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First and Second Laws of Inheritance

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First and Second Laws of Inheritance - Lesson Summary

Gregor Johann Mendel played a vital role in discovering the laws of heredity. He carried out several hybridisation experiments with true-breeding pea lines which had sharply contrasting characters. One such experiment was the crossing of a tall and a dwarf pea plant. Since, the cross was between plants that differed in a single trait, that is, height; in this case, Mendel called it a monohybrid cross. Furthermore, Mendel called the tall and the dwarf plants in this experiment the parental or the ā€˜Pā€™ generation. A monohybrid cross is the study of inheritance of one pair of contrasting characters. Therefore, through his experiments with tall and short pea plants, Mendel found that the character of tallness was more potent over the character of dwarfness in the F one hybrid and that both tallness and dwarfness appeared in a three is to one ratio in the F two hybrid. Mendel called this The Law of Dominance. The law states that in a monohybrid pure line cross between parents with contrasting traits, only one form of the trait, that is, the dominant form, appears in the F one generation and both forms appear in a ratio of three is to one in the F two generation. Also, from his experiments, Mendel observed that the recessive parental trait was expressed without any blending in the F two hybrid. The Law of Dominance states that in a monohybrid, pure line cross between parents with contrasting traits, the dominant form appears in the F one generation and both forms appear in a ratio of three is to one in the F two generation. The Law of Segregation states that alleles do not show any blending but segregate from each other during gamete formation into different gametes.
 
 
After verifying the results of the crossings, it was found that the segregation of alleles is a random process and the chances of a gamete containing either allele is fifty per cent. This is known as the Law of Segregation and it states that alleles do not show any blending but segregate from each other during gamete formation into different gametes. Both the characters governed by these alleles are recovered in the F two generation though one of the characters is not displayed in the F one generation. Therefore, the genotypic ratio is one is to two is to one and, as three-fourths of the plants are tall and one-fourth dwarf, the phenotypic ratio is three is to one. Thus, through his monohybrid cross experiments, Mendel proposed the Laws of Dominance and Segregation.

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