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| Physical Basis of Heredity |
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| According to Mendel, the genotype is made of certain structures called factors. The factors controlled the inheritance of all traits. These factors are present in pairs. The factors are now called genes. Thus the physical basis of heredity are the genes or factors. The different expressions of the same genes are called alleles. Each trait may be represented with an alphabet. If the alphabet T represents height, T represents tallness and t represents shortness. If the letter R represents the colour of the flower, R represents red and r represents white. |
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| General convention is to represent the dominant character or the character expressed in F, generation by capital letter. |
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| Further, each individual for sexual reproduction produces gametes. Gametes are haploid. Each gamete will have only one allele for each trait. Thus the gametes will be of two kinds - having allele T or t. Since each individual is formed by the fusion of gametes, they are represented by writing two letters - TT, Tt or tt depending on their origin. If the plants are pure breeds producing only one type of offspring, they are called homozygous (TT or tt). If the plants produce both tall and short plants among the offspring, then they are heterozygous (Tt). |
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| Mendle's Monohybrid Ratio |
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| In case of height, the tall plants of P generation are represented as TT and the short plants as tt. Each parent produces only one type of gamete. Tall plants produce gametes having allele 'T' and short plants produce allele having 't' gametes. When these two types of gametes fuse, the resultant plants are heterozygous (Tt). However, only tallness is expressed, as all plants of this generation are tall. Thus, tallness is the dominant allele and shortness is the recessive allele. Thus the recessive allele remains hidden in the first generation and becomes expressed on self-pollination in the next generation. |
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| The F1 generation plants are then self-pollinated. This results in tall and short plants in the ratio 3:1. This is called the phenotypic ratio. Of the tall plants, 1/3rd of the plants is homozygous for tallness and the remaining 2/3rd are heterozygous. The heterozygous plants on self-fertilization again result in offspring that show 3:1 ratio for tallness to dwarfness. Thus the genotypic ratio in the F2 generation is 1:2:1. This means that 1/3rd are tall and homozygous, 2/3rd are tall and heterozygous and 1/3rd are dwarf and homozygous. Note that recessive traits are phenotypically expressed only in the homozygous genotype. |
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| Similar crossing experiments were also carried out with more than one trait. However, it is not possible to follow all the traits at the same time. The other traits studies by Mendel in garden pea were colour of the seed, the shape of the seed, colour of the seed coat, the flower colour, flower position, pod colour and pod shape. |
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| The crosses that study two traits together are called the dihybrid crosses. In the dihybrid crosses, it was found that the two traits were inherited independent of each other. The dominant alleles for each of the two traits asserted their dominance independent of the other. |
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