Mendelian Genetics ( Mendel's laws)

1. Introduction:

Genetics, branch of biology deals with the principles and explain the similarities between parents and progenies and differences among the individuals of same species. It is the science of inheritance and variation (Singh, 2019). The term genetics was coined by Bateson, it is derived from Greek word meaning 'to generate'.

Genetics deals with the transfer of biological information from cell to cell, from parents to offspring, and thus from generation to generation (Gardener & Snustad, 1909). Genetics deals with the physical and chemical nature of information transmitted through generation.

Long ago in the history before human discovered the inheritance of genetic materials human wondered about the similarities of the offspring with their parents. Scientists were working on plant improvement through hybridization of plant but they could not know the actual mechanism involved.

There was an immense need to understand the mechanism involved in similarities of offspring to their parents. These understanding have power to change man's outlook on the word and his power over nature, than any other advances in natural knowledge that can be clearly foreseen.

This wise of mankind was fulfilled by Gregor Mendel in 1860s by his discovery of the laws that governs the inheritance of the individual characters from parents to their progeny. This discovery is one of the land marks in the history of biological science. 

This became possible because of selection of proper experimental material i.e. garden pea (Pisum sativum) by Gregor Mendel and proper recording of the data obtained in each generation. Gregor Mendel have great knowledge of mathematics and he used that knowledge to understand and interpret laws governing the inheritance of characters. 

In 1960s scientist of that time could not acknowledge Mendel's work as Mendel has used statistics and probability theory in biological sciences which was new at time and his discovery was contradictory to his contemporaries' theories i.e. Darwin, Galton and others.

Mendel’s resurrection involved a “rediscovery” of his work by botanists Carl Erich Correns (1864-1933) of Tübrgen (Germany), Erich Tschermak von Seysenegg (1871- 1962) of Esslingen (near Vienna, Austria), and Hugo Marie de Vries (1845-1935) of Amsterdam (Netherlands), each of whom claimed to have independently rediscovered and independently published virtually the same results in early 1900 i.e. 16 years after Mendel’s death (Moore, 2001). In 1990s the behavior of chromosome and mechanism involved in cell division was understood which also helped to understand Mendel's findings.

The findings of Mendel opened the new era in the field of biology and given new understanding of biological laws. These laws are the basic laws which should be followed in every crop improvement programs. The laws helps to accumulate the desired characters in the plants to use it for the betterment of the humankind.

2. Materials and methods:

For the preparation of this blog entitled "Mendelian Genetics" different books, thesis, newspaper articles, journal papers and different website providing information about Mendelian Genetics were reviewed

All these resources were thoroughly studied and various information regarding Mendelian Genetics were presented in chronological order in this term paper. Citation and referencing in the text was done with help of software Mendely Desktop version: 1.19.4 in APA referencing format.

3. Result and discussion:

3.1. Biography of Mendel:

Gregor Johann Mendel was born on July 22, 1822 in Morovia near Brunn in Austria, now Czechoslovakia, in the family of poor family. His father Anton was a farmer. Johann did not have a good relationship with his father Anton. The reason why Johann only spoke to his mother Rosine was because his mother was polite and sweet natured. At the age of eleven he was sent away from his family to finish his elementary and high school education at Gymnasium in Troppau. He graduated at the Gymnasium and moved on to the Philosophical Institute, a two-year program for all high achieving students from the high school education before they could pursue to university.  Back at Heizendorf, he continued his struggle with his illness which made him helpless. At the age of 19, he was again bed-ridden for a year for an unknown disease. A year after he recovered from his disability, he received a letter from his former professor at Gymnasium Father Friedrich Fraz recommending him to go for priesthood (Sapp, 1990).

At that time it was difficult for poor families to obtain a good education and the young Mendel saw the only way to escape a life of poverty was to enter the monastery at Brunn in Moravis, (now Brno in Czechoslovakia). He joined the St. Augustinian monastery at Brunn in 1843. Here he was given the name Gregor. This monastery was a teaching order with a reputation as a center of learning and scientific enquiry.

Mendel become priest on August 6, 1847 at the age of 25. In 1851, he went to the University of Vienna where he studied physics, mathematics, and philosophy etc. although Mendel was a science and hardworking student, he did not do very well in studies particularly in physics and mathematics(Iltis, 2018).

After the completion of university studies he returned to Brunn in 1854 where he was appointed as substitute science teacher. Because of his persistence of acquiring scientific knowledge, he engaged himself in reading hybridization experiments of Josef Kolreuter and Karl Gartner. It was Kolreuter who conducted the first hybridization experiment with related members of tobacco family, Nicotiana rustica and Nicotiana paniculata. Garther was well-known for his 10,000 hybridization experiments yielding some 250 hybrid plants. But none of the two great scientists gave Mendel the answer as to how the hybrids acquired characteristics from the parents? In fact, these two scientists were great believers of ‘blending theory’, which theorized that offspring showed a combination of traits and thus roughly midway between the two parents, which Mendel doubted.

In addition as a priest in the local church, he collected pea seeds for his experiment in 1857 from commercial seed grower all over the Europe. He conducted the experiment in the kitchen garden of his own house within the church premises, using the available resources. 

After 7 years of extensive experimentation, he presented his findings before the natural history society of Brunn at meeting of February 8 and March 8, 1865. This paper entitled "experiments on plant hybridization", was published on German language (versuche iiber Pflanzehybriden) in the annual proceeding of the society in 1866. This volume of the proceeding was distributed to many libraries of Europe and North America (Singh, 2019).

After this, Mendel became increasingly involved with the work of monastery but he continued his studies on honey bee, some of the plants and climatology. Mendel died in 1884 at an age of 62 years. His finding become famous only in 1900s primarily as a result of a priority dispute between de Vries and Correns(Moore, 2001).

3.2. Plant hybridization activities before Mendel:

Many scientist before Mendel had worked on plant hybridization during 18th and 19th centuries. In 1716 Cotton Mather described the crossing of different colored varieties of Indian corn (Zea mays). He also reported the pollination of the squash and gourd( Orel, 2009).

In 1717 Thomas Fairchild fertilized the Carnation with the pollen from Sweet Willium. This is the first recorded plant hybrids. Richard Bradley noted spontaneous hybridization in primula. He also noted the effect of foreign pollen upon the appearance and flavor of apple(Vitězslav Orel & Wood, 2000). 

Johannes Wahlborn in 1746 described hybridization in tulip described the hybridization in tulip. He also explained the degeneration of brassica which was previously been recorded by Morison (1680) and Rye (1686) as a result of contamination by pollen of wild varieties (Gardner, Simmons, & Snustad, 2006).

In 1760 Carolus Linnaeus in Disquisitio de sexu Plantarum described he experiments which produced hybrids in Mirabilis, veronica, Delphinium, Hieracium and Tragopogon.

Josef Gottlieb Kölreuter (1733-1806) published series of paper on sex of plant. He cultivated plant for studying fertilization and development. He performed experiment particularly on tobacco plant that included artificial and production of fertile hybrids between plants of different species. On the other hand, he was convinced to have found a method by which one species could be turned into another by repeated backcrossing. His experiments were significant, because they constitute the starting point for an experimental tradition that led up to Gregor Mendel’s (1822–84) famous discovery of genetic laws a century later (Gasking, 1959).

Gaertner (1722-1850) reviewed in details the previous work of Knight, Goss and Seton with peas. In 1829, he started a selfed and a crossed series of peas, using four varieties i.e. Paris Wax (yellow seeds), Dwarf Creeping (white flowers, yellow seeds), Sugar peas (red flowers, wrinkled greenish-yellow seeds), Early Green Brockel (white flowers, green seeds). The result of the immediate effect of the cross showed the dominance of yellow over green in the hybrid seed which was experienced by Knight, Goss and Seton. Identical results were obtained in reciprocal crosses too (Roberts, 1919b).

Darwin (1809-1882) thoroughly investigated the comparative relation of the offspring of the crossed to those of selfed plants with respect to vigor. Darwin also remarks upon the greater power of the cross-fertilized plants in his experiment to stand expo- sure, the crossed plants, enduring sudden removal from greenhouse to out-of-door conditions better than did the self-fertilized, and also resisting cold and intemperate weather conditions more successfully. This was the case with morning glory and with Mimulus (Roberts, 1919a).

3.3. Ignorance of Mendel work:

Mendel in 1866 put forwarded the basic law of heredity governing inheritance of characters which provided the foundation for modern theory of genetics. Mendel (1866) published his now famous (but seldom read) 48-page paper in the society’s journal of its proceedings, Proceedings of the Brünn Society for the Study of Natural Science and distributed to hundred individuals, libraries and university but full significance of his were ignored till 1900 (Moore, 2001). He became famous after 34 years when his findings were rediscovered by other scientists.

Various reasons have been put forwarded for the alleged neglect of the Mendel's findings that it was not distributed to Mendel's contemporaries and it was even overshadowed by Darwin's work as he published his findings in the mid-nineteenth century in 1866, just seven years after Darwin’s On the Origin of Species appeared in print(Collins & Stewart, 1989).

Mendel's experiment demonstrated that the variability among F2 and further hybrids generation could be traced to the original variability in the first parental cross. Many scientist of that time thought that Mendel's work is the duplication of the previous research since scientist before him were also involved in hybridization of the plants (Moore, 2001).

Mendel at that time used forbidden mathematical approach i.e. principle of probability and binomial distribution for interpreting his findings which was something new and biologist believed that biological phenomenon were too complex for this treatment (Singh, 2019). 

Mendel was an amateur scientist and he published his findings in an obscure journal. At that time the phenomenon of fertilization and the behavior of chromosome were not known. So that time was not yet ripe to understand Mendel's idea/findings(Rheinberger, 1995).

Scientist of that time like Darwin, Galton and others were focusing on continuous variation for understanding evolutionary concepts. Mendel used contrasting pair of characters exhibiting discontinuous variation. So the scientist of that time does not give much more importance to Mendel's finding. 

Mendel even does not publicize his work through further writing and he failed to demonstrate similar finding on other species that he worked (Hieraceum and honey bee). Hieracium was a poor choice as an experimental organism because many of its embryo, especially those of hybrids between two different varieties, arise directly from diploid tissue in the ovary without fertilization of gamete (apomixes). Therefore not segregate for different characters among its offsprings(Zwick, Cutler, & Chakravarti, 2000).

This may have created a doubt about the applicability of Mendel's conclusion to other plants. Mendel soon lost confidence and abandoned most of his botanical research. Only years later did he resume this research, studying apples and pears. Those studies were solid work, but produced nothing remarkable(Gasking, 1959).

3.4. Experimental material of Mendel:

Pea as an experimental materials offers several advantages over other plant materials. Pea seeds were available from seed merchants in a wide array of distinct variant shapes and colors that could be easily identified and analyzed. This permitted an easy classification of F2 and F3 progeny from various crosses into clear cut classes on the basis of contrasting forms of the different character.

Pollination could be easily controlled in pea plant. Flower structure of pea plant ensures self-pollination and consequently use of Mendel's main techniques, "selfing" presented no difficulties. When cross fertilization between two pea plants was necessary, Mendel had merely to remove the stamens from one plant and transfer to other destamenized plant.

The pea plant was easy to cultivate and from one generation to another took a single growing season. Pea flowers are relatively large which facilitate emasculation and pollination. 

Pea seed are large and presents no problem in germination. Pea plants are relatively easier to grow and each plant occupies only a small space.

The description of seven pair of contrasting characters of a pea studied by Mendel are described below.

• Difference in the form of the ripe seed of pea plant. These pea seeds were round or roundish in shape, if any depression then only shallow or they have irregularly angular and deeply wrinkled seed (Mendel, 1996).

• To the difference in the color of the seed albumen (endosperm). The albumen of the ripe seeds is either pale yellow, bright yellow and orange colored, or it possesses a more or less intense green tint. This difference of color is easily seen in the seeds as their coats are transparent.

• To the difference in the color of the seed–coat. This is either white, with which character white flowers are constantly correlated; or it is gray, gray–brown, leather– brown, with or without violet spotting, in which case the color of the standards is violet, that of the wings purple, and the stem in the axils of the leaves is of a reddish tint. The gray seed–coats become dark brown in boiling water.

• To the difference in the form of the ripe pods. These are either simply inflated, not contracted in places; or they are deeply constricted between the seeds and more or less wrinkled (P. saccharatum).

• To the difference in the color of the unripe pods. They are either light to dark green, or vividly yellow, in which coloring the stalks, leaf–veins, and calyx participate

• To the difference in the position of the flowers. They are either axial, that is, distributed along the main stem; or they are terminal, that is, bunched at the top of the stem and arranged almost in a false umbel; in this case the upper part of the stem is more or less widened in section (P. umbellatum).

• To the difference in the length of the stem. The length of the stem is very various in some forms; it is, however, a constant character for each, in so far that healthy plants, grown in the same soil, are only subject to unimportant variations in this character. In experiments with this character, in order to be able to discriminate with certainty, the long axis of 6 to 7 ft. was always crossed with the short one of ¾ ft. to 1½ ft.

3.5. Law of segregation of Mendel:

In Mendel experiment with pea plant, he crossed tall and dwarf varieties of garden pea. All the offspring of first filial generation (F1, where F symbolize filial from Latin, meaning progeny) were tall. The dwarf trait disappeared in the F1 generation. However when these F1 were permitted t reproduce by self-fertilization, progeny obtained were of both types i.e. tall and dwarf. Careful classification of plant obtained showed that when large no of plants were considered about three- fourth were tall and remaining one fourth were dwarf. To be an exact, an F2 of 1064 consisted of 787 tall plants and 277 dwarf, a near perfect ratio of 3:1. 

In cross between smooth seed and wrinkled seed, Mendel found that all F1 plants produced smooth seeds. Upon self-fertilization of F1 plants mixture of smooth and wrinkled seed were produced. To be precise, an F2 of 7324 consisted of 5474 smooth seed and 1850 wrinkled seeds.

For all the seven character tested by Mendel, the result appeared to be the following pattern.

• For every character F1 derived from the crosses between two different varieties showed only one of the trait and never the other.

• It does not matter which parent variety provided the pollen and which provided the ova, "reciprocal crosses" in which each of the two varieties used to provide male and female parents gave the same result.

• The trait that has disappeared or hidden in F1 reappeared in the F2, but only in the frequency one-quarter to that of the total number.

Mendel called the determining agent responsible for each trait a "factor". From the evidence of F1 and F2, the factor that determines the appearance of the trait could be hidden but not destroyed. This phenomenon by which one trait appears but others doesn't even though the factor for both are present is called dominance. In the Mendel's cross the factor for tall height were considered dominant and factor for dwarf is considered as recessive. Symbolically we can present "T" for tall height and "t" for dwarf. Similarly "S" stands for a smooth and "s" stands for wrinkled seed shape where smooth shape of seed was a dominant character and wrinkled as a recessive character.

This regular reappearance of the hidden trait was of course a notable contribution to heredity theory but still this doesn't clarify how many factors were involved in the determination of any trait? For understanding the principle laying behind, Mendel further inbreeded F2 plants and found that wrinkled plants in all generation for which the experiment was carried out gave the wrinkled this concludes that there was no smooth factor within them. 

On the other hand, F2 that appears smooth did not always breed true. Out of 565 self-fertilized smooth plants only 193 breed true to smooth i.e. they produce all smooth seed in all the generation for which the experiment was carried out. Rest of 372 produced smooth and wrinkled in 3:1 ratio. In other words F2 smooth plants were of two types, "pure" smooth producer and "hybrids" or mixed smooth wrinkled producer.

From the result it is clear hybrid smooth plants producing both smooth and wrinkled must contain a factor for both smooth and wrinkle determination. The simplest assumption therefore, is that a hybrid plant for seed shape contain two factors, S and s. for the argument to be consistent the pure smooth and pure wrinkled producer must also contain the two factor as well SS and ss respectively.

In case of hybrid containing both S and s factor, during their gamete formation some gamete may carry S and others may carry s i.e. there are two kind of equally frequent pollen and there are two kind of equally frequent ova. The random combination of these gametes to form zygotes will account for the observed ratio. Thus Mendel demonstrated that a hybrid between two different varieties possesses both type of parental factors, which subsequently separates or segregates in the gametes. This fundamental law is known as the principle of segregation. 

In modern genetics we use words which are different than used by Mendel. In modern genetics the factor that determines the biological characteristics of an organism is known as a gene. In diploid organism such as a pea plant, gene exists in a pair. The two individual gene in a particular pair is known as alleles. In some cases these alleles are identical i.e. a wrinkled plant carried both alleles s and s. in some cases these two alleles may be different. A hybrid plant contains one allele S and another allele s. therefore the term allele and gene are interchangeably used with the restriction that allele refers only to gene at a particular gene pair. When the gene pair of an organism contains two identical allele then that organism is homozygous fort that gene pair and is known as homozygote. When two different alleles are present in a single gene pair, the organism is heterozygous for that gene pair and known as heterozygote. Recessive characteristics, according to Mendel's experimental findings, only appears in homozygous condition.

In genetics, two terms i.e. genotypes and phenotypes are widely used in order to distinguish the appearances of the organism from the genetic factor that influence it. Phenotype refers to all the manifold biological appearances including chemical, structural and behavioral attributes that we can observe about an organism. Genotype defines only the complements of genetic materials that an organism inherit from their parents. Therefore although the phenotype changes with the time as the appearance of the organism changes, the genotype remains relatively constant except for rare genetic changes known as mutation.

3.5.1. Validation of Mendel's law of segregation:

In the law of segregation, Mendel has shown that allele of gene pair separates during the process of gamete formation. In order to give evidence for the equal frequency of gamete produced by F1, Mendel has crossed F1 hybrid with their parents.

A cross between the F1 hybrids of a variety with the recessive form of a character in question is known as test cross. Thus in case of seed shape of pea, F1 (Ss) will be crossed with a variety having wrinkled seed (ss). This test will show segregation of a single gene in F1 hybrid producing two types of gametes in equal frequencies. The F1 (Ss) would produce S and s gamete in equal frequencies. The test cross parent ss would produce only one type of gamete i.e. s. Union of the gametes from F1 with those from test cross parent would produce two types of seed: Ss( Round) and ss (wrinkled) in equal frequencies. Mendel found that ratio in the test cross progeny was indeed 1 Round: 1 Wrinkled, indicating that the equal frequency of F1 gamete carrying the S and s alleles. The same result was found in reciprocal crosses too.

Mendel's law of segregation is also validate by selfing of the hybrids or heterozygote. When hybrids were selfed they produced both smooth and wrinkled type of seed. Thus from this also we can conclude that allele of a gene pair don't contaminate each other and they separate at the time of gamete formation.

3.5.2. Testing phenotypes of plant:

The characteristics of the offspring of the Mendel's cross can be predicted through the genotype of the parent if we have a knowledge that which one of the gene is dominant and which one is recessive. As we have seen ss genotype is wrinkle, while Ss and SS individuals are smooth. In case of recessive trait we can determine the genotype of the plant by looking at its phenotype since recessive traits express themselves only in homozygous condition. On the other hand dominant phenotype may be caused by homozygous dominant alleles or heterozygous condition. 

Mendel used two type of test to distinguish between heterozygous dominant and homozygous dominant. First one being the selfing of the smooth plants. SS genotype of course only produce smooth offspring, whereas self-fertilized Ss plants would produce smooth and wrinkled offspring in the ratio of 3:1. Thus the appearances of some wrinkled offspring from the self-fertilize smooth plants is an indication that smooth parents were heterozygote for that character.

3.6. Law of independent segregation of gametes:

Mendel's study of monohybrid cross led his to discover the principle of segregation. By the use of this principle it was possible to predict the segregation of two or more pair of gene. So Mendel crossed two differing in two pair of gene known as dihybrid cross.

In dihybrid cross Mendel crossed plant with smooth and yellow seed with plant having wrinkled and green seeds. As expected all F1 progeny produced was smooth and yellow since these two traits were dominant traits. These smooth yellow F1 progeny was self-fertilized and F2 were obtained. Of total 556 of F2 seeds, 315 were smooth yellow, 108 were smooth green, 101 were wrinkled yellow and 32 were wrinkled green. In term of ration these numbers were very close to 9:3:3:1. How these ratios come? To answer this question we have split the result and analyze them separately with respect to single gene pair.

P1 ……………….. Smooth*Wrinkle                                Yellow*Green

F1 …………………… Smooth                                               yellow

F2……………    423 Smooth: 133 Wrinkled                 416 Yellow: 140 Green

                                Or about                                                 Or about 

                       ¾ Smooth: ¼ Wrinkled                           ¾ Smooth: ¼ Wrinkled

These obtained F2 ratio closely fit the ratio obtained from cross involving single gene pair showing dominance. When we combine these two independent result to single dihybrid cross, we find each other. This means that chance for a plant to be smooth or wrinkle do not interfere for the chance of plants to become Yellow or Green. Mathematically this relationship is expressed by multiplying the individual probability of each trait to occur in the plant. For example, if the seed has the probability of ¾ to become Smooth and ¾ probability to become Yellow then the probability of seed to become smooth and yellow is the multiplication of two probabilities i.e. ¾* ¾ = 9/16. We can therefore obtain the ratio of each phenotypic combination by multiplying the probabilities of the individual phenotypes.

 

¾ smooth* ¾ yellow = 9/16 smooth yellow 

¾ smooth* ¼ green = 3/16 smooth green

¼ wrinkled* ¾ yellow = 3/16 wrinkled yellow

¼ wrinkled*1/4 green = 1/16 wrinkled green

These are the exact ratios that we observed, showing the independent assortment of these two gene pair.

Conclusion: 

The law segregation of gametes and the law of independent assortment given by the Mendel has changed the old concept and thought about the transmission of genetic factors from parents to off-springs. These law at that time was contradictory because it opposed the theory of continuous evolution which was popular and widely accepted by many scientist of that time. So, Mendel's work was not recognized at his time. He became famous after his laws were rediscovered by other scientist only during 1990s. Still in the conventional breeding approach Mendel's law were very useful in predicting the appearance of the particular trait in the offspring.   

4. References:

Collins, A., & Stewart, J. H. (1989). The Knowledge Structure of Mendelian Genetics. The American Biology Teacher, 51(3), 143–149. https://doi.org/10.2307/4448880

Gardner, Simmons, & Snustad. (2006). PRINCIPLES OF GENETICS, 8TH ED. Retrieved from https://books.google.com/books?id=33vi549EIIcC&pgis=1

Gasking, E. B. (1959). Why was Mendel’s Work Ignored? Journal of the History of Ideas, 20(1), 60. https://doi.org/10.2307/2707967

Iltis, H. (2018). Life of Mendel. In Life of Mendel. https://doi.org/10.4324/9780429399794

Mendel, G. (1996). EXPERIMENTS IN PLANT HYBRIDIZATION (1865). Retrieved from http://www.netspace.org./MendelWeb/

Moore, R. (2001). The “Rediscovery” of Mendel’s Work.

Orel, V. (2009). The “Useful Questions of Heredity” before Mendel. Journal of Heredity, 100(4), 421–423. https://doi.org/10.1093/jhered/esp022

Orel, Vitězslav, & Wood, R. J. (2000). Essence and origin of Mendel’s discovery. Comptes Rendus de l’Academie Des Sciences - Serie III, 323(12), 1037–1041. https://doi.org/10.1016/S0764-4469(00)01266-X

Rheinberger, H.-J. (1995). When Did Carl Correns Read Gregor Mendel’s Paper? A Research Note. Isis, 86(4), 612–616. https://doi.org/10.1086/357321

Roberts, H. F. (1919a). Darwin’s Contribution to the Knowledge of Hybridization. The American Naturalist, 53(629), 535–554. https://doi.org/10.1086/279731

Roberts, H. F. (1919b). The Contribution of Carl Friedrich von Gartner to the History of Plant Hybridization. The American Naturalist, 53(628), 431–445. https://doi.org/10.1086/279723

Sapp, J. (1990). The Nine Lives of Gregor Mendel. In Experimental Inquiries (pp. 137–166). https://doi.org/10.1007/978-94-009-2057-6_5

Strickberger, M.W. 1996. Genetics. New Delhi: Prentic-Hall of India Private Limited.

Singh, B. D .2019. Genetics. New Delhi: Kalayani Publisher.

Zwick, M. E., Cutler, D. J., & Chakravarti, A. (2000).  P ATTERNS OF G ENETIC V ARIATION IN M ENDELIAN AND C OMPLEX T RAITS . Annual Review of Genomics and Human Genetics, 1(1), 387–407. https://doi.org/10.1146/annurev.genom.1.1.387