PRINCIPLES OF INHERITANCE AND VARIATION Notes NCERT

Genetics

Genetics is a branch of biology dealing with inheritance and variation of characters from parents of offspring.

Inheritance

Process by which characters are passed on from parent to progeny

Variation

Degree by which the progeny differs from its parents

Mendel’s Experiments

• Gregor Johann Mendel known as the father of genetics proposed the laws of inheritance.
• He used garden pea as his sample.
• Large sampling size gave credibility to his collected data.
• Garden pea plant possessed certain completely opposite traits.

Example − tall and dwarf plants

• He worked on the following seven traits of garden pea:

S. No.	Character	Dominant	Recessive
1	Stem height	Tall	Dwarf
2	Flower colour	Violet	White
3	Flower position	Axial	Terminal
4	Pod shape	Inflated	Constricted
5	Pod colour	Green	Yellow
6	Seed shape	Round	Wrinkled
7	Seed colour	Yellow	Green

 

• True breeding pea lines were obtained by continuous self pollination for several generations.
• Fourteen true breeding pea lines were selected as pairs, which were similar except for one character with contrasting traits.
• Artificial cross pollination (hybridisation) was performed on such varieties to obtain first hybrid generation known as the first filial progeny or F1.

Inheritance of One Gene

• After hybridisation, the F1 generation so obtained resembled only one of its parents (say, all tall; no dwarf).
• When 2 plants from F1 generation were self pollinated, the second filial progeny or F2 generation was obtained.
• Revival of unexpressed trait (dwarf) was observed in some F2 progeny. Both traits, tall and dwarf, were expressed in F2 in ratio 3:1.
• Mendel proposed that something is being passed unchanged from generation to generation. He called these things as ‘factors’ (presently called genes).
• Factors contain and carry hereditary information.
• Alleles − Slightly different form of same factor

Two alleles code for a pair of two contrasting traits. (e.g., tall and dwarf)

Monohybrid CrossCross that considers only

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