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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%
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 Punnetís 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
The result indicates that even though the recessive character
was not seen in hybrid, it was present there and reappeared in
pure form in 25% individuals of the progeny. This result also
enabled Mendel to conclude that the two factors (alleles) come
together in the hybrid but do not mix or fuse with each other.
They simply remain together without diluting or contaminating
each other. In other words, factors maintain their purity.
As the gamete always receives only one factor (alleles) for a
trait, it is always pure for the character. This is called purity
Table of Contents7.0 Introduction
7.1 Gregor Mendel
Mendel's Experiment on Sweet Pea
Law of Dominance
Law of Segregation
Law of Independent Assortment
Test Cross or Back Cross
The Concept of "Factor"