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The below mentioned article will highlight the Mendel’s experiments on heredity including reasons for selecting pea as experimental material.
Mendel’s Experiments on Heredity (Including Reasons For Selecting Pea As Experimental Material):
Combining the art of careful experimentation, founded on clear conceptions of what he wanted to do, with the capacity for deep analysis of the results, Mendel succeeded in discovering those natural laws which have made his name known the world over.
The experiments which Mendel carried out in the garden of the Brunn monastery are basic to the science of genetics.
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He, however, choosed Pisum sativum (garden pea) as material for his classical experiment for several reasons:
1. Plant is annual and short life cycle and it helps to study several generation within very short period.
2. Plant consist bisexual (hermaphrodite) flower, self pollination can be done.
3. Plants are homozygous due to self pollination. It is easy to get pure line for several generation.
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4. Sharply visible seven pairs of contrasting characters are found in the each chromosome of pea plant.
5. Though self pollination takes place artificial pollination can be done by emasculation process to produce hybrid plant.
In all, Mendel collected 34 varieties of peas of which he selected 22 for further investigations. In these varieties he observed several contrasting characters, such as certain varieties were tall and others dwarf; some were having yellow seed coat while others green seed coat, and so on.
Mendel selected the following seven such characters for study:
1. Smooth seed form as against wrinkled seed form.
2. Yellow seed form, as against green seed form.
3. Green cotyledons as against yellow cotyledons.
4. Hard pod as against soft pod.
5. Green pod as against yellow pod.
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6. Axial flowers as against terminally placed flowers.
7. Tall plants as against dwarf plants.
Mendel referred to the contrasting characters as “differentiating characters”. Mendel’s technique of experiments was to cross two plants differing in one of the said contrasting characters. The seeds were planted, and the first generation (F1 generation) plants carefully observed. The hybrid plants were then self fertilized, and the characteristics of their progeny (i.e., F2 generation) carefully classified and counted. The F2 plants were then once more subjected to self-fertilization, and F3 generation obtained.
Mendel’s famous experiments with peas were reported in a paper entitled “Experiments in plant hybridization” in the Proceedings of the Natural History Society (Discussion IV, 1865), in Brunn. For 35 years these pages, now celebrated as a model of concentrated reporting of experiments and keen analysis, rested in obscurity.
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The time, years before the discovery of the mechanism of cell division, chromosomes, and the real process of fertilization, was not yet ripe for the paper in which the gist of these later discoveries was quietly anticipated.
Not until 1900, long after Mendel’s death, were his law rediscovered by Correns in Germany, De Vries in Holland, and Tschermak in Austria, and was Mendel’s original work brought to world attention. The fundamental importance of “Mendelism” was then quickly recognized.
On the Basis of his Carefully Conducted Experiments Mendel Drew Following Conclusions Regarding the Mechanism of Heredity:
1. He pointed out that characteristics appeared in two alternative forms, such as smooth seed or wrinkled seed. (Now we call these “differentiating characters” as allelomorphs or alleles).
2. That every trait in an organism was determined by a single pair of hereditary units. Mendel used the letters A, a, B, b, etc., to represent them (Now these heredity units are known as genes).
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3. That one member of each contrasting pair, the dominant, would completely suppress the other, or recessive, trait in a cross.
4. That the pairs of hereditary units which occur together in a hybrid, separate from each other during the formation of reproductive cells, without in any way being contaminated by their previous association with their opposites. As a result, each reproductive cell has only one such hereditary unit for each trait.
5. That in each generation there is a recombination of the hereditary units carried by the reproductive cell. The combination of these units from the male parent and from the female parent is dependent purely on chance, and therefore, when there are many off-springs, every possible combination of traits is produced.
The aforesaid Mendel’s ideas regarding the mechanism of heredity were expressed in the form of four postulates:
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1. Principle of paired factors (Postulate 1)
2. Principle of Dominance (Postulate 2)
3. Law of Segregation (Postulate 3 or 1st Law of Inheritance)
4. Law of Independent Assortment (Postulate 4 or 2nd Law of Inheritance)
Reasons for the Success of Mendel:
Mendel stands out from all his contemporaries in the clarity of his understanding of these essential experimental requirements.
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Besides the great labour, methodical work and intellectual assessment of the experimental data the following reasons may be listed to account for this great success:
1. Lucky choice of material:
Suitability of Pisum sativum as material for experiments on hybridization has already been told earlier.
2. Study of unit characters:
Mendel thought that organisms are composed of a number of characters which are controlled by factors and the inheritance of any particular factor could be studied. Because of the study of unit character, Mendel automatically avoided several complications of linkage etc., which would have definitely disturbed his results. The object of his experiments was to observe the variations in the case of each pair of differentiating characters and to deduce the law according to which they appear in successive generations.
3. Selection of healthy plants:
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Only healthy plants were chosen by Mendel for his experiments in order to avoid any possibility of uncertainty in results.
4. Reciprocal crosses:
In the reciprocal crosses the variety which served as female parent in the experiment was used as male parent.
5. Control:
In each case control plants were maintained.
6. Introduction of mathematics:
Introduction of mathematics into genetics helped Mendel a lot to postulate definite laws of inheritance on sound basis.
7. Maintenance of pedigree records:
Record of all observations and counts of progeny was properly maintained; this helped Mendel to interpret the results more satisfactorily.
