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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
    1.  
    •  

 

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. 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

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CA Maninder Singh is a Chartered Accountant for the past 12 years and a teacher from the past 16 years. He teaches Science, Economics, Accounting and English at Teachoo