EduroDownload
Class 12 · NCERT Biology

Chapter 4

Principles of Inheritance and Variation

Use Mendelian genetics, linkage, sex determination, and pedigree ideas. Learn the ideas, see how they connect, and practise until you can choose the next step without help.

MendelInheritancePedigree

Reviewed by Eduro educators · Built for independent practice

Study board

Quick reference
Concept 01Mendel
Concept 02Inheritance
Concept 03Pedigree

NCERT chapter notes

Principles of Inheritance and Variation, clearly explained

This chapter explains how characters pass from parents to offspring, starting with Mendel's experiments on garden pea and his laws of inheritance, then extending to incomplete dominance, codominance, multiple alleles, linkage, sex determination, and pedigree analysis.

It closes with Mendelian disorders (haemophilia, sickle-cell anaemia, thalassaemia) and chromosomal disorders (Down's, Klinefelter's, Turner's syndrome). Crosses, ratios, and disorder inheritance are the most-asked board topics.

Core concepts

Key concepts and formulae

01

Mendel's monohybrid cross and laws

A cross between tall (TT) and dwarf (tt) pea plants gives all tall F₁ (Tt), and the F₂ shows a 3:1 phenotypic and 1:2:1 genotypic ratio. This illustrates the Law of Dominance and the Law of Segregation (alleles separate during gamete formation).

02

Dihybrid cross and independent assortment

A cross involving two traits gives a 9:3:3:1 F₂ phenotypic ratio, demonstrating the Law of Independent Assortment — the alleles of one gene assort independently of the alleles of another.

03

Incomplete dominance and codominance

In incomplete dominance the heterozygote is intermediate (e.g. red × white snapdragon gives pink; ratio 1:2:1). In codominance both alleles express fully in the heterozygote (e.g. AB blood group, where both A and B antigens appear).

04

Multiple alleles and ABO blood groups

A gene can have more than two alleles in a population. The ABO blood group is controlled by three alleles I^A, I^B, and i, where I^A and I^B are codominant and both are dominant over i.

05

Sex determination and linkage

In humans sex is determined by the XX (female) / XY (male) system. Genes located close together on the same chromosome tend to be inherited together (linkage), studied by Morgan in Drosophila.

06

Genetic disorders

Mendelian disorders are caused by single-gene change: haemophilia (X-linked recessive), sickle-cell anaemia (autosomal recessive, a point mutation Glu→Val in the β-globin chain), and thalassaemia. Chromosomal disorders are caused by aneuploidy: Down's syndrome (trisomy of chromosome 21), Klinefelter's (XXY), and Turner's (XO).

Exam practice

Important questions, with worked answers

Use each answer to check the method and wording—not as a paragraph to memorise.

Q1Why did Mendel select the garden pea (Pisum sativum) for his experiments?

Answer

Pea plants have several easily distinguishable contrasting traits, are normally self-pollinating (so pure lines are maintained), have a short life cycle, and produce many offspring, making statistical analysis reliable.

Q2Differentiate between incomplete dominance and codominance with one example each.

Answer

In incomplete dominance the heterozygote shows an intermediate phenotype, e.g. pink flowers in snapdragon (Rr). In codominance both alleles are fully and separately expressed in the heterozygote, e.g. the AB blood group, which shows both A and B antigens.

Q3A man with blood group A marries a woman with blood group B. What are the possible blood groups of their children?

Answer

If the man is I^A i and the woman is I^B i, the children can be of blood group A (I^A i), B (I^B i), AB (I^A I^B), or O (ii) — all four groups are possible.

Q4Why is haemophilia more common in males than in females?

Answer

Haemophilia is X-linked recessive. A male has only one X chromosome, so a single recessive allele expresses the disease. A female needs the recessive allele on both X chromosomes to be affected, which is far less likely.

Q5What is the molecular cause of sickle-cell anaemia?

Answer

It is caused by a point mutation in the gene for the β-globin chain of haemoglobin, where glutamic acid is replaced by valine at the sixth position (GAG → GUG), producing abnormal haemoglobin HbS that distorts red blood cells into a sickle shape.

Q6Name the chromosomal abnormality and the main features of Down's syndrome.

Answer

Down's syndrome is caused by trisomy of chromosome 21 (an extra copy). Affected individuals show short stature, a small round head, intellectual disability, and characteristic facial features.

Q7State Mendel's Law of Segregation.

Answer

The Law of Segregation states that the two alleles of a character separate (segregate) during the formation of gametes, so each gamete carries only one allele of each gene; the alleles reunite at fertilisation.

Fast recall

Key terms to remember

Allele: One of two or more alternative forms of a gene for a character.

Genotype: The genetic makeup (allele combination) of an organism, e.g. Tt.

Phenotype: The observable trait of an organism, e.g. tall.

Test cross: Cross of an individual with a homozygous recessive to find its genotype.

Aneuploidy: Gain or loss of one or more chromosomes, e.g. trisomy 21 in Down's syndrome.

A repeatable method

How to learn Principles of Inheritance and Variation properly

  1. 1

    Start with the chapter promise

    Use Mendelian genetics, linkage, sex determination, and pedigree ideas. Before solving, the student should be able to say what the chapter is trying to teach and which kind of problem it helps solve.

  2. 2

    Build the core vocabulary

    The important words for this chapter are Mendel, Inheritance and Pedigree. Eduro should make the student define each one in simple language, then use it in a question or explanation.

  3. 3

    Move from recognition to recall

    Recognition means the answer makes sense after seeing it. Recall means the student can produce the next step independently. This page is built for recall, because that is what tests reward.

  4. 4

    Close the loop with practice

    A strong study session ends with diagram and process recall, not only reading. The student should solve, review the mistake, and then attempt a similar question before moving on.

Before the test

Where students usually lose marks

Look for the first wrong decision—not only the final wrong answer. That is the mistake worth repairing.

Mistake 1

Knowing Mendel only after seeing the solution

This is the most common hidden gap. The student feels confident while reading, but cannot choose the starting step alone. Eduro should ask a short diagnostic question before explaining the method.

Mistake 2

Treating Principles of Inheritance and Variation as a memory chapter

Even memory-heavy chapters need reasoning. A memorised line becomes fragile when the question changes. The student should explain why the answer works, not only what the answer is.

Mistake 3

Skipping the checking step

sequence confusion usually survives because the student finishes the answer and moves on. Eduro should make review part of the answer: what was asked, what was used, and whether the final response fits.

Try it with Eduro

Practice that builds real confidence

Say it naturally, as you would to a patient teacher. Each prompt below is designed to make understanding visible.

Prompt 1

“Ask Eduro to explain Principles of Inheritance and Variation through Mendel before showing any solved answer.”

Prompt 2

“Create five questions that separately test Mendel, Inheritance and Pedigree.”

Prompt 3

“Give one wrong answer from Principles of Inheritance and Variation and ask the student to find the first incorrect step.”

Prompt 4

“End with a mixed mini-test where Eduro does not reveal which skill is being tested.”

For parents

A better way to ask, “Is the chapter done?”

For Class 12 Biology, a good sign is not that the child says 'Principles of Inheritance and Variation is done.' A better sign is that they can explain Mendel, solve one fresh question, and correct one mistake without panic.

Created by

Eduro Editorial Team

CBSE learning research and AI tutoring design

Reviewed by

Eduro Academic Review

Curriculum and accuracy review

Freshness

Reviewed for the 2025-26 CBSE/NCERT study cycle

Reviewed quarterly and after major CBSE/NCERT updates

Quick help

Common questions

How can I study Principles of Inheritance and Variation for NCERT Class 12?

Start with the NCERT examples, understand the key ideas in Mendel, Inheritance, Pedigree, then practice exercise questions and ask Eduro where you get stuck.

Can Eduro help with Principles of Inheritance and Variation?

Yes. Eduro can explain Principles of Inheritance and Variation step by step, answer follow-up doubts, and help students practice related Biology questions.