mendels experiment

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05 HEREDITY AND EVOLUTION

LAW OF INHERITANCE: MENDEL’S CONTRIBUTION

 

Genetic characteristics or traits from parents are acquired by their offspring through inheritance. Both parents equally contribute to the inheritance of traits in humans. Gregor Mendel was the first scientist to make a systematic study of patterns of inheritance. He is known as the father of genetics. He observed the pattern which involved the transfer of characteristic from parents to progeny. Mendel chose pea plants for his experiment to unravel the concept of inheritance. Mendel chose pea plants for his experiment because:

  • Had number of clear cut difference which were easy to tell apart
  • Short life span of the plant
  • Large number of seeds produced
  • Carry out both self-fertilization and cross fertilization

 

The observations resulted in the discovery of three laws of inheritance, famously known as Mendel’s laws of Inheritance: Law of Dominance, Law of Segregation and Law of Independent Assortment.

 

MONOHYBRID INHERITANCE

 

In the first experiment, only single character (plant height) was considered and was known as monohybrid inheritance. Monohybrid cross is the cross between two pea plants with one pair of contrasting characters.

 

Mendel made the following observations:

 

 

On  crossing of gametes of tall pea plant with the gametes of dwarf plant by the process of fertilization.Zygotes are formed, which then produce various progeny in the F1  generation (or first generation). This consists of all tall plants.

All of the F1 generation are tall, since all the plants Iof the F1 generation have the factors Tt.

(ii)Now on crossingtwo hybrids i.e. the tall pea plants (Tt) produced in the first generation (F1) with each other, they will produce second generation (F2) pea plants.

 

 

Here, the pea plants produced have genotype or inheritance factors TT, Tt, Tt and tt.

Observations:

  • Now, the plants having genotype TT, Tt and Tt all contain the factor T for dominant trait ‘tallness’ so all the three plants (TT, Tt and Tt) are tall.
  • The plant having the genotype tt has both factors t for the recessive trait ‘dwarfness’, so it is a dwarf plant.
  • The genotypic ratio in monohydrid cross will be TT : TT : tt = 1 : 2 : 1

 

 

DIHYBRID INHERITANCE AND THE LAW OF INDEPENDENT ASSORTMENT

 

Another experiment was based on two characters (seed shape and color), thus called dihybrid inheritance.In dihybrid inheritance, two pairs of contrasting characteristics or traits are crossed.

 

Mendel made the following observations:

 

(i) Mendel first crossed pure-bred pea plants having round-yellow seeds with pure-bred pea plants having wrinkled-green seeds. Mendel observed that only round-yellow seeds were produced in the first generation. There were no wrinkle-green seeds obtained in the F1 generation.

 

 

(ii)Then Mrndel cross-bred the pea plants of F1 generation pea plants round-yellow seeds by self-pollination., This resulted in the second generation(or F2). Here, four different combinations of shape and colour of seeds were obtained

 

 

 

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