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PinkMonkey Online Study Guide-Biology

Mendel conducted his experiments in three stages.

. Stage-1 : It involved selecting a pair of parents with contrasting characters and obtaining each parent plant in pure condition ,i.e,. breeding true for the characters selected.

For example, Mendel ensured that the plant selected for round seeds produced only round seeds on self-fertilization and the plant selected for wrinkled seeds produced only wrinkled seeds . Such pure, true-breeding parents were obtained by Mendel through repeated self fertilizing, generation after generation.

Stage -2 : It involved crossing of the selected parents. Of the pair, one plant was used as the male parent and the other as the female parent. Pollen from the male was dusted on the stigma of the female parent for cross-pollination. Mendel conducted reciprocal crosses also.

For example : in one cross, the round seeded variety was used as the male parent and the wrinkled seeded variety as the female parent, while in the reciprocal cross of the same parents, the wrinkled variety was used as the male parent and the round seeded variety as the female parent. This may be represented as follows :

                                   Male Parent          Female Parent 
Cross - I	                Rounded seeded      Wrinkled seeded   
Reciprocal Cross   Wrinkledd seeded     Rounded seeded

Such a cross between two parents representing contrasting forms of a single character is called monohybrid cross and the offspring is called a hybrid. The hybrid represents the first filial generation or F1 generation.

Stage - 3 : In the third stage, Mendel allowed each F1 hybrid to self-pollinate and produce the next, i.e., Second Filial generation or F2 generation.

Mendel conducted a similar type of hybridization experiment separately for each of the seven pairs. He meticulously maintained a complete record of the actual number of each type of offspring in every generation (i.e., data of qualitative as well as quantitative results).

Mendel observed that in each of these crosses, all the F1 hybrids resembled only one parent, while the character of the other parent was not seen in the F1 hybrid. For example, in a cross between round X wrinkled type, the F1 were all round seeds only. The wrinkled character was not seen. The character which appears in the F1 hybrid was termed dominant and the other as recessive by Mendel. Thus, in each of the seven pairs, one form is dominant and the other is recessive (See Table 1.1). The F2 progeny showed presence of both parental forms which always appeared in the ratio of 3 dominants : 1 recessive. This 3:1 F2 ratio was termed as the monohybrid ratio.

Table of Contents

7.0 Introduction
7.1 Gregor Mendel
7.2 Mendel's Experiment on Sweet Pea
7.3 Terminology Used
7.4 Law of Dominance
7.5 Monohybrid Ratio
7.6 Law of Segregation
7.7 Dihybrid Ratio
7.8 Law of Independent Assortment
7.9 Test Cross or Back Cross
7.10 The Concept of "Factor"

Chapter 8


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