Mendel, Gregor

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Natural Scientist 1822-1884

Gregor Mendel laid the foundation for the modern understanding of inheritance with his experiments on transmission of traits in garden peas. The ideas he developed are still in use today, and his essential insights into the physical nature of inheritance led directly to the understanding of the gene as a physical entity within the cell.

Mendel was born into a farming family in Heinzendorf, Austria (now Hyncice, Czech Republic). He attended university in Olmutz, but financial difficulties soon persuaded him to enter the Augustinian monastery in Brno, where he received both theological and agricultural training. Mendel remained affiliated with the monastery for the rest of his life. He served briefly as a parish chaplain in the region, and for many years served as a popular and successful teacher at the technical school in Brno. His training in agricultural experimentation, obtained at the University of Vienna, beginning in 1851, prepared him for the experiments that he began in 1856 on peas.

Mendel's experiments were designed to investigate the most widely accepted model of inheritance, blending, which held that the traits of an offspring would be a blend of the parental traits. For example, the theory of blending predicts that a tall and short parent would give rise to a medium-height offspring. Mendel's results showed that for many simple traits, at least, this model was wrong. Instead, the offspring displayed traits in exactly the same form as they appeared in one or the other of the parents.

Mendel chose to study a small group of traits that occur in either of two forms, such as round versus wrinkled pea shape. He began by developing "pure-breeding" lines of each form. In a pure-breeding line, crossing two members gives only offspring that are identical to the parents for that trait. Mendel then crossed pure-breeding parents who had different forms of a trait. For example, he crossed a pea plant that produced only round peas with one that produced only wrinkled peas. All the offspring from this cross-developed only round peas; no wrinkled peas were found. When these off-spring were crossed among themselves, however, both round and wrinkled were observed, in a numerical ratio of three round-pea plants for every one wrinkled-pea plant.

Mendel explained these results by proposing that each visible trait is governed by the presence of two "factors," which may be the same or different in any individual. One of these factors is "dominant," while the other is "recessive." In the above example, the round-producing factor is dominant, and the wrinkled-producing factor is recessive. If two recessive factors are present, the organism will display the recessive trait. If the organism has two dominant factors, or one dominant and one recessive, the dominant trait will be displayed.

To explain the numerical relationships he obtained, Mendel developed the Law of Segregation. He proposed that during the process of egg and sperm formation, the two factors separate, or segregate, so that each egg or sperm contain only one factor. For a parent containing one of each type of factor, this means that half the sperm (or eggs) will contain the dominant factor, and half the recessive factor. During fertilization, these randomly pair up, so that some offspring will have two dominants, some two recessives, and some one of each. Simple algebra shows that the ratio of offspring in such a cross will be 3:1, just as Mendel found.

To show how this works, let 0.5D be the proportion of dominant factors and 0.5r be the proportion of recessive factors. Multiplying (0.5D + 0.5r) times itself gives the offspring ratios, 0.25D² + 0.5Dr + 0.25r². In this expression, 0.25D² indicates that one-quarter of the offspring will have both dominant factors, 0.5Dr means half will have one of each type, and 0.25r²means one-quarter will have both recessive factors. Since both the D² and Dr organisms will show the dominant trait, the ratio of dominant to recessive traits in the offspring will be 0.75:0.25, or 3:1.

Mendel went on to study crosses between peas with multiple sets of traits, such as round seeds plus tall plants crossed with wrinkled seeds plus short plants. He found that the factors for each trait acted independently, so that the offspring of these crosses showed all possible combinations of traits. From the results of these experiments, he formulated his second principle, known as the Law of Independent Assortment, which states that the members of factor pairs assort (segregate) independently of each other during sperm and egg formation, and combine again randomly.

While neither Mendel nor anyone else in his day knew anything about chromosomes or genes, the laws of inheritance he discovered predicted exactly how genes behave on chromosomes during the reproductive process. Indeed, the factors he discovered are genes, which come in pairs and segregate on separate chromosomes during sperm and egg production, just as he suggested. Gene pairs located on different sets of chromosomes will assort independently during the process. While most genes do not exhibit simple dominance-recessiveness relations, and most traits are governed by more than one gene, it is to Mendel's credit that he began by trying to understand simple systems in order to develop generalizable laws.

Mendel published the results of his experiments, "Versuche über Pflanzenhybriden" ("Attempts at Plant Hybridization") in 1866. He did little scientific work after he became abbot of the monastery two years later.His work was ignored by the larger scientific community, in part because it was not published in a widely read journal, and in part because it tackled a problem, the physical basis of heredity, that few other scientists were thinking deeply about at that time.

That changed shortly afterward, when microscopic studies of cells revealed that chromosomes divided when cells divided, provoking speculation that they might be involved in inheritance. Mendel's studies were redis-covered in 1900, sixteen years after his death, by three biologists studying similar phenomena. The importance of his theory of inheritance was immediately recognized and widely accepted, and became the starting point for further investigations of the nature of inheritance that were carried out by Thomas Hunt Morgan, Alfred Sturtevant, and other twentieth-century geneticists. Mendelism, as the theory was called, was merged with Darwinism in the 1930s to form the "New Synthesis," which explained evolutionary theory in modern genetic terms.

Chromosomal Theory of Inheritance, History; Inheritance Patterns; Mendelian Genetics; Nature of the Gene, History; Probability.

Henig, Robin Marantz. The Monk in the Garden: The Lost and Found Genius of Gregor Mendel, the Father of Genetics. Boston: Houghton Mifflin, 2000.

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