Gregor Mendel used the common garden pea in his experiments the results of which became the basis of the science of genetics. It was not by accident that it became his experimental plant. Even from the start, he was already aware that the right experimental plants must be used in order to avoid the “risk of questionable results”. Accordingly, he was descriminate in the selection of testplants for his study.
1. They must possess constant differentiating characters. The parents in each cross differ in one set of character (example smooth or round or roundish seeds vs. wrinkled seeds) and each parent maintains the character in the succeeding generations;
2. The hybrids of such plants must, during the flowering period, be protected from the influence of all foreign pollen, or be easily capable of such protection. The experimental plants must be protected, either as a natural trait or artificially, from pollen contamination; and
3. The hybrids and their offspring must maintain fertility in the fullest in the succeeding generations. Stated another way, both partial and complete sterility must be inexistent. This is because the outcome of the hybridization needs to be strictly monitored from generation to generation.
Thus he procceeded in identifying the most qualified experimental plants. He focused on the members of the family Leguminosae (now alternatively called Fabaceae) because of their peculiar floral structure. Their flowers are clearly visible and the parts, particularly the pistil and the stamen, are easily distinguishable. Finally, he identified the genus Pisum to have possessed the essential qualities.
For two years he preexperimented on 34 varieties of garden pea (mainly Pisum sativum) which he obtained from several seedsmen with more or less distinct characteristics. However, only 22 exhibited constant characteristics. From these he selected the experimental plants (7 P1 plants and 7 P2 plants) for his studies that differed in seven characters.
1. It exhibited characters that are constant and easily
recognizable such as texture of seed, height or stature, color of
specific plant organs, etc. ;
2. Some forms of the genus exhibited contrasting traits such as smooth vs. wrinkled seeds, short vs. tall height, axillary or terminal flowers, etc.;
3. Hybrids and their offspring are fertile, that is, continuous cross- and self-fertilizations were possible and the seeds maintained viability;
4. It is highly naturally self pollinated because the reproductive parts of the flower are covered by the keel (petals) which only opens after pollination has been completed. This phenomenon is now called cleistogamy;
5. These plants are easy to grow either on the ground or in pots;
6. The plants have short maturity; and
7. Although the cross pollination methods are somewhat complicated, the results are largely successful. The procedure consisted of the opening of the young bud, removal of the keel, removal of the stamen using forcep (emasculation), and dusting of the stigma with the pollen from the specified parent plant.
Mendel mentioned some possibilities that might have caused error in the results (Rook 1964). He said that insects, notably the beetle Bruchus pisi, open the flowers of garden pea to lay eggs inside. In so doing, some pollen adhere to their tarsi. This could lead to unintended pollination of some flowers that they may visit. However, he did not mention thrips and this, Prof. W. Bateson noted as he edited the translation of Mendel’s original paper for the Royal Horticultural Society of London, was an error.
Second, he mentioned that in rare cases certain parts of a flower wither resulting to the partial exposure of the reproductive organs. This can lead to the introduction of foreign pollen.
Third, the keel may not be fully developed which may also partially expose the reproductive organs.
Fourth, it was observed that sometimes the pollens are not fully developed. In this case the pistil gradually lengthens until the tip of the stigma protrudes from the keel. However, Mendel observed that out of more than 10,000 plants examined, there occured only a few cases of contamination or “false impregnation” of foreign pollen.
Nevertheless, despite the meticulousness of Gregor Mendel, it is possible that his selection of garden pea as experimental plant was also attended by some element of luck. As already mentioned in the main page, he attempted to show that his derivations are also true to other plants. If only it became successful, it would have possibly lead to the timely recognition of his findings. Unfortunately, he selected the hawkweed as his second experimental plant, an apomict . As such, the offsprings are clones of the plant from which the seeds are derived. There is no fertilization and therefore only the traits of the maternal parents are transmitted (click to read the significance of apomixis in plant propagation).
ROOK A (ed.). 1964. The Origins and Growth of Biology. Harmondsworth, Middlesex: Penguin Books, Ltd. p. 294-311.
(Last edited Nov. 8, 2013)