NEET / BOARD EXAM NOTES
Principles of Inheritance
and Variation
Genetics • Mendel to Molecular Basis of Heredity
Mendel's Laws Monohybrid & Dihybrid Cross Chromosomal Theory
Linkage & Recombination Sex Determination Mutation Genetic Disorders
Compiled from NCERT Biology (Chapter 4) & Aakash Board & Competitive Exams Module (Level-I)
, Introduction to Genetics
Genetics is the branch of biology that deals scientifically with inheritance and variation of characters from parents to offspring.
DEFINITIONS
Inheritance — the process by which characters are passed on from parent to progeny; it is the basis of heredity.
Variation — the degree by which progeny differ from their parents.
Humans knew as early as 8000–1000 B.C. that variation is hidden in sexual reproduction, and used this to selectively breed plants/animals (e.g., Sahiwal cows in
Punjab) — but had no idea of the scientific basis.
1. Mendel's Laws of Inheritance
Gregor Johann Mendel (born 22 July 1822) worked on Pisum sativum (garden/edible pea) for 7 years (1856–1863) and proposed the laws of inheritance. He
was not the first to hybridize plants — but the first to correctly interpret the results using statistics and mathematics.
Why Pea Plant? AAKASH
1. Many distinct, easily distinguishable contrasting traits.
2. Produces a large number of seeds and completes life cycle in one season.
3. Flowers are self (bud) pollinating → gives true-breeding lines.
4. Easy to perform artificial cross-pollination (emasculation + hand pollination).
REMEMBER
A true-breeding line = a line that, after continuous self-pollination, shows stable trait inheritance for several generations. Mendel initially took 34 pea
varieties, later worked with 22, and finally used 7 pairs from 14 true-breeding varieties.
Seven Contrasting Traits Studied (with Chromosome location)
S.No. Character Dominant Trait Recessive Trait Chromosome No.
1 Seed shape Round Wrinkled 7
2 Seed colour Yellow Green 1
3 Flower colour Violet White 1
4 Pod shape Full/Inflated Constricted 4
5 Pod colour Green Yellow 5
6 Flower position Axial Terminal 4
7 Stem height Tall Dwarf 4
⚡ IMPORTANT NOTES
• Term "Pure line" was coined by Johannsen (1900).
• Term "gene" was given by Johannsen; term "allele" (allelomorphs) was given by Bateson.
• Term "factor" was Mendel's own word — later renamed gene.
Reasons for Mendel's Success
Applied statistical methods and mathematical logic to analyse results.
Kept accurate records — necessary in genetic studies.
Used a large sample size, reducing chances of error.
Tested his theories across successive generations (F₁, F₂, F₃...), proving general rules rather than one-off observations.
2. Inheritance of One Gene (Monohybrid Cross)
Principles of Inheritance
and Variation
Genetics • Mendel to Molecular Basis of Heredity
Mendel's Laws Monohybrid & Dihybrid Cross Chromosomal Theory
Linkage & Recombination Sex Determination Mutation Genetic Disorders
Compiled from NCERT Biology (Chapter 4) & Aakash Board & Competitive Exams Module (Level-I)
, Introduction to Genetics
Genetics is the branch of biology that deals scientifically with inheritance and variation of characters from parents to offspring.
DEFINITIONS
Inheritance — the process by which characters are passed on from parent to progeny; it is the basis of heredity.
Variation — the degree by which progeny differ from their parents.
Humans knew as early as 8000–1000 B.C. that variation is hidden in sexual reproduction, and used this to selectively breed plants/animals (e.g., Sahiwal cows in
Punjab) — but had no idea of the scientific basis.
1. Mendel's Laws of Inheritance
Gregor Johann Mendel (born 22 July 1822) worked on Pisum sativum (garden/edible pea) for 7 years (1856–1863) and proposed the laws of inheritance. He
was not the first to hybridize plants — but the first to correctly interpret the results using statistics and mathematics.
Why Pea Plant? AAKASH
1. Many distinct, easily distinguishable contrasting traits.
2. Produces a large number of seeds and completes life cycle in one season.
3. Flowers are self (bud) pollinating → gives true-breeding lines.
4. Easy to perform artificial cross-pollination (emasculation + hand pollination).
REMEMBER
A true-breeding line = a line that, after continuous self-pollination, shows stable trait inheritance for several generations. Mendel initially took 34 pea
varieties, later worked with 22, and finally used 7 pairs from 14 true-breeding varieties.
Seven Contrasting Traits Studied (with Chromosome location)
S.No. Character Dominant Trait Recessive Trait Chromosome No.
1 Seed shape Round Wrinkled 7
2 Seed colour Yellow Green 1
3 Flower colour Violet White 1
4 Pod shape Full/Inflated Constricted 4
5 Pod colour Green Yellow 5
6 Flower position Axial Terminal 4
7 Stem height Tall Dwarf 4
⚡ IMPORTANT NOTES
• Term "Pure line" was coined by Johannsen (1900).
• Term "gene" was given by Johannsen; term "allele" (allelomorphs) was given by Bateson.
• Term "factor" was Mendel's own word — later renamed gene.
Reasons for Mendel's Success
Applied statistical methods and mathematical logic to analyse results.
Kept accurate records — necessary in genetic studies.
Used a large sample size, reducing chances of error.
Tested his theories across successive generations (F₁, F₂, F₃...), proving general rules rather than one-off observations.
2. Inheritance of One Gene (Monohybrid Cross)
