Chapter 8 of 13

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Rajasthan (RBSE)Class 10 Science★ 4–5 marks · RBSE Class 10 Science board (Unit — Biology)

Heredity

RBSE Class 10 Science Chapter 8 — how traits pass from parents to offspring. Covers Mendel's experiments, dominant and recessive traits, monohybrid and dihybrid crosses (3:1 and 9:3:3:1), genotype vs phenotype, and sex determination in humans, with Punnett-square worked problems and board-pattern questions.

⏱ 26 min readMedium difficulty✓ Free · NCERT-aligned

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

1Explain Mendel's experiments and dominant vs recessive traits
2Work out monohybrid crosses and the 3:1 ratio using a Punnett square
3State the dihybrid 9:3:3:1 ratio and independent assortment
4Distinguish genotype from phenotype
5Explain sex determination in humans

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Why this chapter matters

A logic-based biology chapter that reliably carries a Punnett-square cross problem and short definitions (dominant/recessive, genotype/phenotype) plus a near-certain sex-determination question.

Before you start — revise these

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

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How do Organisms Reproduce? (Class 10)

Sexual reproduction and the source of variation.

Heredity — RBSE Class 10 (Science)

Children resemble their parents — yet are never exact copies. Why do some traits skip a generation and reappear? A quiet monk named Gregor Mendel, counting pea plants in a garden, uncovered the rules of inheritance long before anyone had seen a gene. This chapter is his elegant logic, plus how a baby's sex is decided.

1. Traits, genes and variation

Offspring inherit traits (characters) from both parents. The instructions are carried by genes on chromosomes; each trait is controlled by a gene that may exist in different forms (alleles). Sexual reproduction reshuffles these, giving variation.

2. Mendel's experiments (garden pea)

Mendel crossed pure-breeding pea plants differing in one trait (e.g. tall × short).

F₁ generation: all offspring showed one trait (tall) — the dominant trait; the hidden one (short) is recessive.
F₂ generation (self-pollinate F₁): both traits reappeared in the ratio 3 : 1 (3 tall : 1 short).

This showed traits are controlled by pairs of factors (alleles), one from each parent, and that a recessive trait is masked but not lost.

3. Monohybrid cross (one trait)

Represent tall as T (dominant), short as t (recessive). Pure tall = TT, pure short = tt.

TT × tt → all Tt (tall) in F₁.
Tt × Tt → TT : Tt : tt = 1 : 2 : 1 (genotype); tall : short = 3 : 1 (phenotype).

Genotype = the genetic makeup (TT, Tt, tt). Phenotype = the visible trait (tall/short). Tt is tall because T is dominant.

4. Dihybrid cross (two traits)

Crossing plants differing in two traits (e.g. round-yellow × wrinkled-green seeds) gives an F₂ ratio of $9 : 3 : 3 : 1$ The appearance of new combinations (round-green, wrinkled-yellow) shows the two traits are inherited independently — Mendel's law of independent assortment.

5. Sex determination in humans

Humans have 23 pairs of chromosomes; one pair is the sex chromosomes. Females are XX, males are XY.

The mother's egg always carries X.
The father's sperm carries X or Y (50:50).
Sperm with X → girl (XX); sperm with Y → boy (XY).

So the father's sperm determines the child's sex, and the chance of a boy or girl is 50% each — blaming the mother has no scientific basis.

6. Closing thought

Mendel's insight — traits come in dominant/recessive pairs that segregate and assort independently — explains the 3:1 and 9:3:3:1 ratios that fall straight out of a Punnett square. Keep genotype and phenotype distinct, and remember sex is decided by the father's X/Y sperm. In the RBSE board this chapter reliably sets a cross/Punnett-square problem and short definitions worth 4–5 marks.

Key formulas & results

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

Monohybrid F₂ ratio

phenotype 3:1; genotype 1:2:1

Tt × Tt cross.

Dihybrid F₂ ratio

9:3:3:1

Independent assortment of two traits.

Genotype vs phenotype

TT/Tt/tt vs tall/short

Genetic makeup vs visible trait.

Dominant/recessive

T masks t; tt needed to show recessive

Recessive appears only when homozygous.

Sex determination

mother XX, father XY; sperm X→girl, Y→boy

Father decides sex; 50:50.

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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 allele pair (TT, Tt, tt); phenotype is the visible trait (tall, short). Tt is genotype, 'tall' is phenotype.

WATCH OUT

✗ Saying the mother determines the child's sex

✓ The mother always gives X; the father's sperm (X or Y) decides — so the FATHER determines sex.

WATCH OUT

✗ Writing the F₁ as a mix of traits

✓ In a monohybrid cross F₁ shows only the DOMINANT trait; both traits reappear in F₂ (3:1).

WATCH OUT

✗ Wrong genotype ratio

✓ F₂ genotype is 1 TT : 2 Tt : 1 tt, giving the 3:1 phenotype (3 tall : 1 short).

WATCH OUT

✗ Mislabelling the dihybrid ratio

✓ Two-trait F₂ is 9:3:3:1, not 3:1; the new combinations show independent assortment.

NCERT exercises (with solutions)

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

Intext questions

Traits, dominance, Mendel's crosses, sex determination

Questions

Ex s

Exercises

Cross problems, genotype/phenotype, applications

Questions

Practice problems

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

Q1EASY· Definition

Define a dominant trait.

▶ Show solution

Step 1 — A dominant trait is the one expressed in the F₁ generation and that masks the recessive allele.

✦ Answer: the trait expressed even when only one allele for it is present.

Q2EASY· Sex

What are the sex chromosomes of a human male and female?

▶ Show solution

Step 1 — Male: XY; Female: XX.

✦ Answer: male XY, female XX.

Q3EASY· Ratio

State the phenotypic ratio of a monohybrid cross in F₂.

▶ Show solution

Step 1 — 3 dominant : 1 recessive.

✦ Answer: 3 : 1.

Q4MEDIUM· Monohybrid

Cross TT × tt and give the F₁ genotype and phenotype.

▶ Show solution

Step 1 — All offspring receive one T and one t → Tt.

Step 2 — T is dominant, so all are tall.

✦ Answer: F₁ all Tt, all tall.

Q5MEDIUM· Genotype/phenotype

Differentiate genotype and phenotype with an example.

▶ Show solution

Step 1 — Genotype = genetic makeup (e.g. Tt); phenotype = observable trait (e.g. tall).

Step 2 — Both TT and Tt are tall (same phenotype, different genotype).

✦ Answer: genotype is the allele pair; phenotype is the visible trait.

Q6MEDIUM· Punnett

Cross two Tt pea plants. Give the genotypic and phenotypic ratios.

▶ Show solution

Step 1 — Punnett of Tt × Tt gives TT, Tt, Tt, tt.

Step 2 — Genotype ratio 1 TT : 2 Tt : 1 tt.

Step 3 — Phenotype ratio 3 tall : 1 short.

✦ Answer: genotype 1:2:1, phenotype 3:1.

Q7HARD· Dihybrid

What F₂ ratio did Mendel get in a dihybrid cross, and what does it prove?

▶ Show solution

Step 1 — The F₂ ratio is 9:3:3:1.

Step 2 — New trait combinations appear, showing the two traits are inherited independently.

✦ Answer: 9:3:3:1, proving independent assortment.

Q8HARD· Sex determination

Explain how the sex of a child is determined and why the father is responsible.

▶ Show solution

Step 1 — Eggs always carry X; sperm carry X or Y.

Step 2 — X-sperm + egg → XX (girl); Y-sperm + egg → XY (boy).

Step 3 — Since the sperm's X or Y decides, the father determines the sex (50:50 chance).

✦ Answer: the father's X or Y sperm decides the sex, with equal probability.

Q9HARD· Recessive

A tall plant is crossed with a short one and all offspring are tall. What are the likely genotypes of the parents?

▶ Show solution

Step 1 — All F₁ tall means tall is dominant and the short parent is tt.

Step 2 — For all offspring to be tall, the tall parent must be homozygous TT.

✦ Answer: TT (tall) × tt (short).

Q10MEDIUM· Variation

How do traits get expressed? Give the role of genes.

▶ Show solution

Step 1 — Genes carry instructions to make proteins (e.g. enzymes) that produce a trait.

Step 2 — Alleles inherited from both parents determine which version of the trait appears.

✦ Answer: genes code for proteins that create traits; inherited alleles decide the version shown.

5-minute revision

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

•Traits are carried by genes (alleles) on chromosomes.
•Mendel: F₁ shows the dominant trait; F₂ gives 3:1.
•Monohybrid genotype ratio 1:2:1, phenotype 3:1.
•Dihybrid F₂ ratio 9:3:3:1 → independent assortment.
•Genotype = allele pair; phenotype = visible trait.
•Humans: female XX, male XY.
•Father's X/Y sperm determines the child's sex (50:50).

Rajasthan (RBSE) marks blueprint

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

Typical chapter weightage: 4–5 marks

Question type	Marks each	Typical count	What it tests
MCQ / very short	1	1	Dominant/recessive, sex chromosomes, ratios
Short answer	2	1	Genotype vs phenotype; monohybrid F₁
Long answer / cross	3	1	Punnett square or sex determination

Prep strategy

Practise drawing Punnett squares for monohybrid crosses
Memorise the 3:1 and 9:3:3:1 ratios
Keep genotype and phenotype clearly separate
Be ready to explain sex determination fully

Where this shows up in the real world

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

Plant and animal breeding

Mendelian ratios guide selective breeding for desired traits.

Genetic counselling

Predicting inheritance of traits and disorders in families.

Medicine

Understanding recessive disorders (e.g. sickle-cell) relies on these rules.

Forensics and ancestry

Inheritance principles underpin DNA-based identity and lineage analysis.

Exam strategy

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

Always draw a labelled Punnett square for cross problems.
State genotype and phenotype ratios separately.
Use clear allele symbols (T/t) and define them.
For sex determination, show both X and Y sperm outcomes.
Quote the correct F₂ ratio (3:1 or 9:3:3:1) explicitly.

Going beyond the textbook

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

Incomplete dominance and co-dominance (e.g. blood groups).
Test cross and back cross.
Linkage and recombination frequency.
Sex-linked inheritance (colour blindness, haemophilia).

Where else this chapter is tested

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

RBSE Class 10 Board (BSER Ajmer)High — a cross problem and sex determination every year

NTSE / state scholarshipMedium — genetics MCQs

NEET FoundationHigh — Mendelian genetics is core NEET biology

Science Olympiad (NSO)Medium — inheritance problems

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Questions students ask

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Yes — RBSE (BSER, Ajmer) prescribes the NCERT Science textbook. In the rationalised syllabus this chapter is 'Heredity' (the evolution content has been removed); RBSE sets its own exam pattern.

Genotype is the pair of alleles an organism carries (e.g. Tt); phenotype is the observable trait (e.g. tall). Different genotypes (TT, Tt) can give the same phenotype.

The mother's egg always carries an X chromosome. The father's sperm carries either X or Y, so whichever fertilises the egg decides whether the child is XX (girl) or XY (boy).

It appears in a dihybrid cross and shows that two different traits are inherited independently of each other (law of independent assortment).

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