🧬 CBSE · Class 12 · Biology · Chapter 5

Principles of Inheritance
and Variation

Complete chapter resources for CBSE Class 12 Biology — topic breakdown, key concepts, sample questions, previous year board questions, and instant AI question paper generation.

4Topics
6–8Board marks
8Sample questions
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Key Concepts — Chapter 5
  • Monohybrid ratio: 3 : 1 (dominant : recessive)
  • Dihybrid ratio: 9 : 3 : 3 : 1 (two independent traits)
  • Incomplete dominance: F2 = 1 : 2 : 1 (no blending in genotype)
  • Codominance (ABO): IA, IB codominant · i recessive
  • Sex linkage: X-linked recessive — more frequent in males (XY)
  • Hardy–Weinberg: p² + 2pq + q² = 1; p + q = 1
Chapter Overview

What this chapter covers

Chapter 5 of NCERT Class 12 Biology begins with Mendel's laws of inheritance derived from his pioneering experiments on garden peas (Pisum sativum). The chapter establishes the Law of Dominance, the Law of Segregation (purity of gametes), and the Law of Independent Assortment, explaining how traits are transmitted from parents to offspring across generations. Students learn to predict phenotypic and genotypic ratios for monohybrid (3:1) and dihybrid (9:3:3:1) crosses using Punnett squares.

The chapter then explores deviations from Mendelian ratios through incomplete dominance, codominance (illustrated by ABO blood groups and the multiple allele system), polygenic inheritance, and pleiotropy. Sex determination mechanisms in humans (XX/XY), birds (ZW/ZZ), and grasshoppers (XO), along with sex-linked traits — especially haemophilia and colour blindness — are high-frequency board topics. The chromosomal theory of inheritance, Sutton and Boveri's contribution, and Morgan's work on linkage and crossing over in Drosophila are covered as extensions of Mendelian principles.

The chapter concludes with the Hardy–Weinberg principle, which describes allele frequency equilibrium in an ideal population and lists the five forces (mutation, gene flow, genetic drift, non-random mating, natural selection) that disturb it. This section bridges classical genetics and population genetics, and is regularly tested in CBSE board exams as a 2-mark conceptual question.

Topics

What's inside Chapter 5

As per NCERT Class 12 Biology (CBSE syllabus)

Topic 1
Mendel's Laws of Inheritance
Experiments on Pisum sativum — choice of traits, monohybrid and dihybrid crosses. Law of Dominance, Law of Segregation, Law of Independent Assortment. Punnett square construction with genotypic and phenotypic ratios.
Topic 2
Inheritance Patterns & Deviations
Incomplete dominance (snapdragon 1:2:1), codominance and multiple alleles (ABO blood groups), polygenic inheritance (skin colour), pleiotropy (sickle cell anaemia), and epistasis — all altering classical Mendelian ratios.
Topic 3
Sex Determination & Sex-Linked Inheritance
XX/XY (humans), ZW/ZZ (birds), XO (grasshopper) sex-determination systems. X-linked recessive traits — colour blindness and haemophilia. Criss-cross inheritance pattern. Pedigree analysis for sex-linked disorders.
Topic 4
Chromosomal Theory, Linkage & Hardy–Weinberg
Sutton–Boveri chromosomal theory, Morgan's linkage and recombination in Drosophila, chromosomal mapping. Hardy–Weinberg equilibrium: p² + 2pq + q² = 1 and the five microevolutionary forces that disturb it.
Connections

How this chapter fits in

Useful for setting question difficulty and cross-chapter papers.

Builds on
Ch 8 · Cell Division (Class 11)
Meiosis produces gametes; segregation of chromosomes is the physical basis of Mendel's laws
Ch 3 · Cell Structure (Class 11)
Chromosomes, nucleus, and chromatin — prerequisite vocabulary for chromosomal theory
Chapter 5 Principles of
Inheritance
Leads to
Ch 6 · Molecular Basis of Inheritance
DNA as the hereditary material; genes as segments of DNA that Mendel's factors map to
Ch 7 · Evolution
Hardy–Weinberg principle directly underpins population genetics and natural selection concepts
Board Blueprint

Marks & question-type breakdown

Typical pattern based on CBSE Class 12 Biology board papers from the last five years.

Question type Marks Typical count What's usually tested
MCQ / Assertion–Reason 1 1–2 Mendelian ratio identification, dominance type, Hardy–Weinberg allele frequency
Very Short Answer 2 1 Define incomplete dominance vs codominance, state Hardy–Weinberg conditions, name sex-linked disorders
Short Answer 3 1 Dihybrid cross Punnett square, ABO blood group crosses, pedigree analysis for haemophilia
Long Answer / Case-Based 5 1 Explain Mendel's laws with experimental evidence, chromosomal theory linkage to Mendelian genetics
Total (approximate) 6–8 4–5 Weightage varies across paper sets and years
Sample Questions

8 sample questions — generated by MarksZen AI

Aligned to CBSE Class 12 Biology Chapter 5. Covers all question types across Easy, Medium, and Hard difficulty.

Q1 Easy 1 mark MCQ
The phenotypic ratio obtained in the F2 generation of a monohybrid cross is: (a) 1 : 1 (b) 1 : 2 : 1 (c) 3 : 1 (d) 9 : 3 : 3 : 1
Q2 Easy 2 marks Short Answer
Distinguish between incomplete dominance and codominance. Give one example of each.
Q3 Medium 2 marks Short Answer
A woman with normal vision whose father was colour blind marries a man with normal vision. What is the probability that their son will be colour blind? Show the cross with genotypes.
Q4 Medium 3 marks Word Problem
In a dihybrid cross between plants with round yellow seeds (RRYY) and wrinkled green seeds (rryy), draw the Punnett square for the F2 generation. State the phenotypic ratio obtained and name the law illustrated.
Q5 Medium 3 marks Short Answer
What is the ABO blood group system? A man with blood group AB and a woman with blood group O have children. List all possible blood groups of their children. Show the cross clearly with allelic notation.
Q6 Hard 4 marks Word Problem
In a population of 10,000 individuals, 9% show the recessive phenotype for a particular trait. Using the Hardy–Weinberg principle: (i) Calculate the allele frequencies p and q. (ii) Calculate the expected number of heterozygous individuals. (iii) State any two conditions required for Hardy–Weinberg equilibrium.
Q7 Hard 5 marks Long Answer
Explain Mendel's Law of Segregation with a suitable example. How did the rediscovery of Mendel's work and Sutton–Boveri's observations help establish the chromosomal theory of inheritance?
Q8 Hard 5 marks Case-Based
Study the following pedigree chart showing the inheritance of haemophilia in a family. Assume all individuals in generation I are carriers or affected as indicated. (i) Identify the pattern of inheritance shown. (ii) Give the genotypes of the affected male in generation II and his mother. (iii) What is the probability that the next child of the carrier female (generation II) will be an affected son? (iv) Why is haemophilia more common in males than females?
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Previous Year Questions

From CBSE board examinations

Actual questions from past Class 12 Biology board papers — Principles of Inheritance and Variation chapter.

Board 20223 marks
In Antirrhinum (snapdragon), a cross was made between plants with red flowers and those with white flowers. The F1 plants all had pink flowers. When F1 plants were self-crossed, the resulting F2 plants showed red, pink, and white flowers. (a) What is the phenomenon illustrated here? (b) Give the ratio of red : pink : white in F2. (c) How is it different from Mendelian dominance? (All India 2022)
Board 20232 marks
A colour-blind man marries a woman with normal vision whose father was colour blind. Work out the cross and state the probability of their children being colour blind. (Delhi 2023)
Board 20205 marks
Explain the law of independent assortment. How did the study of Drosophila by Morgan support the chromosomal theory of inheritance? Mention the significance of linkage in genetics. (CBSE 2020)
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FAQ

Questions teachers ask

How many marks does Principles of Inheritance and Variation carry in the CBSE Class 12 board exam? +
This chapter typically carries 6–8 marks in the CBSE Class 12 Biology board exam, spread across 2–3 questions. Expect one 2-mark short-answer on Mendel's laws or dominance, one 3-mark question on dihybrid cross or codominance, and occasionally a 5-mark question linking chromosomal theory to Mendelian inheritance. The exact weightage shifts by paper set and year.
Which topics from this chapter are highest-probability for the CBSE board exam? +
The highest-frequency board topics are: (1) Dihybrid cross and the 9:3:3:1 ratio — asked almost every year; (2) Codominance and incomplete dominance with human blood group examples; (3) Sex-linked inheritance — colour blindness and haemophilia pedigree questions; (4) Chromosomal theory of inheritance linking Mendel's factors to chromosomes via Morgan's linkage experiments. Punnett square drawing with genotypic and phenotypic ratios is a standard 3-mark ask.
What is the difference between incomplete dominance and codominance? +
In incomplete dominance, the heterozygote shows a blended intermediate phenotype — for example, a cross between red (RR) and white (rr) snapdragons produces pink (Rr) offspring. Neither allele is fully dominant. In codominance, both alleles are fully expressed simultaneously in the heterozygote — the classic example is the ABO blood group system where IA and IB alleles together produce the AB phenotype. The key distinction is blending (incomplete dominance) versus full co-expression (codominance).
How should students draw a Punnett square for a dihybrid cross in board exams? +
Write the parent genotypes (e.g., RrYy x RrYy), list all four gametes for each parent (RY, Ry, rY, ry) along the top and side of a 4x4 grid, then fill in all 16 combinations. State the phenotypic ratio 9:3:3:1 (round yellow : round green : wrinkled yellow : wrinkled green) and the genotypic distribution. Examiners award full marks for a correctly labelled grid — skip it and you lose 1–2 marks even if the ratio is right. CBSE mark schemes explicitly credit the Punnett square diagram.
How do I generate a custom question paper for Principles of Inheritance and Variation using MarksZen? +
Sign up for a free MarksZen account, choose CBSE Class 12 Biology, select Chapter 5 (Principles of Inheritance and Variation), set your preferred question-type mix (MCQ, short answer, case-based) and total marks — the AI generates a complete board-aligned paper with answer key in under 2 minutes, ready for PDF export.