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| Monohybrid Inheritance |
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| In the initial set of experiments, Mendel concentrated only on the pattern of inheritance of a single pair of contrasting characters. This pattern of inheritance involving only one pair of contrasting characters is known as monohybrid inheritance. |
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| In the first set of experiments, Mendel conducted cross-pollination between a pure breeding tall plant and a pure breeding dwarf plant. He collected the seeds from this cross pollination and allowed them to germinate. All the resulting plants were found to be tall. |
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| fig. 18.7 Mendel's Experiments with Tall and Short Plants |
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| In a similar pollination between a pure breeding plant with axial flowers and a pure breeding plant with terminal flowers, all the resulting plants of the next generation produced only axial flowers. |
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| fig. 18.8 Plants with Axial and Terminal Flowers |
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| Similar results were obtained with reference to all the pairs of contrasting characters. Based on these results, Mendel came to the conclusion that in a cross-involving two contrasting characters, only one character expresses itself in the next generation. Mendel called the character, which expressed as dominant character and the character, which failed to express, as recessive character. This idea came to be known as the principle of dominance (first law). |
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| At this stage, Mendel wanted to know whether the tall plants resulting from a cross between tall and dwarf plants, were similar to the tall plants of the P1 generation. Hence, he allowed the tall plants of the F1 generation to undergo self-pollination. In the next generation, Mendel found both tall plants and dwarf plants, approximately in the ratio 3:1. The results were most surprising since the recessive character dwarfness had reappeared in the next generation. (F2 generation) |
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| fig. 18.9 Monohybrid Inheritance |
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| From the results, it was clear that the tall plants of the F1 generation were different genetically from the tall plants of the P1 generation. Similar results were obtained by Mendel for the other contrasting characters also. Based on these results, Mendel came to the conclusion that certain factors are involved in the expression of each of these contrasting characters. He presumed that if the F1 tall plants on self pollination could give rise to both tall and dwarf plants, this plant should have contained two factors, one responsible for tallness and the other responsible for dwarfness. Similarly, he presumed that the P1 tall plants also should have contained two factors, both responsible for tallness. He represented these ideas by using the letters of English alphabet to represent the factors. He represented the factor for dominant character by a capital letter and the factor for recessive character by a small letter. |
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| Eg : Factor for tallness ------------T |
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| Factor for dwarfness -------- t |
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| Based on these results. Mendel arrived at another conclusion, which is known as the second law of inheritance or law of segregation or the law of purity of gametes. The law states that: In a cross involving a pair of contrasting characters, the factors responsible for the two opposite characters stay together in the F1 generation but segregate (separate) during the formation of gametes. |
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| Certain specific terminologies are now used to describe the various conditions in any cross involving opposite characters. |
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 Homo Heterozygous Condition is a condition, which describes the nature of a plant based on its genetic composition. A homozygous plant is one, which has both the factors responsible for the same characters. Such a plant produces only one type of gametes. |
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| Example: |
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| A pure breeding tall plant of the P1 generation can be described as Homozygous tall plant : It carries two factors both of which are responsible for tallness (TT). Similarly, the dwarf plant of the P1 generation can be described as homozygous dwarf plant. It carries two factors, both responsible for dwarfness (tt) |
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| Heterozygous condition is a condition in which a plant carries two factors, one responsible for the dominant character and the other responsible for the recessive character. A heterozygous plant produces more than one type of gamete. |
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| Example: |
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| Tall plant of F1 generation (Tt). It carries two factors, one responsible for tallness (T) and the other responsible for dwarfism(t) |
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 Phenotype is the term used to describe the external expression of a particular character. |
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 Genotype is the term used to describe the actual genetic composition. |
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 The phenotypes of two plants may be similar but their genotypes may be different. |
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| Example: |
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| The tall plants of both P1 and F1 generation have the same phenotype, both are tall plants. However, their genotypes are different. The tall plant of the P1 generation is a homozygous tall plant whereas the tall plant of the F1 generation is heterozygous tall plant. |
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| Thus, in the monohybrid inheritance, the F2 generation ratio is of two types phenotypic ratio (3:1) for dominant and recessive characters and genotypic ratio (1:2:1) for the homozygous dominant, heterozygous dominant and homozygous recessive conditions. |
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