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

Important Terms in Genetics

• Genetics: Study of inheritance, heredity, and variation of characters, or the 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 parents and 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 a 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 a chromosome pair carries dissimilar alleles of a gene. E.g., Tt, Yy, etc.
• Dominant character: The character which is expressed in heterozygous condition. It is indicated with a capital letter.
• Recessive character: The character which is suppressed in heterozygous condition. It is indicated with a 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 the probability of all genotypes of offspring in a genetic cross.

Mendel’s Laws of Inheritance

• Gregor Mendel is the Father of genetics.
• He conducted hybridization experiments on garden peas (Pisum sativum) for 7 years (1856–1863).

Steps in Making a Cross in Pea

• Selection of 2 pea plants with contrasting characters.
• Emasculation: Removal of anthers of one plant to avoid self-pollination. This is the female parent.
• Pollination: Collection of pollen grains from the male parent and transferring to the female parent.
• Collection & germination of seeds to produce offspring.

Mendel’s Selected Seven Pairs of True Breeding Pea Varieties

• 1. Stem height:
  • Dominant: Tall
  • Recessive: Dwarf
• 2. Flower colour:
  • Dominant: Violet
  • Recessive: White
• 3. Flower position:
  • Dominant: Axial
  • Recessive: Terminal
• 4. Pod shape:
  • Dominant: Inflated
  • Recessive: Constricted
• 5. Pod colour:
  • Dominant: Green
  • Recessive: Yellow
• 6. Seed shape:
  • Dominant: Round
  • Recessive: Wrinkled
• 7. Seed colour:
  • Dominant: Yellow
  • Recessive: Green

Select a Topic 👇

Topic 2: Inheritance of One Gene Topic 3: Inheritance of Two Genes Topic 4: Other Patterns of Inheritance Topic 5: Chromosomal Theory, Morgan's Experiment Topic 6: Sex Determination Topic 7: Mutation and Pedigree Analysis Topic 8: Genetic Disorders (Mendelian and Chromosomal)

Download PDF