Principles of Inheritance and Variation - Notes | Class 12 | Part 1: Mendel's Experiments

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5. PRINCIPLES OF INHERITANCE AND VARIATION

IMPORTANT TERMS

  • Genetics: Study of inheritance, heredity and variation of characters or Study of genes and chromosomes.
  • Inheritance: Transmission of characters from parents to progeny. It is the basis of Heredity.
  • Variation: Difference between parents and offspring.
  • Character: A heritable feature among the parents & offspring. E.g. Eye colour.
  • Trait: Variants of a character. E.g. Brown eye, Blue eye.
  • Allele: Alternative forms of a gene. E.g. T (tall) and t (dwarf) are two alleles of a gene for the character height.
  • Homozygous: The condition in which chromosome pair carries similar alleles of a gene. Also known as pure line (True breeding). E.g. TT, tt, YY, yy etc.
  • Heterozygous: The condition in which chromosome pair carries dissimilar alleles of a gene. E.g. Tt, Yy etc.
  • Dominant character: The character which is expressed in heterozygous condition. It indicates with capital letter.
  • Recessive character: The character which is suppressed in heterozygous condition. It indicates with small letter.
  • Phenotype: Physical expression of a character.
  • Genotype: Genetic constitution of a character.
  • Hybrid: An individual produced by the mating of genetically unlike parents.
  • Punnett square: A graphical representation to calculate probability of all genotypes of offspring in a genetic cross.

MENDEL’S LAWS OF INHERITANCE


Gregor Mendel is the Father of genetics.

He conducted some hybridization experiments on garden peas (Pisum sativum) for 7 years (1856-1863).

Steps in making a cross (Deliberate mating) in pea:
  • Selection of 2 pea plants with contrasting characters.
  • Emasculation: Removal of anthers of one plant to avoid self-pollination. This is female parent.
  • Pollination: Collection of pollen grains from the male parent and transferring to female parent.
  • Collection & germination of seeds to produce offspring.
Mendel selected 7 pairs of true breeding pea varieties:

7 Characters

Contrasting Traits

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

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