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Mendel's Experiments
- Mendel's Experimental Material. Mendel selected Garden pea ( = Edible Pea, Pisum sativum ; 2n = 14 ) for his experiments.
Advantages of Selecting Pea Plant
- Pure varieties of Pea were available.
- Pea plants showed a num ber of easily detectable contrasting characters.
- The flower structure of Pea is such as to allow controlled breeding. Though plant is self pollinated . but it can be cross bred manually.
- Pea flower normally remains closed and undergoes self - pollina- tion.
- It is an annual plant with short life span and gives results within 3 months
- A large number of seeds are produced per plant.
- The plant is grown easily and does not require after - care except at the time of pollination.
- F1 hybrids are fertile.
Selection of Traits.
- The term character is used for a feature of individual such as flower colour that varies among individuals.
- An inherited character, such as purple or white colour for flowers is called a trait.
- Mendel selected 7 pairs of con trasting traits The traits which always appear in two opposing conditions, one domi nant and other recessive, are called the contrasting traits.
- Three steps were involved in Mendel's experimental technique.
- These were selection of pure parent plants , hybridization of pure plants for F1 generation and self breeding in hybrid plants for F2 and F3 generations.
- Mendel selected 34 varieties for his experi obtaining pure varieties.
- The number was ments. He allowed them to self breed for reduced to 22 and finally Mendel selected 7 pairs of pure or true breeding varieties of pea as the starting material for his experiments On self pollination or self breeding, a pure variety gives rise to offspring having similar trait, e.g. , tall variety with tall offspring, a violet flowered variety with violet flowered offspring, etc.
- All the characters of selected varieties had easily distinguishable alternate traits, e.g. , tallness and dwarfness, violet flow ers and white flowers. Mendel satisfied himself as to true - breeding nature of the variety through self - pollination.
- Any offspring not true to the form of the trait was eliminated.
- True breeding plants were then used for the next step. They formed the parent (P) generation.
2. Hybridisation of Pure Plants For F Generation
- Crossing or mating of two varieties of plants or animals is known as hybridisation.
- Plants with contrasting traits were cross pollinated. 50 % of flowers of same plant were made to function as female parent while remaining 50 % were made male parent for that trait.
- Cross pollination involved dusting off pollen grains of one trait over the stigma of second trait. The various steps were as follows.
- Emasculation. Emasculation means removal of male sex organs.
- In the flowers which were to function as female parent, stamens (male reproductive organs of a flower stamen consists of two parts filament and anther ) were removed before the pollen grains matured.
- The pistils (female reproductive organs - pistil has three parts, stigma, style and ovary) were also removed from those flowers which were to function as male parent.
- Bagging. The flowers were covered with paper bags in order to avoid contami nation from foreign pollens.
- Dusting. Pollens were collected from flowers selected to function as male parent They were dusted with the help of fine brush over the stigma of emasculated flowers. Soon after dusting , the flowers were covered.
- Tagging. Tags (labels) were attached to the plants indicating the crosses.
- Crosses. Mendel performed separate crosses involving traits of one character, two characters and three characters which were respectively called monohybrid crosses, dihybrid crosses and trihybrid crosses.
- Collection of Seeds. The seeds of the cross or crosses were collected and som next year. The hybrid offspring including the seeds constitute the first generation. The first generation of Mendel's cross was named the first filial generation, indicated by the symbol F1, by Bateson and Saunders in 1902. Filial refers to offspring.
- Self breeding in Hybrid plants for F2 and F3 Generations.
- The plants of generation were allowed to perform self pollination (sibcrossing or selfing).
- In order to avoid contamination from foreign pollens, the flowers were covered with paper bags from the beginning.
- Mendel collected the seeds and raised a new generation of plants.
- The seeds and plants raised from them constitute the second filial or F generation.
- Further self pollinatin produced F3 or third filial generation.
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