Email Page  Printer Friendly

In 1856 Gregor Mendel began an extensive series of experiments upon culinary peas, with the aim of determining general laws governing the development of specific traits in hybrid species.

Gregor Mendel ©Copyright California Institute of Technology All Rights Reserved

A monk who had studied agriculture and botany, Mendel selected 22 varieties of peas to cross-breed in the garden of the monastery where he lived and worked. He chose to follow seven distinct traits—length of stem, seed-coat color, and so on—over the course of eight years. When his experiments were completed, he had examined some 10,000 plants. Mendel worked with care, meticulously recording his results and quantifying them.

In 1865 Mendel delivered two long lectures that were published in 1866 as "Experiments in Plant Hybridisation." This paper established what eventually became formalized as the Mendelian laws of inheritance:

• The law of independent assortment. Specific traits operate independently of one another. A pea plant might have a stem that is tall or short, but in either case may produce white or gray seed coats.

When Mendel cross-bred true-breeding purple-flowered peas with white-flowered peas, all of the offspring bloomed purple. When he cross-bred the offspring, three fourths bloomed purple and one fourth bloomed white. ©2000 GNN Mary S. Gibbs

• The law of independent segregation. Inherited characteristics (such as stem length in Mendel's pea plants) exist in alternative forms (tallness, shortness)—today known as alleles. For each characteristic, an individual possesses two paired alleles—one inherited from each parent. Correspondingly, these pairs segregate (or separate) in germ cells and recombine during reproduction so that each parent transmits one allele to each offspring.

• The law of dominance. For each characteristic, one factor is dominant and appears more often, in a definite 3:1 ratio. The alternative form is recessive. In Mendel's peas, tallness was dominant, shortness recessive. Therefore, three times as many plants were tall as were short. This constant ratio represents the random combination of alleles during reproduction. Any combination of alleles that includes the dominant allele will express that form of the trait.

Mendel's laws represent a theory of particulate inheritance that describes how the germ cells of most organisms transmit characteristics from one generation to the next. Discrete "factors" (as Mendel called them) represent, as it was ultimately understood, specific genes.

The significance of Mendel's work was not recognized in his lifetime. His good luck with peas did not hold up when he tried to work with milkweed, which reproduces in an unusual way. Mendel abandoned his researches and eventually became abbot of his monastery in the ancient Moravian city of Brünn—today Brno, in the Czech Republic. There he died in 1884.

Mendel's paper remained obscure for about 35 years. But in 1900 it was rediscovered. Independently, Hugo de Vries of The Netherlands, Carl E. Correns of Germany, and Erich Tschermak of Austria each found in Mendel's "Experiments in Plant Hybridisation" confirmation of his own research. Mendel's meticulous experiments and powerful mathematical model ensured his priority of discovery.

Back to GNN Home Page

Genome News Network is an editorially independent online publication of the J. Craig Venter Institute. © 2000 - 2004 J. Craig Venter Institute. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.