7.5 Monohybrid Ratio

The phenotypic ratio of different types of individuals occurring in the F2 generation of the monohybrid cross is called the monohybrid ratio. In the Mendelian monohybrid experiments, this ratio was always 3:1( i.e., 75% is dominant and 25% is recessive).

For example, for one of his monohybrid crosses, Mendel selected true breeding homozygous parents showing contrasting characters for the height of the plant. He performed the experiment in three stages as described. The result obtained is shown in Figure 7.2 below.

Figure 7.2 Tall x Dwarf Monohybrid cross showing the result obtained by Mendel

According to Mendel, each sexually reproducing diploid organism possesses two 'factors' (genes) for each character; one factor is received (inherited) from male parent and the other factor is inherited from the female parent. These two factors for a particular character are called alleles or allelomorphs. When an offspring receives identical alleles from both parents, it is called homozygous, pure or true breeding for the character. On the other hand, when the offspring receives dissimilar alleles from two parents, it is called heterozygous, impure or a hybrid for that character.

The pure tall is crossed with the pure dwarf parent. According to Mendel, when the diploid individual (having both the alleles/factors) produces gametes, each gamete receives only one of the two factors/alleles of a character. No gamete receives both the alleles of a character. Thus, pure tall parent produces only one type of gametes, i.e. all the gametes possess only (T) factor for tallness. Similarly, all gametes produced by pure dwarf are of one type only and possess (t) factor. The fusion of (T) and (t) gametes (fertilization) results in the F1 offspring with (Tt) genotype. It is heterozygous or a hybrid. Its phenotype (external appearance) is tall because the factor for tallness (T) is dominant and expresses itself. The factor for dwarfness (t) is present in F1 hybrid but, being recessive, does not express itself (remains hidden).

Mendel allowed hybrids to self-fertilize or inbreed to raise F2 generation. The F1 hybrid has dissimilar alleles (Tt). Therefore, it will produce two types of gametes in equal number i.e. 50% gametes will have (T) factor and remaining 50% will have (t) factor. Since the pea flower is bisexual, it produces both male and female gametes. Thus, the F1 hybrid will produce two types of male gametes (T) and (t) in equal numbers. Similarly, there will be two types of female gametes (T) and (t) in equal numbers. During self fertilization, the fusion between these male and female gametes occurs at random. For example, each type of male gamete has an equal chance to fuse with either (T) or (t) female gametes and vice-versa. This chance fusion, between two types of male and two types of female gametes will produce a maximum of four combinations (genotypes) in the F2 progeny. This is shown in the checker board or Punnet’s Square (Figure 7.2). These four combinations fall into three categories of the genotypes as follows : 1 (TT), 2 (Tt) and 1 (tt) i.e.

1 Pure tall : 2 Hybrid tall : 1 Pure dwarf

      (TT)           2(Tt)                (tt)

This is called 1:2:1 genotypic ratio of a monohybrid cross. However, phenotypically, the progeny shows 3 Tall and 1 Dwarf individuals (75% Dominant and 25% recessive characters) or 3:1 ratio. This is called monohybrid ratio or phenotypic ratio of a monohybrid cross.

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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