![]() Principle or Law of Independent Assortment: Based upon it, Mendel proposed a second set of generalisations (postulate) which is now called law of independent assortment. It was found that inheritance of one pair of alleles (one character) does not interfere in the inheritance of other pair of alleles (second character). This helped him to understand inheritance of two genes (i.e., two pairs of alleles) at a time. To verify his results of monohybrid crosses, Mendel also crossed pea plants differing in two characters (di-hybrid cross). It is also known as law of non-mixing of alleles. ![]() As a result gametes are always pure for a character. Some workers like Bateson call the principle of segĀregation as the principle of purity of gametes because segregation of the two mendelian factors of a trait results in gametes receiving only one factor out of a pair. ![]() The principle of segregation is the most fundamental principle of heredity that has uniĀversal application with no exception. Their random fusion results in the following: Since, only one of the two factors passes into a gamete, 50% of the male and female gametes formed by F 1 plant possess the factor for tallness while the remaining 50% carry the factor for dwarfness. This is possible only when (a) The two mendelian factors present in the F 1, plants segregate during gamete formation, (b) Gametes carry a single factor or allele for a character, 50% of one type and 50% of the second type, (c) The factors get distributed randomly in the offspring due to random or chance fusion of gametes during fertilization. F 2 generation consists of three types of plants- pure tall, hybrid tall and dwarf. (iii) F 2 generation is produced by self breeding of the F 1 plants. As they carry the gene complement of Tt, the fusing gametes must bring in only one factor each (T from TT and t from tt parent). (ii) F, plants are a product of fusion of male and female gametes. Therefore, F 1 plants are genetically hybrid, in the above case Tt. (i) Though F 1 plants show only one alternative or dominant trait of a character, it actually carries factors or alleles of both the traits of the character because the second alternative or recessive trait appears in the F 2 generation. Hence, the factor for tallness is dominant over the factor for dwarfness. However, the factor for dwarfness is unable to express itself in the presence of factor for tallness. Therefore, in F 1 plants both the factors for tallness and dwarfness are present. That the factor for dwarfness is present in F 1 plants can be tested by self breeding them when individuals of F 2 generation will be both tall and dwarf in the ratio of 3: 1. ![]() All plants of F, generation are tall (height 1.2-2.0 m) though they have also received a factor for dwarfness. ![]() Cross the two and raise their progeny called first filial or F, generation. Take two Pea plants, one pure or homozygous tall (height 1.2-2.0 m) and the other pure or homozygous dwarf (height 0.25-0.5m Fig. In each case he found that one expression or trait of the character, (e.g., T or tallness in case of height) is dominant over the other expression or trait of the character. Mendel experimented with Pisuin sativum for seven characters only. A corresponding small or lower case letter is assigned to the recessive factor, e.g., T (tallness) and t (dwarfness). It is given a capital or upper case letter of the alphabet. The letter symbol refers to the dominant factor. ![]()
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