Principles of Inheritance and Variation – NEET Notes with Charts & Tricks

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🧬 What Is Inheritance?

Inheritance refers to the transmission of genetic material from parents to offspring.
Variation is the differences in traits among individuals.


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🌱 Mendel’s Experiments

Mendel studied inheritance using garden pea (Pisum sativum) because:

Short life cycle

Clearly visible traits

Easy to cross-pollinate


🧠 Trick to remember: “TASS” – Traits, Annual, Self-pollinating, Simple growth


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🧪 Mendel’s Laws of Inheritance

1️⃣ Law of Dominance

Dominant allele masks recessive allele in heterozygous condition


2️⃣ Law of Segregation

Alleles separate during gamete formation (Anaphase I of meiosis)

No blending of traits


3️⃣ Law of Independent Assortment

Alleles of different genes assort independently



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🌼 Monohybrid Cross (Single Trait)

Parent Genotype TT (Tall) × tt (Dwarf)

F1 Generation All Tt (Tall)
F2 Generation 3 Tall : 1 Dwarf


🧠 Genotypic Ratio = 1:2:1
🧠 Phenotypic Ratio = 3:1


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🌾 Dihybrid Cross (Two Traits)

Traits Involved Seed shape & color (RrYy × RrYy)

F2 Phenotypic Ratio 9:3:3:1 (Round Yellow : Round Green : Wrinkled Yellow : Wrinkled Green)


🧠 Mnemonic: “9-3-3-1 = Dihybrid Deal”


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🧬 Key Genetic Terms

Term Meaning

Gene Unit of inheritance
Allele Alternative form of a gene
Homozygous Identical alleles (TT or tt)
Heterozygous Different alleles (Tt)
Genotype Genetic makeup
Phenotype Observable trait



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🧬 Chromosomal Theory of Inheritance

Proposed by Sutton and Boveri

Genes are located on chromosomes

Chromosomes segregate and assort just like Mendel’s factors



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🧬 Variation: Types

1. Somatic: Not inherited (e.g., body weight, language)


2. Germinal: Inherited through gametes (e.g., eye color)

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