Mendel Proposes Laws of Heredity Summary

  • Last updated on November 10, 2022

Gregor Mendel’s experiments with pea plants refuted the then-accepted theory of “blending inheritance” and suggested instead that inheritance is particulate. Because he performed extensive cross-fertilizations and produced and examined numerous offspring, Mendel’s laws of heredity constituted the first mathematically robust theory of inheritance.

Summary of Event

The understanding of heredity during the nineteenth century was at a crossroads. Since the days of Greek antiquity, there had been a debate over how offspring were formed. Hippocrates Hippocrates (c. 460-c. 370 b.c.e.) proposed that the seminal fluids of both the male and female contained a collection of particles from all over the body. When these fluids were brought together during copulation, they fused together and became the fetus. This theory is often called “pangenesis” and is the forerunner of the nineteenth century theory of blending inheritance. Mendel, Gregor Genetics;laws of heredity Botany;and laws of heredity[Laws of heredity] [kw]Mendel Proposes Laws of Heredity (1865) [kw]Proposes Laws of Heredity, Mendel (1865) [kw]Laws of Heredity, Mendel Proposes (1865) [kw]Heredity, Mendel Proposes Laws of (1865) Mendel, Gregor Genetics;laws of heredity Botany;and laws of heredity[Laws of heredity] [g]Austria;1865: Mendel Proposes Laws of Heredity[3780] [c]Genetics;1865: Mendel Proposes Laws of Heredity[3780] [c]Science and technology;1865: Mendel Proposes Laws of Heredity[3780] [c]Mathematics;1865: Mendel Proposes Laws of Heredity[3780] Napp, Cyrill Franz

Aristotle Aristotle (384-322 b.c.e.) proposed an alternative model in which menstrual fluid in the female contained particles from throughout her body and semen from the man was the active principle that shaped these particles into a fetus. Later theories dispensed with particles entirely and proposed that the female contained miniature, preformed embryos in her ovaries and that semen from the male acted as nourishment for the developing fetus. A variation of this preformation theory held that the preformed embryo was in the sperm and the woman simply nourished it as it grew. Eventually, the recognition that offspring typically display a combination of traits from both parents removed support for preformation theories and led to an almost universal acceptance of pangenesis, and thus of blending inheritance.

The validity of blending inheritance was considered self-evident. Offspring, for the most part, contain a blending of traits from their parents. Breeders, though, had always been aware of exceptions called “sports.” A sport possessed a new trait not seen in either parent. Rather than seeing this as a challenge to blending inheritance, they assumed the new trait to be a defect resulting from an error in the joining of parental particles. More careful, scientific breeding experiments revealed more troubling challenges to the theory of blending inheritance. Sometimes, when parents had differing traits, their offspring displayed the trait of one parent instead of a blending of the traits of both. Such results awaited an explanation at the time that Gregor Mendel began his experiments in 1856.

Mendel was a university-educated Augustinian novitiate at a monastery in Brno, Austria. Although he never successfully passed the teaching examinations, he had taught natural history and physics at the Oberrealschule in Brno for sixteen years. With this background and the support of his abbot, Cyrill Franz Napp Napp, Cyrill Franz , Mendel began conducting crossbreeding studies on garden peas (Pisum sativum). He was not the first plant breeder to study peas. Others had discovered that crossing certain purebreeding varieties of peas resulted in offspring with just one form of a trait. No one, however, had yet offered an adequate explanation for this result.

Why did Mendel succeed where others had failed? The primary reason was probably his meticulous and careful experiments. During the seven years that he conducted these experiments, he produced and analyzed approximately twenty-eight thousand plants. This large number of experimental results allowed for more accurate mathematical Mathematics;and genetics[Genetics] calculations. He also crossbred plants over several generations, noting the frequency of a given trait in each generation. To make his data easier to manage, he focused on only seven traits that were consistently easy to differentiate. He was also lucky in that the genes for five of the seven traits he tracked are now known to reside on separate chromosomes Chromosomes;and heredity[Heredity] and are therefore not linked. The remaining two traits have their genes on the same chromosome and are therefore linked, but they are so far apart that when a cross-fertilization involving both traits is done, they behave like unlinked genes. Had some of the genes been closely linked, Mendel might have been confused by the progeny ratios he observed.

Mendel’s experiments yielded similar results for all seven of the traits he studied. For example, when he crossbred pea plants grown from round seeds with pea plants grown from wrinkled seeds, all of the resulting offspring produced pods with round seeds. When he allowed these plants to self-fertilize, both round-seeded plants and plants with wrinkled seeds resulted, and there were consistently three times as many round-seeded offspring as there were offspring with wrinkled seeds. These results were expressed by Mendel as a 3:1 ratio of round to wrinkled.

It was Mendel’s interpretation of these results that proved revolutionary. He hypothesized that the inheritance of each trait was controlled by discrete particles that were contributed by each parent. The observable expression of a particular trait in a particular offspring (now known as its phenotype) was determined by which specific particles that offspring had inherited (now referred to as its genotype). When there were different particles, as, for example, in the case of seed shape, one of the particles would be dominant over the other. In the case of seed shape, the particle for round seeds was dominant over the particle for wrinkled seeds.

To account for the patterns he observed, Mendel concluded that the genotype of each plant cell comprised a specific pair of particles controlling the expression of each trait. The original, purebreeding parents had only one type of particle. Thus they contained either two round particles or two wrinkled particles. The first-generation offspring each possessed both one round and one wrinkled particle, but because the round particles were dominant, all first-generation offspring were of the round-seeded phenotype.

Mendel expanded upon this model to produce his law of segregation, which described how the hereditary particles were passed to offspring. Eggs and sperm (gametes) contained only one hereditary particle for each trait. The particles segregated during the formation of gametes, and when the egg and sperm joined together during fertilization, the normal condition of two particles per trait was restored in the offspring.

Mendel went on to perform dihybrid and trihybrid cross-fertilizations, that is, fertilizations crossbreeding two and three contrasting traits, respectively. One of Mendel’s dihybrid crosses was between smooth-seeded, purple-flowered (smooth, purple) plants and wrinkle-seeded, white-flowered (wrinkled, white) plants. The first-generation offspring were all smooth, purple. Allowing these first-generation plants to self-fertilize produced four different phenotypes in the following ratio: 9:3:3:1 (smooth and purple: smooth and white: wrinkled and purple: wrinkled and white). Mendel noted that within the dihybrid ratio, each of the individual traits taken alone still formed a 3:1 ratio. The same observation held true for his trihybrid crosses.

Mendel generalized his conclusions from the dihybrid and trihybrid crosses and formulated his law of independent assortment. This law stated that the particles governing the expression of each trait followed the law of segregation and that each trait was expressed independently of all others. In other words, the pattern of inheritance for one trait had no effect on the inheritance of any other trait. It was in formulating this law that Mendel’s luck in choosing traits controlled by unlinked genes is apparent.

Mendel presented his findings in 1865 at the meeting of the Society for Natural Sciences. They was published in Verhandlungen des Naturforschenden vereines (proceedings of the society for natural sciences) as Versuche über Pflanzenhybriden (1865; Experiments in Plant Hybridization, 1901). His paper was received with little fanfare, and it is likely that few in attendance actually understood what Mendel was proposing. It was rare at the time to use the type of mathematics Mathematics;and genetics[Genetics] and probability calculations that Mendel had in breeding studies, and his conclusions were dependent on understanding the math. Being published in the journal of a small, isolated scientific society meant that few others took notice either.

Mendel died in 1884, and his great discovery of the laws of segregation and independent assortment languished in obscurity, essentially forgotten. His paper was later independently “rediscovered” near the turn of the century by three different biologists: Erich Tschermak von Seysenegg, Carl Correns, and Hugo de Vries. They helped to popularize Mendel’s model, which would form the basis for all modern genetics.

Significance

Considering Mendel as the founder of genetics is entirely appropriate, given that his basic laws are still useful to geneticists in the twenty-first century. Although Mendel had no knowledge of the inner workings of cells and knew nothing of deoxyribonucleic acid (DNA) DNA;and heredity[Heredity] or chromosomes Chromosomes;and heredity[Heredity] , his two laws are entirely consistent with the way genes behave. Consequently, many modern textbook accounts use the language of genes and chromosomes to describe Mendel’s work and findings.

If Mendel’s paper had received wider attention in his day, it is likely that the field of genetics would have expanded several decades earlier than it did. If Charles Darwin had read Mendel’s paper, he might have realized that Mendel’s model of inheritance provided the specific mechanism for natural selection that was missing from Darwin’s own theory. Ironically, Darwin did own a copy of Mendel’s paper, but he never read it. The pages were still uncut. It was left to later generations, then, to acknowledge Mendel’s gift to science.

Further Reading
  • citation-type="booksimple"

    xlink:type="simple">Carlson, Elof Axel. Mendel’s Legacy: The Origin of Classical Genetics. Woodbury, N.Y.: Cold Spring Harbor Laboratory Press, 2004. A history of classical genetics that shows how Mendel’s research laid the primary groundwork for modern genetics.
  • citation-type="booksimple"

    xlink:type="simple">Edelson, Edward. Gregor Mendel: And the Roots of Genetics. Oxford, England: Oxford University Press, 2001. Part of the Oxford Portraits in Science series, this basic introduction to Mendel explains how he developed his laws of genetics.
  • citation-type="booksimple"

    xlink:type="simple">Henig, Robin Marantz. The Monk in the Garden: The Lost and Found Genius of Gregor Mendel, the Father of Genetics. New York: Mariner Books, 2001. A recent biography of Mendel that gives insights into the founder of genetics and discusses the “lost and found” nature of his discoveries.
  • citation-type="booksimple"

    xlink:type="simple">Sturtevant, A. H. History of Genetics. Reprint. Woodbury, N.Y.: Cold Spring Harbor Laboratory Press, 2001. Classic text by a geneticist who was present for many of the genetic discoveries of the first half of the twentieth century. Provides a comprehensive history, beginning before Mendel, that places the Austrian monk’s work in its larger scientific context.
  • citation-type="booksimple"

    xlink:type="simple">Wood, Roger J., and Vitezslav Orel. Genetic Prehistory in Selective Breeding: A Prelude to Mendel. Oxford, England: Oxford University Press, 2001. Focuses on the developments in animal and plant breeding from 1700 to 1860; a good introduction to the ideas surrounding inheritance when Mendel began his work.

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