Mendel’s Experiment and Contribution

Gregor Johann Medel aimed at finding how traits are passed from one generation to another.

• At that time it was believed that the traits of the parents were mixed and appeared in the progeny.
• But a series of experiments conducted by him, helped to understand how traits were passed.

Characteristics of a pea plant

Mendel conducted his experiments on garden peas ( Pisum sativum ) .

But why did he choose pea plant?

This is because:

• They are easy to grow
• They can breed (produce offsprings) at a very fast rate
• They have many easily observable characteristics

Mendel took pea plants with different characteristics of the same criteria ;

• He took a tall plant and a short plant and crossed them (made them to mate).
• He calculated the percentage of short and tall variants produced in the next generation.

Let us understand how this happens.

• Mendel took a purebred tall and a purebred short plant for his experiment.

NOTE - A purebred or true-breeding organism always passes down a particular phenotypic trait to its offspring over many generations.

• Genotype of the plants:
• Tall plant - TT - Homozygous dominant trait
• Short plant - tt - Homozygous recessive trait

• First, the chosen parents were made to pollinate;
• The homozygous tall parent is capable of passing down only 1 form of allele ‘T’ which represents tall plants.
• The homozygous short parent is capable of passing down only 1 form of allele ‘t’ which represents short plants
• In the F ₁ generation (Filial 1 - first generation), all plants get one copy of ‘T’ and one copy of ‘t’ from each parent and have heterozygous dominant genotypes ‘Tt’.
• These plants are heterozygous tall plants.
• He also observed that no ‘in-between’ traits were observed . Either the plants were tall or short. They were not of medium height.

• Then, the F ₁ tall plants were self pollinated ;
• In this cross, the plants in the F ₁ generation pass the traits to the F ₂ (Filial 2 - Second generation).
• All plants in the F ₁ generation are heterozygous tall .
•  This means they can pass down a copy of ‘T’ allele or ‘t’ allele to the offspring .
• There are three possibilities:

1. If both plants in F ₁ generation produce ‘T’ allele;

• • • The plant in F ₂ generation will be homozygous dominant .
    • Genotype : TT
    • Phenotype: Tall plant

2. If one plant in F ₁ generation produces ‘T’ allele and other produces ‘t’ allele;

• • • The plant in the F ₂ generation will be heterozygous dominant .
    • Genotype : Tt
    • Phenotype: Tall plant

1. 1. 1. If both plants in F ₁ generation produce ‘t’ allele;

• • • The plant in the F ₂ generation will be homozygous recessive .
    • Genotype : tt
    • Phenotype: Short plant

3. If both plants in F ₁ generation produce ‘t’ allele;

• • • The plant in the F ₂ generation will be homozygous recessive .
    • Genotype : tt
    • Phenotype: Short plant

• • Now let us find out the ratio; 
    • In F ₁ generation , all plants were tall and heterozygous (Tt)
    • But in the F ₂ generation we saw different genotypes and phenotypes.

• • • Genotypes - TT, Tt, tt
      • Genotypic ratio → TT : Tt : tt → 1 : 2 : 1
      • This implies:

1 Homozygous Dominant : 2 Heterozygous Dominant : 1 Homozygous dominant

• • Phenotypes - Tall, Short
    • Phenotypic ratio → Tall : Short → 3 : 1
    • This implies:

3 Tall : 1 Short