Review Article

How Mendel uncovered the mechanism of inheritance overlooked by

his predecessors?

S. Sreekumar

Former faculty, Department of Zoology, University College, Thiruvananthapuram, Kerala, India

Corresponding Author: S. Sreekumar, Email: ssreekumar53@gmail.com

Journal of Experimental Biology and Zoological Studies. 2(2): p 114-27, Jul-Dec 2026.

Received: 27/05/2026; Revised: 10/06/2026; Accepted: 12/06/2026; Published: 05/07/2026

_____________________________________________________________________________________________

Abstract

Gregor Mendel conducted a series of hybridization experiments on the garden pea, Pisum sativum,

to elucidate the mechanisms of heredity. For his studies, he selected seven pairs of contrasting

traits, each occurring in alternative forms. Crosses involving any single pair of traits are known as

monohybrid crosses, while those in which two or three pairs of traits were followed

simultaneously are termed dihybrid and trihybrid crosses, respectively. Mendel tracked these

crosses for two generations, meticulously recording the number of progeny phenotypes during

each stage. He observed that these traits followed a predictable pattern: the F1 generation

expressed only one of the parental phenotypes, whereas the F2 generation saw the reappearance

of all parental traits in specific, predictable ratios. Based on these observations, he postulated the

fundamental laws of heredity: the Law of Dominance, the Law of Segregation, and the Law of

Independent Assortment. This review paper explains these laws and their corresponding ratios

using principles of probability. Furthermore, it provides a historical account of similar

experiments carried out by Mendel’s predecessors, evaluating their merits and shortcomings to

examine how they ultimately helped Mendel formulate his pioneering concepts that later laid the

foundation of classical genetics.

Keywords: Alleles, dominance, independent assortment, Mendelian laws, Mendelian ratios,

Product rule, segregation.

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Introduction

It was in 1868 that Gregor Mendel published his findings on hybridization experiments. However,

the significance of his work was recognized only in 1900, laying the foundation for modern

genetics. Hence, the year 1900 is considered the birth year of genetics. The preceding century is

often referred to as the pre-Mendelian era in the history of genetics. Even during the pre-Mendelian

period, there were several predecessors of Mendel, many of whom conducted plant hybridization

experiments. Some of them made observations, obtained results, and arrived at conclusions that

were similar or very close to those later proposed by Mendel. The progress in elucidating the

mechanism related to heredity was, however, retarded during the pre-Mendelian period, largely

due to the prevailing concepts and doctrines of heredity established by renowned philosophers

and biologists of that time. This article attempts to explore these concepts and doctrines of heredity

and trace the historical development of our understanding of hereditary mechanisms by examining

the experiments and observations of Mendel’s predecessors. Furthermore, it provides a

comprehensive account of Mendel’s subsequent experiments and his monumental contributions to

elucidating the definitive principles of inheritance, explaining how Mendel succeeded where

earlier investigators failed.

Fixity of species

The concept of the fixity of species was formally proposed by Carl Linnaeus (1707-1778).

According to this doctrine, all species are fixed and immutable, perpetually retaining their original

morphological characteristics. The central precept of this view is encapsulated in the phrase nullae

species novae ("no new species"), asserting that no additional species have been produced since the

original creation.[1,2] In terms of heredity, Linnaeus argued for the constancy of characters,

maintaining that the "essential characters" used to identify a genus or species remained permanent

and were not subject to evolutionary progression or change.[3] This perspective effectively

precludes the possibility of organic evolution and suggests a static model of heredity where variation

is negligible. Consequently, because species were viewed as the result of a single, divine creative

act, the total number of species in existence was considered defined, finite, and unchanging. The

concept of fixity of species, in fact, reflected the belief of special creation.[4]

Naturphilosophie

Naturphilosophie was a dominant philosophical movement during the late 18th and early 19th

centuries, championed by German thinkers such as F.W.J. von Schelling, J.W. von Goethe, and

G.W.F. Hegel. It advocated a holistic approach to understanding the natural world, conceptualizing

nature as a dynamic, organic whole rather than a mere collection of mechanical parts.[5]

Naturphilosophie significantly influenced early biology, particularly the early plant hybridization

experiments. Proponents argued that isolated parts removed from an organism, as was done in

hybridisation experiments, did not represent the integrated nature of the whole organism.

Consequently, they maintained that interpretations derived from such reductionist experiments

were unreliable, as they failed to account for the vital unity and “archetypal” form of the organism

as a whole. However, by the mid-19th century, Naturphilosophie fell out of favour with the rise of

strictly empirical and mechanistic methodologies and critics dismissed the movement as overly

"speculative" and lacking in experimental rigor. [5,6]

Cell theory and atomic theory

The cell theory, primarily developed by Matthias Jakob Schleiden and Theodor Schwann, is a

foundational principle of biology. It establishes that all living organisms are composed of cells,

which serve as the basic structural and functional units of life. A critical later addition, popularized

by Rudolf Virchow, clarified that all cells arise from pre-existing cells.[7] Parallel to these

biological advancements was the atomic theory proposed by John Dalton in the early nineteenth

century (1803–1808). This was the first scientific framework to define the nature of matter,

asserting that all substances are composed of minute, indivisible particles called atoms.[8] While

some German scientists were inspired by the "oneness" of nature—a core tenet of

Naturphilosophie—the cell and atomic theories ultimately helped transition science towards

Materialism and Empiricism. By focusing on discrete, physical building blocks, these theories

moved scientific thought gradually away from the more mystical and speculative aspects of

"Nature Philosophy."

Pre-Mendelian insights into the mechanism of heredity

During the pre-Mendelian era, the scientific community lacked a unified, mathematically sound

framework for understanding how physical traits were transmitted across generations.[9] Highly

speculative and often competing theories dominated the landscape, and contemporary experiments

frequently led to ambiguous or indecisive conclusions.[9] One of the most widely accepted views

during this period was blending inheritance, which speculated that parental traits mix during

fertilization to produce an intermediate physical appearance in the offspring.[10] Similarly, when

Charles Darwin proposed natural selection acting on individual variations as a mechanism for

evolution, he lacked a definitive understanding of how these variations originated or persisted. To

address this, he introduced a provisional hypothesis known as Pangenesis in 1868.[11] According

to this model, every organ and tissue in the body sheds minute organic particles called gemmules

into the bloodstream, which carry somatic characteristics and accumulate within the gametes

during their formation. While numerous hereditary concepts and breeding experiments designed

to follow hereditary traits emerged during the pre- Mendelian era, this review focuses exclusively

on those that directly aligned with or laid the conceptual groundwork for Mendel’s laws.

Plant hybridization experiments by Joseph Gottlieb Kölreuter

Joseph Gottlieb Kölreuter was a pioneering 18th-century German botanist who conducted more

than 500 large- scale scientific plant hybridization experiments using Nicotiana species

(tobacco).[12] In 1760, he performed the first successful artificial hybridization between two

tobacco species, Nicotiana rustica and Nicotiana paniculata, with the former serving as the female

parent and the latter as the male parent. Kölreuter observed that the resulting hybrids often exhibited

superior physical characteristics compared to their parents, a phenomenon now known as heterosis,

although the hybrids were frequently sterile. He regarded these hybrids as unnatural and unstable.

In experiments with carnations (Dianthus), crosses between red- and white-flowered varieties

produced pink offspring. When these pink hybrids were self-pollinated, the next generation yielded

a mixture of red, white, and pink plants. Kölreuter noted that hybrids were often unstable and

tended to “revert” to parental forms in subsequent generations rather than maintaining a stable

hybrid state. This observation led him to consider hybrids as “unnatural” deviations from the

natural order, a view consistent with the school of Naturphilosophie.

Kölreuter’s experiments provided strong evidence for the theory of plant sexuality. Through

reciprocal crosses, he demonstrated that similar results were obtained regardless of which species

served as the pollen donor or the seed parent. He concluded that both male (pollen) and female

(egg) parents contribute to the traits of the offspring, thereby supporting the concept of biparental

inheritance. Although he did not formulate the laws of inheritance, his meticulous hybridization

experiments laid the foundation for the later work of Gregor Mendel.[12]

Hybridisation experiments on garden peas by Knight

In 1799, Thomas Andrew Knight conducted experiments using the genus Pisum (garden pea). He

fertilized a white-seeded variety with pollen from a grey-seeded variety.[13] The hybrid plants

produced in the first generation exhibited only grey seeds. This demonstrated the dominance of

one character over another, a phenomenon later formulated by Mendel as the “principle of

dominance”.[13,14] However, in the second generation raised from the first-generation grey-seeded

plants, both parental seed colours reappeared. This observation demonstrated that traits were not

“lost” in hybrids but could re-emerge,[13] and that the two traits—grey and white—did not blend.

Mendel later observed a similar pattern of inheritance, on the basis of which he proposed the law

of purity of gametes. Although Knight observed these inheritance patterns, he neither counted the

offspring belonging to each trait category in successive generations nor attempted to determine a

numerical ratio between them.[14]

John Goss (1800–1880)

Goss’s work during the 1820s provided some of the clearest early evidence of what is now

recognised as Mendelian segregation. He pollinated a blue-seeded pea variety (“Blue Prussian”)

with pollen from a yellowish- white variety (“Dwarf Spanish”).[14] To his surprise, all the seeds

produced in the first filial generation (F1) were yellowish-white, resembling the male parent.[13,14]

In the second filial generation (F2), raised from these hybrids, the pods contained a mixture of both

blue and white seeds. Goss further observed that the blue seeds, representing the recessive trait,

bred true and consistently produced only blue seeds, whereas some of the white-seeded plants

produced only white seeds while others yielded a mixture of blue and white seeds.[13,14] Although

these observations closely paralleled the inheritance patterns later described by Gregor Mendel,

Goss lacked a mathematical framework for analysing heredity and, like Knight, did not record the

numerical proportions of the different seed types.[14]

Charles Victor Naudin

Charles Victor Naudin conducted several hybridization experiments during the 1850s and 1860s,

providing a more theoretical understanding of hybridity.[15] He worked with various plant species,

including members of the gourd family (Cucurbitaceae) and the nightshade family (Solanaceae).

Naudin observed that the first generation of hybrids was often uniform and resembled one of the

parents, a pattern previously observed by Knight and Goss, and later systematically studied by

Mendel.[16]

One of the most significant outcomes of Naudin’s experiments was his proposal that gametes

contain distinct “essences” responsible for different characters, and that these essences separate

during the formation of pollen and ovules in hybrids. He believed that this process of “disjunction”

was responsible for the reappearance of the original parental forms in subsequent generations.[15,16]

Mendel later referred to these hereditary units as “factors.” Naudin’s concept of “disjunction”

closely anticipated the law of segregation later formulated by Mendel. What prevented Naudin

from proposing a proper mechanism for the inheritance of characters was his adherence to the

contemporary concept of the “fixity of species,” according to which the tendency of hybrids to

revert to parental forms represented nature’s mechanism for preventing the formation of new,

permanent species through hybridization.[15]

Gartner’s plant hybridization experiments

Carl Friedrich von Gärtner (1772–1850) was a German botanist who carried out one of the largest

studies on plant hybridization before the work of Mendel. Inspired by the earlier experiments of

Kölreuter, Gärtner spent more than 25 years performing systematic plant-crossing experiments.

He conducted nearly 10,000 artificial cross- pollination experiments involving about 700 plant

species from more than 80 genera, producing around 250–350 hybrids.[17] His studies included

plants such as Nicotiana (tobacco), Dianthus, Aquilegia, Verbascum, Oenothera, and Malva. To

prevent self-fertilization, he carefully removed the male parts of flowers before pollination and

then manually transferred pollen from one plant species to another. He closely recorded

characteristics such as flower shape, plant growth, fertility, and the inheritance of parental traits.

His main aim was to understand whether hybrids could return to their original parental forms or

develop into new stable species. One of Gärtner’s important observations was that hybrids usually

showed characteristics intermediate between the two parents. He also noticed that some hybrids

were more vigorous than their parents, producing larger flowers and stronger growth. This is the

same phenomenon of hybrid vigour (heterosis) mentioned earlier. However, he observed that this

vigour often decreased in later generations. Gärtner found that many hybrids between different

species were partly or completely sterile, which supported the nineteenth-century belief that

species were fixed and distinct. He also observed that intraspecific hybrids often reverted to

parental forms in later generations, especially when repeatedly crossed back with one of the parent

species. He called this gradual return of parental characters “transformation.” According to his

experiments, repeated backcrossing could eventually produce offspring almost identical to one

parent species.

In corn, Gärtner crossed varieties with different seed colours, such as yellow, white, and striped

seeds. He observed that pollen from one variety could directly influence the colour of the grain and

suggested that the male parent could visibly affect the female reproductive structures. Gärtner’s

experiments with corn and garden pea (Pisum sativum) were especially important because they

later influenced Mendel’s work on inheritance. Mendel carefully studied Gärtner’s writings and

cited them in his own famous paper. Gärtner noticed that hybrids often showed a stronger

expression of one parent’s traits than the other, an observation related to what later became known

as dominance. He also observed that hybrid characteristics often disappeared in later generations,

with plants returning to the parental forms, a process he described as the “Law of Reversion.”[17]

Although Gärtner collected a huge amount of experimental data, he did not develop a clear theory

of inheritance. Unlike Mendel, Gärtner provided mainly qualitative descriptions of “types” and

no quantitative data, such as numerical counts of offspring. A major conceptual limitation of his

work was his belief that fertilization involved a qualitative struggle between parental “essences”

rather than the inheritance of discrete hereditary units. Similarly, like many botanists of his time,

he believed in the fixity of species and interpreted sterility and reversion as natural mechanisms that

preserved species identity. Therefore, he rejected the idea that hybridization could permanently

create new species. Mendel owned a copy of Gärtner’s book, Versuche und Beobachtungen über

die Bastarderzeugung im Pflanzenreich.[11] Gärtner’s difficulty in explaining the reappearance of

parental traits inspired Mendel to search for a mathematical explanation of inheritance. Mendel

admired the scale and precision of Gärtner’s experiments;[18] but differed from him by proposing

that inheritance was controlled by the transmission of discrete hereditary factors (later called

alleles) rather than by blending inheritance and reversion.

Gregor Mendel

Gregor Mendel (1822–1884), who is recognized as the “Father of genetics,” was an Austrian monk

and scientist whose pioneering experiments on pea plants laid the foundation for modern genetics.

He was born on 20 July 1822 in Heinzendorf, a village in the Austrian Empire (now Hynčice in the

Czech Republic), to a farming family. From an early age, Mendel showed a passion for nature and

learning. Although financial difficulties frequently interrupted his education, he persevered with

the support of his family and teachers. In 1843 he entered the Augustinian monastery of St.

Thomas in Brünn (now Brno), where he adopted the name Gregor. The monastery was an

important intellectual centre that encouraged scientific study.[19, 20]

While Mendel studied theology and philosophy to prepare for the priesthood, his interests remained

deeply rooted in mathematics and the natural sciences. He attempted to obtain a high school

teaching license but failed the formal examination. Recognizing his potential, the monastery sent

him to the University of Vienna from 1851 to 1853. There, he studied physics, mathematics, and

chemistry, gaining invaluable exposure to scientific methods that emphasized rigorous

observation, experimentation, and statistical analysis. Upon returning to Brünn, Mendel worked as

a substitute teacher. It was in the monastery’s garden that he conducted his famous hybridization

experiments on pea plants (Pisum sativum), ultimately discovering the fundamental laws of

inheritance that changed science forever.

Between 1856 and 1863, Mendel conducted extensive experiments on the garden pea, Pisum

sativum, in the monastery garden. He selected pea plants because they possessed several clearly

contrasting traits, were easy to cultivate, had short generation times, and could either self-pollinate

or be artificially cross-pollinated.[19,21] For his experiments, Mendel selected seven characters, each

occurring in two alternative forms. For example, with respect to plant height, he studied tall-

stemmed and dwarf-stemmed varieties. Other characters included seed shape (round vs. wrinkled),

seed colour (yellow vs. green), flower colour (purple vs. white), pod shape (inflated vs.

constricted), pod colour (green vs. yellow), and flower position (axial vs. terminal). For the

experiments, Mendel first established true-breeding lines through repeated self-pollination. He then

carried out controlled cross-pollination between plants possessing contrasting traits.

Monohybrid cross

In one set of experiments, Mendel considered only a single character existing in alternative forms.

For example, when true-breeding tall plants were crossed with dwarf plants, all the offspring in

the first filial (F₁) generation were tall. However, when these F₁ plants were allowed to self-

pollinate, the second filial (F₂) generation produced both tall and dwarf plants in an approximate

ratio of 3:1. This type of experiment is known as a monohybrid cross because only a single character

is studied to trace the pattern of inheritance.

Dihybrid cross

In another set of experiments, Mendel studied the inheritance of two characters simultaneously.

Such experiments are known as dihybrid crosses. [19,21] For example, Mendel crossed true-breeding

tall plants having round seeds with dwarf plants producing wrinkled seeds. In the first filial (F₁)

generation, all the offspring were tall plants with round seeds, indicating that tallness and round

seed shape were dominant traits. When the F₁ plants were self- pollinated, the second filial (F₂)

generation produced four different types of plants. These included the two parental combinations—

tall plants with round seeds and dwarf plants with wrinkled seeds—as well as two new

combinations, namely tall plants with wrinkled seeds and dwarf plants with round seeds. The

numerical data showed that tall round, tall wrinkled, dwarf round, and dwarf wrinkled plants

appeared in the ratio 9:3:3:1.

Mendel’s Laws of Inheritance

Based on his monohybrid and dihybrid crosses, Mendel proposed the below-mentioned laws of

inheritance.[19,21]

Law of Segregation: According to Mendel, the expression of a particular character is controlled by

a pair of hereditary factors, one contributed by the male parent through the pollen and the other by

the female parent through the ovule. During gamete formation, the two factors controlling a

character separate from each other so that each gamete receives only one factor, either of maternal

origin or paternal origin, and not both. These factors do not blend or alter one another but remain

unchanged through successive generations. Hence, this principle is also referred to as the law of

purity of gametes.

Law of Dominance: When two contrasting factors controlling a character are present together in

an organism, one factor may mask the expression of the other. The factor that is expressed is called

the dominant factor, whereas the factor whose expression is masked is called the recessive factor.

Law of Independent Assortment: Factors controlling different characters assort independently

during gamete formation, resulting in the production of new combinations of traits in the offspring.

In other words, various segregation processes are independent. Thus, segregation of one pair of

factors occurs independently of the segregation of other pairs of factors. This law is applicable to

crosses involving more than one pair of contrasting characters, such as a dihybrid cross.

Genetic Terminology

The hereditary factors referred to by Mendel are now known as genes. The different alternative

forms of a gene are called alleles.[19] For example, the gene controlling seed shape exists in two

allelic forms: the dominant round allele (denoted by a capital letter, R) and the recessive wrinkled

allele (denoted by a small letter, r). Mendel also proposed that each character is controlled by a

pair of factors, meaning that an individual possesses two alleles for a particular character. When

both alleles for a trait are identical (e.g., RR or rr), the condition is called homozygous. When the

two alleles are different (e.g., Rr), the condition is referred to as heterozygous. The observable

characteristics of an individual constitute its phenotype, whereas the genetic constitution or

combination of alleles is known as the genotype.

Why the 3:1 Ratio and Not a 2:1 Ratio?

According to Mendel's Law of Segregation, the F1 parents in a monohybrid cross produce an equal

number of gametes carrying alternative traits. For example, in a cross involving true-breeding tall

and dwarf plants, the F1 progeny are heterozygous, containing both the alleles for tallness (T) and

dwarfness (t). During gamete formation, these alleles segregate so that each gamete receives only

one allele—either T or t. Fertilization involves the random fusion of these gametes. Because both

types of gametes (T and t) are produced in equal numbers, one might initially expect an equal

distribution of three zygotic combinations: (1) Pure tall (TT); (2) Pure dwarf (tt); and (3) Hybrid

(Tt).

Since the T allele is completely dominant over the t allele, both TT and Tt express the tall

phenotype. If these three combinations occurred in equal numbers, the resulting phenotypic ratio

could be assumed as 2:1 (2 tall: 1 dwarf).

The Reality: Probability and the Punnett Square

However, Mendel’s actual experiments consistently yielded a 3:1 ratio.[19,21] This happens because

the outcomes are determined by the laws of probability. In other words, the ratios are determined

by the probability of events. Thus, in a monohybrid cross, there are actually four distinct, equally

probable fertilization events, as given below:



An ovule with T fertilized by a pollen with T, producing TT homozygote



An ovule with T fertilized by a pollen with t, producing Tt heterozygote



An ovule with t fertilized by a pollen with T, producing Tt heterozygote



An ovule with t fertilized by a pollen with t, producing tt homozygote

These events can be depicted using a Punnett square (named after the British geneticist R. C.

Punnett) in which the alleles carried by the male and female gametes are arranged along the top

and side of the square, respectively, and the possible allele combinations resulting from

fertilization are then determined (Figure 1).[19]

Figure 1: Punnet square depiction of probable combinations of alleles during fertilization.

Final Ratios

Because the hybrid genotype (Tt) can be formed in two different ways—through the union of a T-

bearing gamete with a t-bearing gamete or vice versa—it occurs twice as frequently as either

homozygous genotype (TT or tt). Consequently, the offspring exhibit the following predictable

ratios: Genotypic Ratio: 1TT: 2Tt: 1 tt (or 1:2:1) and Phenotypic Ratio: 3Tall: 1Dwarf (or 3:1).

Probability rule governing dihybrid ratios

The phenotypic and genotypic ratios in a dihybrid cross can be derived by constructing a Punnett

square. Alternatively, these ratios can be calculated using the probability rule known as the

multiplication rule. [19,21]

The multiplication rule states that the probability of two independent events occurring together is

equal to the product of their individual probabilities. For example, consider a cross between tall

round plants and dwarf wrinkled plants. In the F₂ generation, the probability of obtaining tall

plants, as observed in a monohybrid cross, is ¾, while the probability of obtaining round plants is

also ¾. Therefore, the probability of obtaining tall round plants is: ¾ × ¾ = 9/16. Similarly, the

probability of obtaining tall wrinkled plants is: ¾ × ¼ = 3/16. The probability of obtaining dwarf round

plants is: ¼ × ¾ = 3/16. The probability of obtaining dwarf wrinkled plants is: ¼ × ¼ = 1/16.

Thus, the phenotypic ratio for tall round, tall wrinkled, dwarf round, and dwarf wrinkled plants in

the F₂ generation will be 9:3:3:1.

Genotype probabilities

The genotype probabilities can also be calculated using the same product method.[19,21] For

example, consider the probability of obtaining the genotypes TT Rr and Tt Rr in the F₂ generation

of a dihybrid cross, where T and t represent the alleles for tall and dwarf traits, and R and r represent

the alleles for round and wrinkled traits respectively.

Probability of the TT Rr genotype: In a standard monohybrid cross, the probability of obtaining the

homozygous dominant TT genotype is ¼, while the probability of obtaining the heterozygous Rr

genotype is ½. Therefore, as per the multiplication rule, the combined probability of obtaining TT,

Rr is: ¼ × ½ = 1/8.

Probability of the Tt Rr genotype: Similarly, for the heterozygous Tt, Rr genotype in the F₂

generation, the probability of obtaining the Tt genotype is ½ and that of obtaining the Rr genotype

is also ½. Consequently, the combined probability of obtaining Tt, Rr: ½ × ½ = ¼.

Table 1: Phenotypic Ratios in Mendel's F₂ Dihybrid Cross: Comparing the actual and expected number of

F₂ phenotypes from a cross between plants with round-yellow seeds and wrinkled-green seeds (n=556).

Phenotypes

Actual number of F₂

phenotypes obtained

Expected number as per

Mendelian dihybrid ratio

Ratio based on actual

data

Round yellow

315

313 (556 x 9/16)

Round green

101

104 (556 x 3/16)

Wrinkled yellow

108

104 (556 x 3/16)

Wrinkled green

35 (556 x 1/16)

Total

556

A dihybrid cross involves different combinations of alleles. In fact, a dihybrid cross is actually two

independent monohybrid crosses occurring simultaneously; hence the ratio 9:3:3:1 = (3:1) x (3:1).

Therefore, analysing any single phenotype independently will yield the exact ratio of a standard

monohybrid cross. For example, the actual data obtained by Mendel in his classic dihybrid cross

experiment is presented in Table 1.[19]

If we isolate and consider the ratio of just a single trait, for example, seed shape (round vs. wrinkled),

the classic 3:1 monohybrid ratio emerges as follows: Total seeds = 556



Round seeds: 315 + 101 = 416

(Expected monohybrid number: 556 x ¾ = 417)



Wrinkled seeds: 108 + 32 = 140

(Expected monohybrid number:556 x ¼ = 139).

Similarly, seed colour (yellow vs. green) also yields the same 3:1 ratio:



Yellow seeds: 315 + 108 = 423

(Expected monohybrid number: 556 x ¾ = 417)



Green seeds: 101 + 32 = 133

(Expected monohybrid number: 556 x ¼ = 139).

What helped Mendel formulate the laws of heredity?

As mentioned earlier, several early hybridization experiments were remarkable and contributed

greatly to the understanding of the mechanisms of heredity. Mendel learned hybridization

techniques and gained important insights from these studies. Thomas Andrew Knight, John Goss,

and Carl Friedrich von Gärtner all conducted independent experiments using the genus Pisum

(garden pea), the same plant used by Mendel.[11,13,14,17] By selecting plants with two distinct seed

colours to track inheritance patterns, they observed that the F1 generation exhibited only one of

the two parental colours. However, crosses involving these F1 plants produced offspring

displaying both parental colours. This crucial observation indicated that traits do not blend and that

one colour is dominant over the other. Gärtner also noted that hybrids frequently showed a stronger

expression of one of the parent’s traits over the other. Mendel was also significantly influenced by

the work of Kölreuter, who conducted more than 500 extensive plant hybridization experiments

using Nicotiana (tobacco) species. Through elaborate hybridization experiments, Charles Naudin

concluded that traits are determined by specific "essences"—a concept that foreshadowed Mendel’s

own postulation of discrete hereditary "factors."[15] He came remarkably close to modern genetic

theory by suggesting that these essences segregate without influencing one another during gamete

formation, postulating this as the concept of "disjunction."

Despite these breakthroughs, Mendel’s predecessors were ultimately unable to arrive at definitive,

logical conclusions regarding the mechanisms of heredity. Their interpretations were often

constrained by a preconceived belief in the "fixity of species" or influenced by Naturphilosophie.

They largely maintained that hybrids are inherently unstable and possess a natural tendency to

revert to parental forms—viewing this reversion as a biological mechanism designed to prevent

the formation of new, permanent species through hybridization.[15]

Mendel’s experience in plant breeding

Being an experienced plant breeder who had studied the hybridization experiments of his

predecessors, including Kölreuter and Gärtner, Mendel was well aware of the techniques required

for controlled crossing. To ensure the true-breeding nature of his subjects, he tested each pea strain

by selfing the plants and growing their progeny for two years;[19] this confirmed that the plants

remained identical with respect to selected traits generation after generation. In this manner, he

established and maintained several true-breeding strains of pea plants, routinely repeating his

experiments to verify his results and confirm trait consistency.

Departure from conventional ideas and concepts

In a significant departure from his predecessors, Mendel applied a rigorous mathematical treatment

to his biological data, deriving distinct statistical ratios and providing robust explanations for them.

He firmly rejected the prevailing theory of blending inheritance, postulating instead that discrete

units—which he termed "factors"— were responsible for the expression of traits. Above all, his

scientific insights remained unhampered by contemporary doctrines and dogmas such as the fixity

of species or the theory of special creation.

Mathematical approaches to interpreting results

During his university education in Vienna, Mendel’s scientific temperament was greatly

influenced by two eminent professors, Christian Doppler and Franz Unger.[20] Doppler adopted a

highly practical and innovative teaching approach, demonstrating experiments to students and

encouraging them to conduct their own investigations. Mendel also attended Doppler’s course on

higher mathematics. From Unger, Mendel learned plant anatomy and physiology. Unger’s interests

were broad and included cell biology, plant anatomy and physiology, and palaeontology. He

believed in evolution and proposed that all plants could be traced back to an “original plant,” even

before Charles Darwin formulated his evolutionary theory. Unger was convinced that cells were

the fundamental units of life. He observed relationships in size and structure among plants and

collected data on a large scale. The mathematical training and experimental approach to problem-

solving that Mendel acquired from Doppler, together with Unger’s studies on plant characteristics,

had a decisive influence on Mendel’s later research in plant hybridization and numerical data

analysis. This ultimately led him to apply extensive statistical analyses to his findings—a

methodology that was revolutionary and highly unusual among biologists of his time.

Mendel’s ratios – Too perfect or biased?

Mendel was extra cautious about the ratios. His observed results exhibit a remarkably close fit to

the expected values (Table 1). Sir Ronald A. Fisher, a renowned British statistician and geneticist,

examined Mendel’s data across all reported experiments using a Chi-Square goodness-of-fit test.19]

He discovered that the observed results were extraordinarily close to the expected Mendelian ratios,

meaning the deviations between the observed numbers and the theoretical expectations were

improbably small. Consequently, Fisher suggested that the data may have been systematically

adjusted—either consciously or unconsciously—to better fit the expected ratios. However, this

controversial finding does not imply that Mendel was dishonest. Modern historians and

statisticians offer a few key possibilities for this "too-good-to-be-true" data.

Confirmation Bias: Mendel may have been exceptionally careful while recording results and may

have unconsciously favoured observations that agreed with his expectations. For example, he

might have stopped counting once the ratios appeared sufficiently close to the predicted values or

classified ambiguous traits (such as slightly wrinkled versus round seeds) in a manner consistent

with his hypothesis.

Assistant hypothesis: Fisher himself proposed that assistants involved in tending the thousands of

pea plants may have omitted doubtful observations or unintentionally adjusted counts to satisfy

Mendel’s expectations, especially if they were aware of the anticipated results.

Selective Reporting: The published data may represent only a selected portion of the many

experiments Mendel conducted, while experiments showing larger natural statistical deviations

may not have been included in the final report.

Between 1854 and 1856, Mendel cultivated and tested thousands of pea plants, operating under the

principle that a larger sample size of independent measurements would effectively rule out random

phenomena. Although he began his investigations with 34 varieties of garden peas, he eventually

narrowed his selection to 22 varieties with highly consistent characteristics. However, in his final

paper, he reported the results of only seven monohybrid crosses, one dihybrid cross, and one

trihybrid cross.[19] This selective reporting of only the crosses that best fit his expectations has

drawn criticism from modern commentators, as it can make the data appear biased towards his

hypotheses.

Why Mendel’s ratios remain undistorted by linkage?

The garden pea (Pisum sativum) is a diploid organism with seven homologous pairs of

chromosomes (2n = 14 chromosomes). For the hybridization studies, Mendel selected only seven

pairs of alleles representing different traits. Earlier it was believed that these selected alleles were

located on seven different chromosomes. However, in 1975, a Swedish geneticist named Stig Blixt

published a famous paper in Nature mapping out exactly which chromosomes held the genes for

Mendel's seven traits.[22,23] The actual distribution looks like this:



Chromosome 1: Seed coat/flower colour (A) and Cotyledon colour (I)



Chromosome 4: Stem length (LE), Pod form (V), and Flower positio (FA)



Chromosome 5: Pod colour (GP)



Chromosome 7: Seed shape (R)

Mendel postulated the law of independent assortment based on the results of his dihybrid crosses.

A limitation of dihybrid and trihybrid crosses is that when two genes are located very close together

on the same chromosome, they tend to be inherited as a single unit, a phenomenon known as

linkage. In such cases, the genes do not assort independently, resulting in deviations from the

expected Mendelian ratios. However, Mendel did not encounter major difficulties due to linkage.

This was because, compared with his monohybrid crosses, he performed relatively few crosses

involving more than one pair of contrasting traits and published the results of only one dihybrid

and one trihybrid cross among them.[19] By a fortunate coincidence, the traits he studied together

were either located on different chromosomes or were positioned far apart on Chromosome 1 and

Chromosome 4, allowing them to assort almost independently. Because of this wide separation,

frequent genetic recombination occurred during meiosis, producing nearly 50% recombination

frequency.[24] Since 50% is the maximum possible recombination frequency, such alleles behave

as though they assort independently, yielding the classic 9:3:3:1 dihybrid phenotypic ratio without

revealing genetic linkage. However, in the case of Chromosome 1, the genes controlling seed coat

colour and cotyledon colour are relatively close together, therefore showing a recombination

frequency of about 30% rather than 50%. Mendel, however, never published the results of a

dihybrid cross involving these two traits together. As a result, linkage never showed up in his

experimental data.

Non-Mendelian patterns of inheritance

The pairs of alleles for the traits Mendel selected exhibited complete dominance, which is precisely

why he obtained such clear, predictable ratios for his monohybrid and dihybrid crosses. However,

many other types of dominance relationships exist between alleles.[19,25-27] These include

incomplete dominance, where the heterozygous phenotype appears intermediate between the

phenotypes of individuals homozygous for either allele.[19] In codominance, both phenotypes of

the two homozygotes are simultaneously expressed, a classic example being human ABO blood

group alleles.[25] Furthermore, some traits are controlled by more than two alleles. In such cases,

a population’s gene pool may contain multiple alleles for a specific trait, although an individual

possesses only a single pair of alleles for that trait.[26] Similarly, quantitative traits such as weight

and skin colour are determined by a complex set of multiple genes acting additively, which is

known as polygenic inheritance.[27] Gene expression is also influenced by several other genetic

mechanisms. For instance, an allele at one locus may block or mask the phenotypic expression of

an allele at another gene locus, a process known as epistasis.[26] In other instances, mutant alleles

of two entirely different genes may produce identical phenotypes. Additionally, sex-linked alleles

show distinct inheritance patterns because they are carried on sex chromosomes.[19] Collectively,

these diverse patterns are referred to as non-Mendelian inheritance, as they deviate from typical

Mendelian phenotypic ratios.

Publication and neglect

Mendel presented his findings in 1865 to the Natural History Society of Brünn and published them

in 1866 in a paper titled Experiments on Plant Hybridization (Versuche über Pflanzen-

Hybriden).[28] However, his work attracted little attention at the time because most biologists

favoured blending theories of inheritance and failed to appreciate the importance of mathematical

analysis in biology.[20] As a result, Mendel remained largely outside the mainstream scientific

community. Even the distinguished plant physiologist Carl Wilhelm von Nägeli, with whom

Mendel maintained extensive correspondence between 1866 and 1873, failed to recognize the

significance of his discoveries. Although Nägeli possessed the expertise to understand the work,

he dismissed Mendel’s conclusions as “only empirical”.[20] Historians have further noted that

Nägeli fundamentally believed Mendel’s conclusions to be incorrect and regarded him more as an

amateur investigator than as a professional scientist. [11,29]

In 1868, Mendel was elected abbot of the St. Thomas Monastery, and the administrative

responsibilities associated with this position greatly reduced his scientific activities. Nevertheless,

he continued to pursue his scientific interests through detailed studies in meteorology and bee

breeding many more years. He died on January 6, 1884, in Brünn. Before his death, Mendel

reportedly told one of the novices: “My scientific work has brought me a great deal of satisfaction,

and I am convinced that it will be appreciated before long by the whole world.”[21] Indeed, sixteen

years later, his work received worldwide recognition.

The Rediscovery of Mendel’s Findings

In 1900, Hugo de Vries, Carl Correns, and Erich von Tschermak, each working independently in

different European countries, published the results of their experiments that agreed with Mendel's

findings. Their publications drew attention to Mendel’s original paper and rightly attributed the

priority of discovery to him. Since then, the patterns of trait expression and the ratios described by

Mendel have been validated in numerous instances and are widely used in pedigree analysis. They

form the foundation of transmission genetics, implying the statistical principles governing the

inheritance of genes from one generation to the next. Consequently, the hereditary transmission

involving dominant-recessive alleles, Mendelian ratios, and laws is universally known as

Mendelian genetics.[21]

Conclusion

When Mendel published his paper on inheritance, no one knew of the existence of chromosomes

or genes. The mechanism of heredity was a black box to him.[20] Despite this, by focusing entirely

on data representing the plant phenotypes of parental, F1 and F2 generations, he correctly

concluded that each trait is controlled by two unit factors—one inherited from each parent. His

experiments are widely appreciated for their brilliant planning, careful observations, rigorous

numerical analysis of data to derive ratios, and insightful interpretation of results. They were

explicitly designed to answer specific questions regarding how offspring characteristics relate to

those of their parents. The principles of heredity he proposed were later seamlessly integrated into

the chromosome theory of inheritance and modern genetics.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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