’s mission led to his ordination as a priest in 1847. The year 1851 marked a turning point, as the monastery directed him to the University of Vienna, where he delved into subjects ranging from physics to philosophy. While Mendel’s dedication was unquestionable, his academic achievements, especially in physics and mathematics, did not necessarily reflect his ardor.

Figure: St. Augustinian monastery at Brunn

Following the completion of his studies, Mendel returned to Brunn in 1854, assuming the role of a substitute science teacher. This role served as the fertile ground where his latent genius began to flourish. Adeptly balancing his teaching duties with his responsibilities as a priest, he inhabited a house adjacent to the local church, where he conducted his pioneering experiments.

In 1857, Mendel embarked on a mission that would ultimately alter the course of scientific history. He meticulously collected pea seeds from seed growers across Europe, amassing a diverse array of materials for his experiments. Armed with determination and his own resources, he transformed his kitchen garden into a laboratory of innovation. Over the span of seven years, his relentless pursuit of knowledge led to the formulation of groundbreaking insights.

In 1865, Mendel unveiled his findings to the Natural History Society of Brunn. The paper, titled “Experiments in Plant Hybridization,” was presented with eloquence and supported by a wealth of empirical evidence. Although its significance was not fully grasped at the time, this classic presentation provided a comprehensive account of his meticulous experiments. Sadly, Mendel’s remarkable discoveries remained largely unappreciated during his lifetime.

Figure: Mendel’s Original Paper- Experiments with Plant Hybridization

As his scientific endeavors continued, Mendel’s involvement in monastery life grew deeper, culminating in his appointment as an abbot in 1868. Yet, amidst these responsibilities, he still found time to delve into studies on honey bees, other plant species, and climatology. Tragically, Mendel passed away in 1884, never witnessing the full realization of his contributions to the realm of life sciences.

Ironically, it was only after his death that the world would awaken to the profound significance of Mendel’s work. Sixteen years later, in 1900, three independent scientists—de Vries in Holland, Correns in Germany, and Tschermak in Austria—rediscovered Mendel’s forgotten paper. This rediscovery ignited a resurgence of interest, as the significance of Mendel’s groundbreaking principles became clear. Although Mendel’s genetic principles were articulated in 1865, they languished in obscurity for 35 years until their triumphant revival.

Gregor Mendel’s legacy serves as a testament to the power of perseverance and the capacity for revolutionary insight that can emerge from even the most unassuming circumstances. His work has fundamentally altered our understanding of heredity, genetics, and the intricate mechanisms governing life’s diversity. It stands as a reminder that true brilliance often shines beyond its own time, illuminating the path for generations to come.

4. Pea: A Strategic Choice of Mendel for Genetic Exploration (Why Mendel selected Pea plants for his experiment?)

The selection of pea as the model organism for his groundbreaking hybridization experiments was not an arbitrary decision for Gregor Mendel. Rather, it stemmed from a profound grasp of the intricacies and challenges inherent in such scientific pursuits. Mendel recognized several distinctive advantages that the pea plant offered as a prime experimental material, strategically positioning it at the heart of his revolutionary investigations. The remarkable advantages that pea brought to the realm of genetic experimentation are meticulously detailed below.

Figure: A pea plant twig with pod

Distinct and Discernible Contrasts: Among the commercially accessible pea varieties, a plethora of characters existed in two distinctly contrasting forms (Table below). This dichotomy was readily distinguishable, rendering the process of classification unambiguous and straightforward. For instance, the presence of round and wrinkled seeds in different pea varieties enabled Mendel to categorize seeds into these clear-cut groups. This inherent feature facilitated the classification of both F2 and F1 progeny, establishing a foundational basis for his ensuing genetic analyses.

Image Source: LadyofHats, reworked by Sciencia58, CC0, via Wikimedia Commons

The seven characters of Pea plants taken by Mendel along with Dominant and Recessive Traits

Character	Dominant Trait	Recessive Trait
Flower Color	Purple	White
Flower Position	Axial	Terminal
Seed Color	Yellow	Green
Seed Shape	Round	Wrinkled
Pod Color	Green	Yellow
Pod Shape	Inflated	Constricted
Stem Length	Long	Short

Self-Pollination and Floral Configuration: The architecture of the pea flower ensured a remarkable degree of self-pollination, an attribute that Mendel systematically verified through experimentation. This inherent tendency significantly eased the generation of F2 and F1 progeny. Furthermore, this natural self-pollination mechanism safeguarded the process against external pollen contamination, preserving the purity of his experimental outcomes.

Facilitated Hybridization through Floral Anatomy: The relatively ample size of pea flowers facilitated the meticulous processes of emasculation and pollination. These pivotal steps in artificial hybridization were executed with relative ease, fostering controlled genetic crosses. The convenience of these procedures ensured precision in Mendel’s experiments and amplified the accuracy of his findings.

Cyclical Crop Duration: Pea’s relatively short life cycle, confined to a single growing season, afforded Mendel the opportunity to cultivate and study a fresh generation each year. This cyclical progression expedited the accumulation of data over multiple generations, a crucial element in deriving comprehensive genetic conclusions.

Generous Seed Characteristics: The size of pea seeds played an essential role in facilitating their germination, eliminating any potential complications in the initial stages of experimentation. Moreover, the manageable size of pea plants meant that multiple individuals could be cultivated within a limited space. This efficient space utilization translated into the cultivation of a substantial number of pea plants within a compact area, amplifying the statistical significance of Mendel’s results.

In addition to his exhaustive studies on pea, Mendel extended his inquiry to the common bean, scientifically known as rajma (Phaseolus vulgaris L.). This complementary experimentation on rajma complemented the data from the pea investigations and was concurrently documented within the same scientific discourse. This holistic approach enriched Mendel’s insights and lent further credibility to his revolutionary discoveries.

In the annals of scientific history, the choice of pea as Mendel’s experimental medium emerges as a strategic masterstroke. The careful consideration of its inherent advantages facilitated the unprecedented revelations that forever transformed our comprehension of genetic inheritance. Mendel’s methodical analysis of pea plants and their hereditary traits remains a testament to the power of astute selection in scientific inquiry, profoundly shaping the course of genetics for generations to come.

5. How Mendel Succeeded in Understanding Genetics (Reasons for Mendel’s success)

Mendel, a scientist from the past, discovered important things about how traits are passed down from parents to children. He did something special that many before him couldn’t do. Here are some reasons why Mendel’s work was a success:

• Learning from Others’ Mistakes: Mendel looked at what other scientists had done wrong before him. He saw where they went wrong in their experiments and made sure he didn’t make the same mistakes. This helped him a lot in his work.
  
  
• Taking One Step at a Time: Mendel started by studying only one thing at a time. This made things easier to understand. Once he figured out how one thing worked, he moved on to study two or three things together.
  
  
• Using Different Peas for Clear Results: Mendel chose special pea plants that had very different features. For example, some peas had round seeds, while others had wrinkled seeds. These differences were easy to see. He called these differences “contrasting features.” This helped him see how traits were passed down clearly.
  
  
• Being Really Careful and Writing Things Down: Mendel was very careful in what he did. He looked at lots of plants and made sure to count them correctly. He wrote down how many plants had each feature in every new generation. This made his findings strong and believable.
  
  
• Doing Experiments in a Special Way: Mendel did his experiments very well. He grew special pea plants for two seasons to make sure they were the same. He also checked for any mixing of plant parts. This way, his results were not messed up by accidents.
  
  
• Using Math and Common Sense: Mendel was good with numbers. He used math to understand his results better. He also knew that nature can be a bit messy and not always exact. So, he was okay with results that were close but not perfect.
  
  
• Thinking and Testing: Mendel thought about why things happened the way they did in his experiments. He came up with ideas to explain them. But he didn’t just guess. He tested his ideas with more experiments to make sure he was right.
  

Mendel’s work was really smart and had a bit of luck too. He picked traits that were easy to study, and he got good results. His discoveries have helped us understand how parents pass down traits to their kids. He was like a detective, figuring out a mystery in nature. And because of his hard work, we know a lot more about how living things inherit their features.

6. Factors Contributing to the Neglect of Mendel’s Discoveries (Why Mendel’s work remained unnoticed?)

The historical overlooking of Gregor Mendel’s groundbreaking findings in genetics can be attributed to a convergence of factors that cast a shadow over his pioneering work. These factors, while not inherently insurmountable, created an environment where the true significance of Mendel’s contributions remained obscured for a considerable period.

• Mathematical Unorthodoxy in Biology: Mendel introduced a novel approach by employing mathematical principles of probability and distribution to elucidate biological phenomena. However, this unconventional use of mathematics within the realm of biology was met with skepticism. Many biologists of his time believed that the intricate and complex nature of biological processes could not be reduced to mathematical formulas. As a result, Mendel’s innovative mathematical perspective faced resistance from a community ingrained in traditional biological thought.
  
  
• Divided Focus on Trait Types: A notable divergence in the scientific focus of Mendel and his contemporaries played a role in obscuring the importance of his work. While Mendel studied pairs of characteristics that exhibited clear-cut differences, his contemporaries such as Charles Darwin and Francis Galton were preoccupied with traits displaying continuous variations. The latter group believed that traits demonstrating discontinuous variations were of limited significance in understanding heredity and evolution. This division in emphasis led to a skewed perception of the applicability of Mendel’s findings.
  
  
• Apparent Stability of Species Traits: Mendel’s meticulous work appeared to suggest that traits within a species remained constant across generations. In the F2 generation resulting from a cross, only parental traits seemed to emerge, and the introduction of new traits was conspicuously absent. This seeming lack of variation ran counter to the prevailing notion of evolution, which relied on the introduction and selection of novel traits for natural selection to act upon. As a result, Mendel’s conclusions seemed incongruent with the established narrative of biological evolution.
  
  
• Lack of Understanding of Cellular Processes: A critical hindrance in comprehending Mendel’s work was the lack of knowledge about crucial cellular processes at the time. The phenomena of fertilization and the intricate behavior of chromosomes during cell division (mitosis and meiosis) remained uncharted territories. Mendel’s findings required an understanding of these fundamental processes to be fully appreciated. His conceptual leap into the world of genetics exceeded the contemporary scientific understanding of cellular mechanisms.
  
  
• Limited Dissemination and Publication: Mendel’s findings, while groundbreaking, lacked extensive publicity beyond his initial paper. His work was not widely circulated, and its limited dissemination contributed to its obscurity. The scientific community’s awareness of his ideas was constrained, preventing broader engagement and validation of his insights.
  
  
• Cross-Species Verification Challenges: Mendel’s challenges extended beyond the boundaries of his chosen experimental organisms. His unsuccessful attempts to validate his conclusions in species other than his well-studied pea and rajma raised doubts about the universality of his findings. The unfortunate selection of organisms such as Hieraceum (a facultative apomict) and honey bees (with haploid males) hindered the confirmation of his principles in diverse contexts.
  

• Doubt Cast by Correspondence: Mendel’s extensive correspondence with contemporaries, including the renowned botanist Nageli, inadvertently sowed seeds of doubt about the broad applicability of his findings. He candidly communicated his inability to replicate his conclusions in certain plant species, adding an element of uncertainty to the external validity of his principles beyond pea and rajma.
  

In sum, the neglect of Mendel’s discoveries is a complex interplay of factors stemming from an unconventional mathematical approach, divergent focus on trait types, the apparent stasis of species traits, gaps in understanding cellular processes, limited dissemination, and cross-species verification challenges. Mendel’s work was ahead of its time, and only when the scientific landscape evolved did his revolutionary insights find the recognition they truly deserved.

7- Key Points from Mendel’s Paper Unveiled (What were the key findings from the Mendel’s original paper?)

In Mendel’s groundbreaking paper, he brought forth several important points that form the cornerstones of modern genetics. Let’s delve into these highlights in detail:

• Understanding Hybrid Development: Mendel challenged the prevailing belief of his time by asserting that both male and female gametes contribute equally to the development of hybrids (F1). This countered the notion that females merely provided nourishment, similar to how seeds nourish plants. The true fusion of male and female gametes, validated later, remained elusive until years after Mendel’s insight.
  
  
• Discovery of Genes: Mendel introduced the concept of “factors,” which we now call genes. These genes hold the responsibility for the development of various traits. Importantly, they pass from one generation to the next, transmitting the genetic information that shapes individuals.
  
  
• Particulate Nature of Genes: Mendel distinctly posited that genes are discrete entities. This contrasted with the prevailing idea that hereditary material was fluid and possibly located in blood, especially in animals. Mendel’s clarity in this matter laid the foundation for his groundbreaking laws of inheritance.
  
  
• Phenotype vs. Genotype: Mendel drew a clear distinction between the visible appearance (phenotype) of an organism and its genetic makeup (genotype). He categorized F₂ individuals showing dominant traits into pure and hybrid forms based on their phenotypic traits exhibited in their F₁ offspring.
  
  
• Formulas for Genetic Ratios: Mendel provided formulas to calculate the number of
  (i) various types of gametes produced by F₁,
  (ii) different genotypes in F₂,
  (iii) homozygous genotypes, and
  (iv) individuals in a perfectly segregated F₂ generation with a certain number of genes.
  
  
• Homozygosity Through Selfing: Mendel delved into the effects of continued self-pollination on homozygosity for a single pair of alleles (a gene with two variants). He explored the stability of homozygosity, a concept applicable to multiple pairs of alleles as well.
  
  
• Introduction of Dominance and Recessiveness: Mendel introduced the concepts of dominant and recessive traits, which underpin our understanding of how certain traits mask others in genetic inheritance.
  
  
• Elaboration of Laws of Inheritance: Mendel provided insight into the laws of segregation and independent assortment, foundational principles that govern how traits are inherited from parents to offspring.
  
  
• Validation Across Species: Mendel’s rigorous approach extended to other plants, including rajma. He explored inheritance patterns for pod color, pod shape, and plant height in rajma, confirming the universality of his findings.
  
  
• Inheritance of Complex Traits: In rajma, Mendel ventured into studying flowering time and noted the intermediate appearance of F1 and the influence of environmental factors on this trait. He postulated that these traits followed the same laws of inheritance.
  
  
• Complex Traits and Multiple Genes: Mendel’s work on rajma revealed numerous new flower colors, hinting at the complexity of traits governed by multiple genes, which Mendel suggested were inherited according to the same laws.
  
  
• Importance of Sample Size: Mendel recognized that larger progeny populations lead to more accurate observations. He acknowledged that biological ratios may not be as precise as mathematical calculations.
  
  
• Assumptions Behind Explanations: Mendel’s conclusions rested on two critical assumptions: equal proportions of different gametes produced by F₁ and equal chances for these gametes to fertilize. He backed these assumptions with experimental evidence.
  
  
• Interspecific Hybrid Studies: Mendel explored F₂ from interspecific hybrids in Phaseolus, noting deviations from the 3:1 ratio. He attributed these to F₁ sterility and limited F₂ progeny, positing that these cases still adhered to his laws.
  
  
• Perplexing Permanent Hybrids: Mendel attempted to explain peculiar hybrids in plants like Heiraceum reported by Gaertner. Unbeknownst to him, these species were apomictic, with unique inheritance patterns that he could not anticipate.
  

Mendel’s paper laid the groundwork for our understanding of genetics, setting the stage for the fascinating journey into the intricate world of inheritance and heredity.

8. Mendel’s Principle of Dominance: The Role of Dominant and Recessive Alleles

In the realm of genetics, Mendel’s insights shed light on how traits are inherited. In organisms like familiar animals and certain plants with paired chromosomes, each having two versions, the law of dominance unfolds. These organisms are diploid, carrying a maternal and paternal chromosome, which unite during fertilization. On the other hand, ovum and sperm cells, known as gametes, contain just one copy of each chromosome, marking them as haploid. Mendel’s law of dominance reveals that in a heterozygote, one trait will mask the presence of another trait for the same characteristic. Instead of both alleles contributing to the physical appearance (phenotype), the dominant allele takes the spotlight and fully expresses itself. Meanwhile, the recessive allele remains hidden but gets passed down to future generations in the same manner as the dominant allele. Only when offspring inherit two copies of the recessive allele will the recessive trait manifest, and these offspring will consistently exhibit the trait in subsequent self-crossing.

Let’s clarify the terms “dominant” and “recessive.” These terms pertain to how alleles interact genetically to shape the phenotype of the heterozygote. The central idea is genetic in nature: which of the two alleles within the heterozygote ultimately shapes the phenotype, mirroring one of the two homozygotes. Although it’s sometimes easier to refer to the dominant allele’s trait as “dominant” and the hidden allele’s trait as “recessive,” this approach can confuse the understanding of the concept from a phenotypic perspective. For instance, saying “green peas dominate yellow peas” muddles the relationship between inherited genetic makeup and observable traits. This confusion might subsequently complicate discussions about the molecular basis underlying these phenotypic differences. Dominance isn’t a fixed attribute; an allele might be dominant over one allele, recessive to another, and even codominant to a third.

When a trait is recessive, a person must inherit two copies of the gene for the trait to manifest. This means that both parents must carry the recessive trait for a child to exhibit it.

While Mendel’s experiments with pea plants laid a foundation, later researchers uncovered that the law of dominance doesn’t apply universally. Different patterns of inheritance have emerged, showcasing the complexity of genetic traits.

Key Takeaways

• Dominant alleles shine in a heterozygote, while recessive traits step into the spotlight only if the organism is homozygous for the recessive allele.
  
• A single allele’s dominance isn’t set in stone; it could be dominant relative to one allele but recessive relative to another.
  
• Not all traits obey the straightforward rules of dominance in inheritance; intricate forms of inheritance are prevalent.

Important Terms

Dominant: A relationship between gene alleles where one allele conceals the expression of another allele at the same genetic location (locus).

Recessive: Capable of being masked by a dominant trait.

References

https://upload.wikimedia.org/wikipedia/commons/3/39/Gregor_mendel.jpg

https://upload.wikimedia.org/wikipedia/commons/b/be/Gregor_Mendel_-_characteristics_of_pea_plants_-_english.png

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Matalova, E., 2022. Johann Gregor Mendel: Born to be a scientist?. PLoS Biology, 20(7), p.e3001703.De Castro, M., 2016. Johann Gregor Mendel: paragon of experimental science. Molecular Genetics & Genomic Medicine, 4(1), p.3.

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