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The History of Genetics - A Farmer's Son Becomes The Father Ofgenetics Study

mendel plants tall generation

Gregor Mendel was born on July 22, 1822, into a peasant family in a small town in Heinzendorf, Austria (now the Czech Republic), and spent much of his youth working in his family's orchards and gardens. (See Figure 1.2.) At the age of twenty-one he entered the Abbot of St. Thomas, an Augustinian monastery, where he studied theology, philosophy, and science. His interest in botany (the scientific study of plants) and aptitude for natural science inspired his superiors to send him to the University of Vienna to study to become a science teacher. However, Mendel was not destined to become an academic, despite his abiding interest in science and experimentation. In fact, the man who was eventually called the "father of genetics" never passed the qualifying examinations that would have enabled him to teach science at the highest academic level. Instead, he instructed students at a technical school. He also continued to study botany and conduct research at the monastery, and from 1868 until his death in 1884 he served as its abbot.

From 1856 to 1863 Mendel conducted carefully designed experiments with nearly 30,000 pea plants he cultivated in the monastery garden. He chose to observe pea plants systematically because they had distinct, identifiable characteristics that could not be confused. Pea plants were also ideal subjects for his experiments because their reproductive organs were surrounded by petals and usually matured before the flower bloomed. As a result, the plants self-fertilized and each plant variety tended to be a pure breed. Mendel raised several generations of each type of plant to be certain that his plants were pure breeds. In this way, he confirmed that tall plants always produce tall offspring and plants with green seeds and leaves always produce offspring with green seeds and leaves.

FIGURE 1.2
Gregor Mendel. The Library of Congress.

His experiments were designed to test the inheritance of a specific trait from one generation to the next. For example, to test inheritance of the characteristic of plant height, Mendel self-pollinated several short pea plants, and the seeds they produced grew into short plants. Similarly, self-pollinated tall plants and their resulting seeds, called the first or F1 generation, grew to be tall plants. These results seemed logical. When Mendel bred tall and short plants together and all their offspring in the F1 generation were tall, he concluded that the shortness trait had disappeared. But when he self-pollinated the F1 generation, the offspring, called the F2 generation (second generation), contained both tall and short plants. After repeating this experiment many times, Mendel observed that in the F2 generation there were three tall plants for every short one—a 3:1 ratio.

Mendel's attention to rigorous scientific methods of observation, large sample size, and statistical analysis of the data he collected bolstered the credibility of his results. These experiments prompted him to theorize that characteristics, or traits, come in pairs—one from each parent—and that one trait will assume dominance over the other. The trait that appears more frequently is considered the stronger, or dominant, trait, while the one that appears less often is the recessive trait.

Focusing on plant height and other distinctive traits, such as the color of the pea pods, seed shape (smooth or wrinkled), and leaf color (green or yellow), enabled Mendel to record accurately and document the results of his plant breeding experiments. His observations about his pure-bred plants and their consistent capacity to convey traits from one generation to the next represented a novel idea. The accepted belief of inheritance described a blending of traits, which, once combined, diluted or eliminated the original traits entirely. For example, it was believed that crossbreeding a tall and a short plant would produce a plant of medium height.

During about the same time period, Darwin was performing similar experiments using snapdragons, and his observations were comparable to those made by Mendel. Although Darwin and Mendel both explained the units of heredity and variations in species in their published works, it was Mendel who was later credited with developing the groundbreaking theories of heredity.

Mendel's Laws of Heredity

[T]he constant characters which appear in the several varieties of a group of plants may be obtained in all the associations which are possible according to the [mathematical] laws of combination, by means of repeated artificial fertilization.

—Gregor Mendel, "Versuche über Pflanzen-Hybriden" ("Experiments in Plant Hybridization"), 1866

From the results of his experiments, Mendel formulated and published three interrelated theories in the paper "Experiments in Plant Hybridization" (originally published in German in 1866 and first translated into English in 1901). This work established the fundamental tenets of heredity:

  • Two heredity factors exist for each characteristic or trait.
  • Heredity factors are contained in equal numbers in the gametes.
  • The gametes contain only one factor for each characteristic or trait.
  • Gametes combine randomly, no matter which hereditary factors they carry.
  • When gametes are formed, different hereditary factors sort independently.

When Mendel presented his paper, it was virtually ignored by the scientific community, which was otherwise engaged in a heated debate about Darwin's theory of evolution. Years later, well after his death in 1884, Mendel's observations and assumptions were revisited and became known as Mendel's Laws of Heredity. His first principle of heredity, the law of segregation, stated that hereditary units, now known as genes, are always paired and that genes in a pair separate during cell division, with the sperm and egg each receiving one gene of the pair. As a result, each gene in a pair will be present in half the sperm or egg cells. In FIGURE 1.3
Mendel's law of segregation. Hans & Cassidy, Thomson Gale.
other words, each gamete receives from a parent cell only one-half of the pair of genes it carries. Since two gametes (male and female) unite to reproduce and form a new cell, the new cell will have a unique pair of genes of its own, half from one parent and half from the other.

Diagrams of genetic traits conventionally use capital letters to represent the dominant traits and lower-case letters to represent recessive traits. Figure 1.3 uses this system to demonstrate Mendel's law of segregation. The pure red sweet pea and the pure white sweet pea each have two genes—RR for the red and rr for the white. The possible outcomes of this mating in the first generation are all hybrid (a combination of two different types) red plants (Rr)—plants that all have the same outward appearance (or phenotype) as the pure red parent, but which also carry the white gene. As a result, when two of the hybrid first-generation plants are bred, there is a 50% chance that the resulting offspring will be hybrid red, a 25% chance the offspring will be pure red, and a 25% chance the offspring will be pure white.

Mendel also provided compelling evidence from his experiments for the law of independent assortment. This law established that each pair of genes is inherited independently of all other pairs. Figure 1.4 shows the chance distribution of any possible combination of traits. The F1 generation of tall flowering red and dwarf white sweet pea plants produced four tall hybrid red plants with the identical phenotype. However, each one has a combination of genetic information different from that of the original parent plants. The unique combination of genetic information is known as a genotype. The F2 generation, bred from two tall red hybrid flowers, produced four different phenotypes: tall with red flowers, tall with white flowers, dwarf with red flowers, and dwarf with white flowers. Both Figure 1.3 and Figure 1.4 demonstrate that recessive traits that disappear in the F1 generation may reappear in future generations in definite, predictable percentages.

The law of dominance, the third tenet of inheritance identified by Mendel, asserts that heredity factors (genes) act together as pairs. When a cross occurs between organisms pure for contrasting traits, only one trait, the dominant one, appears in the hybrid offspring. In Figure 1.3 all of the first-generation offspring are red—an identical phenotype to the parent plant—though they also carry the recessive white gene.

FIGURE 1.4
Mendel's law of independent assortment. Hans & Cassidy, Thomson Gale.

Mendel's contributions to our understanding of heredity were not acknowledged during his lifetime. When his efforts to reproduce the findings from his pea plant studies using hawkweed plants and honeybees did not prove successful, Mendel was dispirited. He set aside his botany research and returned to monastic life until his death in 1884. It was not until the early twentieth century, nearly forty years after he published his findings, that the scientific community resurrected Mendel's work and affirmed the importance of his ideas.

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