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Modern Genetic Analysis Chapter 4. The Inheritance of Genes Inheritance Patterns of Individual Genes
Genetics In Process 4-4: Mendel, originator of the gene concept

In his 1866 paper "Experiments on Plant Hybridization," the Moravian monk Gregor Mendel laid down the basis for the entire subject of genetics by postulating for the first time the existence of the discrete hereditary determinants we now call genes. The purpose of Mendel's study was summarized by the introductory statement of his paper:

Artificial fertilization undertaken on ornamental plants to obtain new color variants initiated the experiments to be described here. The striking regularity with which the same hybrid form always reappeared whenever fertilization between like species took place suggested further experiments whose task it was to follow the development of hybrids and their progeny.

Mendel's experiments started with pure-breeding lines of garden peas that showed contrasting phenotypes for certain characters. For example, two lines had yellow and green seeds, contrasting phenotypes of the character seed color. Upon crossing these lines he obtained "hybrid" seeds that were all yellow. However, when the hybrids were grown up and self-pollinated, 3/4 of the seeds in the next generation were yellow and 1/4 were green.

Of the 3/4 of the seeds that were yellow, 2/4 were just like the original yellow hybrids, whereas 1/4 were pure-breeding like the original pure-breeding yellow line.

To explain these and similar ratios from contrasting pairs of phenotypes for other characters, he devised the following hypothesis:

1. The difference between yellow and green is caused by differences in discrete hereditary determinants he called factors (what we call "genes" today).
2. The factors exist in pairs, one pair for each character.
3. At gamete formation the members of a pair separate, each into 1/2 of the gametes.
4. Male and female gametes fuse randomly.
5. Yellowness is dominant over greenness, which is recessive.

In his analysis he represented genes with letters, the beginning of the practice still used today. Letting Y stand for the yellow-determining gene, and y for the greendetermining gene, the breeding analysis was presented as follows:

The next generation, after random fertilization of hybrid female by hybrid male gametes, would yield the following results:

Furthermore, by making crosses between lines differing in two characters, he observed 9:3:3:1 ratios in the second generation of descendants. He realized that these were two 3:1 ratios combined randomly, and from this he deduced that the different gene pairs he studied behaved independently at gamete formation.

Therefore, he had elegantly explained the constitution and breeding behavior of his hybrids and, without knowing it, had devised rules of inheritance that were to be shown to apply in virtually all eukaryotic organisms.

© 1999 by W. H. Freeman and Company. All rights reserved.