Principles of Inheritance and Variation | Class 12 Biology Notes

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🔹 What is Inheritance?

Inheritance is the process by which characters or traits are transferred from one generation to the next.
Example: Eye color, blood group, flower color in pea plants.


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🔹 What is Variation?

Variation is the difference in characters among individuals of the same species.
Example: Height differences in humans, different seed shapes in pea plants.

👉 Inheritance ensures similarities, while variation ensures diversity.

High CPC keywords: inheritance in biology, variation in biology, NEET genetics notes, class 12 genetics


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🔹 Mendel – Father of Genetics

Gregor Mendel performed experiments on pea plants (Pisum sativum) and explained how traits are inherited.

Why pea plant?

Short life cycle

Easily distinguishable traits

Can self-pollinate and cross-pollinate



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🔹 Mendel’s Experiments

1. Monohybrid Cross (One trait studied):

Example: Tall (TT) × Dwarf (tt)

F1 generation: All Tall (Tt)

F2 generation: Ratio 3 Tall : 1 Dwarf


👉 Mendel’s Law of Dominance


2. Dihybrid Cross (Two traits studied):

Example: Round Yellow (RRYY) × Wrinkled Green (rryy)

F2 generation: Ratio 9:3:3:1


👉 Mendel’s Law of Independent Assortment



High CPC keywords: Mendel’s laws, monohybrid cross, dihybrid cross, law of dominance, law of segregation


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🔹 Mendel’s Laws of Inheritance

1. Law of Dominance – One factor (allele) dominates over the other.


2. Law of Segregation – Alleles separate during gamete formation.


3. Law of Independent Assortment – Traits are inherited independently.




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🔹 Important Terms

Gene: Unit of inheritance.

Allele: Alternative form of a gene (T/t).

Genotype: Genetic makeup (TT, Tt, tt).

Phenotype: Physical expression (Tall, Dwarf).

Homozygous: Same alleles (TT or tt).

Heterozygous: Different alleles (Tt).



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🔹 Variations Beyond Mendel

Mendel’s laws were not enough to explain all traits. Later, scientists discovered:

Incomplete Dominance: Neither allele is completely dominant.
Example: Red (RR) × White (rr) → Pink (Rr).

Co-dominance: Both alleles express equally.
Example: Blood group AB (IAIB).

Multiple Alleles: More than two alleles control a trait.
Example: Blood groups (IA, IB, i).

Polygenic Inheritance: Traits controlled by many genes.
Example: Human skin color, height.


High CPC keywords: incomplete dominance, codominance, multiple alleles, blood groups inheritance, polygenic inheritance


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🔹 Human Genetics and Inheritance

Blood Groups (ABO system): Determined by multiple alleles.

Sex Determination: XX = Female, XY = Male.

Genetic Disorders:

Mendelian Disorders: Hemophilia, Sickle cell anemia, Cystic fibrosis.

Chromosomal Disorders: Down’s syndrome, Turner’s syndrome, Klinefelter’s syndrome.




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🔹 NEET & Exam Tips

Practice Punnett squares for crosses.

Revise laws of inheritance with examples.

Don’t forget incomplete dominance and co-dominance – frequently asked in NEET.

Learn human genetic disorders with causes.


👉 Example NEET Question:
Q: A cross between red (RR) and white (rr) flowers gives pink (Rr). This is an example of?
Ans: Incomplete Dominance.


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Final Words

The Principles of Inheritance and Variation explain how traits pass from one generation to another and why variations occur. For Class 12 Biology exams and NEET preparation, this chapter is scoring if you practice diagrams, genetic crosses, and terms thoroughly.

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