Chapter 18 of 23

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Tamil Nadu (TNBSE)Class 10 Science★ 5-9 marks in the TN SSLC Class 10 Science public exam through MCQs, cross diagrams and short/long answers

Genetics

Samacheer Kalvi (TN State Board) Class 10 Science — Biology Chapter 18, Genetics. Mendel's experiments and laws, the monohybrid cross with genotype and phenotype ratios, dominant and recessive traits, DNA and genes, sex determination, and mutation. Notes with crosses, common mistakes, practice and quick revision.

⏱ 20 min readMedium difficulty✓ Free · NCERT-aligned

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By the end of this chapter you'll be able to…

1Define heredity and variation
2State Mendel's laws of dominance and segregation
3Work out a monohybrid cross and its ratios
4Distinguish genotype and phenotype
5Explain sex determination and mutation

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Why this chapter matters

Heredity explains how traits pass from parents to offspring through Mendel's laws and genes. The monohybrid cross and its 3:1 and 1:2:1 ratios are classic, high-scoring questions in the TN SSLC exam.

Before you start — revise these

A 5-minute refresher here will save you 30 minutes of confusion below.

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Chapter 17 — Reproduction in Plants and Animals

Gametes and fertilisation.

Genetics — Class 10 Science (Samacheer Kalvi)

TN State Board (Samacheer Kalvi) Class 10 Science, Biology — Chapter 18. How characters are passed from parents to offspring.

1. About this chapter

This chapter covers heredity — Mendel's laws, the monohybrid cross, dominant and recessive traits, DNA and genes, sex determination, and mutation.

2. Mendel and his laws

Heredity is the transfer of characters from parents to offspring; variation is the differences among them. Gregor Mendel worked on pea plants.
Law of dominance: in a pair of contrasting characters, only the dominant one is expressed in the first generation.
Law of segregation: the two factors (alleles) of a character separate during gamete formation and one passes to each gamete.

3. The monohybrid cross

Cross of tall (TT) × dwarf (tt) pea plants:
- F₁ generation: all Tt (tall) — only the dominant trait shows.
- F₂ generation: TT : Tt : tt = 1 : 2 : 1 (genotype); tall : dwarf = 3 : 1 (phenotype).
Genotype = genetic makeup (TT, Tt, tt); phenotype = visible trait (tall/dwarf).

4. DNA, genes and sex determination

DNA carries genetic information; a gene is a segment of DNA that controls a character.
Sex determination in humans: females XX, males XY. The sperm decides the sex of the child (X → girl, Y → boy).

5. Mutation

A mutation is a sudden, heritable change in the DNA/gene. Mutations create variation and can be a raw material for evolution; some cause disorders.

6. Common mistakes

Mistake: Confusing genotype and phenotype. Fix: Genotype = genetic makeup (TT/Tt/tt); phenotype = visible trait.
Mistake: Saying the F₂ phenotype ratio is 1:2:1. Fix: 1:2:1 is the genotype ratio; the phenotype ratio is 3:1.
Mistake: Thinking the mother determines the child's sex. Fix: The father's sperm (X or Y) determines it.

7. Practice (book-back style)

Define heredity and variation.
State Mendel's law of segregation.
In a monohybrid cross, give the F₂ genotype and phenotype ratios.
How is sex determined in humans?
What is a mutation?

8. Answer key

Heredity: transfer of characters from parents to offspring; variation: differences among individuals.
The two alleles of a character separate during gamete formation, one entering each gamete.
Genotype 1 : 2 : 1 (TT : Tt : tt); phenotype 3 : 1 (dominant : recessive).
Females are XX and males XY; the sperm (X or Y) determines the child's sex.
A sudden, heritable change in the DNA or a gene.

9. Quick revision

Biology Ch 18 · genetics, Mendel, DNA, sex determination.
Mendel (pea): laws of dominance and segregation.
Monohybrid F₂: genotype 1:2:1, phenotype 3:1.
Genotype = makeup (TT/Tt/tt); phenotype = visible trait.
Sex: XX (female), XY (male); sperm decides sex. Mutation = sudden heritable DNA change.

Key formulas & results

Everything you need to memorise, in one card. Screenshot this for revision.

Law of dominance

dominant trait masks the recessive in F₁

Mendel's first principle.

Law of segregation

alleles separate during gamete formation

One allele per gamete.

Monohybrid F₂ ratios

genotype 1:2:1, phenotype 3:1

From Tt × Tt.

Sex determination

XX female, XY male

Sperm (X/Y) decides the sex.

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Common mistakes & fixes

These are the exact errors that cost students marks in board exams. Read them once, save yourself the trouble.

WATCH OUT

✗ Confusing genotype and phenotype

✓ Genotype is the genetic makeup (TT/Tt/tt); phenotype is the visible trait.

WATCH OUT

✗ Saying the F₂ phenotype ratio is 1:2:1

✓ 1:2:1 is the genotype ratio; the phenotype ratio is 3:1.

WATCH OUT

✗ Thinking the mother determines the child's sex

✓ The father's sperm (X or Y) determines it.

NCERT exercises (with solutions)

Every NCERT exercise from this chapter — what it covers and how many questions to expect.

Multiple choice questions

MCQs on Mendel, ratios and sex determination.

Questions

Cross / short answer

Monohybrid cross, genotype/phenotype, mutation.

Questions

Long answer questions

Mendel's laws and sex determination.

Questions

Practice problems

Try each one yourself before tapping "Show solution". Active recall > rereading.

Q1EASY· Concept

Define heredity and variation.

▶ Show solution

Heredity is the transfer of characters from parents to offspring; variation is the differences among individuals.

Q2MEDIUM· Concept

State Mendel's law of segregation.

▶ Show solution

The two alleles of a character separate during gamete formation, so each gamete receives only one allele.

Q3HARD· Cross

In a monohybrid cross (Tt × Tt), give the F₂ genotype and phenotype ratios.

▶ Show solution

Genotype 1 : 2 : 1 (TT : Tt : tt); phenotype 3 : 1 (tall : dwarf).

Q4MEDIUM· Concept

How is sex determined in humans?

▶ Show solution

Females are XX and males XY; the sperm carrying X gives a girl and Y gives a boy, so the father's sperm determines the sex.

Q5EASY· Concept

What is a mutation?

▶ Show solution

A sudden, heritable change in the DNA or a gene.

Q6EASY· Concept

Differentiate genotype and phenotype.

▶ Show solution

Genotype is the genetic makeup (e.g. Tt); phenotype is the observable trait (e.g. tall).

5-minute revision

The whole chapter, distilled. Read this the night before the exam.

•Biology Chapter 18 of Samacheer Kalvi Class 10 Science.
•Mendel (pea plant): laws of dominance and segregation.
•Monohybrid F₂: genotype 1:2:1, phenotype 3:1.
•Genotype = makeup (TT/Tt/tt); phenotype = visible trait.
•Sex: XX female, XY male; the sperm decides the sex.
•Mutation = sudden heritable change in DNA/gene.

Tamil Nadu (TNBSE) marks blueprint

Where the marks come from in this chapter — so you can plan your prep.

Typical chapter weightage: 5-9 marks across MCQ, cross and short/long answers

Question type	Marks each	Typical count	What it tests
MCQ	1	1-2	Mendel, ratios, sex determination
Cross / Short	2-3	1-2	Monohybrid cross and ratios
Long Answer	3-5	0-1	Mendel's laws / sex determination

Prep strategy

Practise the monohybrid cross with a Punnett square
Memorise 1:2:1 (genotype) and 3:1 (phenotype)
Separate genotype and phenotype
Learn human sex determination (XX/XY)

Where this shows up in the real world

This chapter isn't just an exam topic — it lives in the world around you.

Genetic counselling

Understanding inheritance helps predict and explain genetic disorders.

Plant and animal breeding

Mendel's principles guide the breeding of better varieties.

Understanding variation

Heredity and mutation explain diversity within species.

Exam strategy

Battle-tested tips from teachers and toppers for this chapter.

Draw a Punnett square for crosses
Quote both F₂ ratios correctly
Keep genotype and phenotype distinct
State sex determination clearly

Going beyond the textbook

For olympiad aspirants and curious learners — topics that build on this chapter.

Work out a dihybrid cross and its 9:3:3:1 ratio.
Explain how mutations contribute to evolution.

Where else this chapter is tested

CBSE board isn't the only one — other exams test this chapter too.

TN SSLC Class 10 Public ExamHigh

Foundation / NTSE BiologyMedium

School unit testsHigh

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Questions students ask

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Pea plants have clear contrasting traits, a short life cycle, and can be easily self- or cross-pollinated, making them ideal for studying inheritance.

The mother always contributes an X chromosome, while the father's sperm carries either X or Y; the sperm therefore decides whether the child is a girl (XX) or boy (XY).

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