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CBSE • Class X • Science • Ch 8

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Heredity

In Class 10 Science, "Heredity" provides an authoritative, curriculum-verified master resource aligned with the 2026–27 NCERT syllabus.

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🧬 Have You Ever Wondered?

Why do children inherit their father's eye color or their mother's curly hair, but neither parent can pass on a surgically altered nose? Gregor Mendel...

Why do children inherit their father's eye color or their mother's curly hair, but neither parent can pass on a surgically altered nose? Gregor Mendel's pea plant experiments unlocked the universal genetic code of inheritance.

Why This Chapter Matters

In Class 10 Science, "Heredity" provides an authoritative, curriculum-verified master resource aligned with the 2026–27 NCERT syllabus.

Before You Begin (Prerequisites)

Asexual vs sexual reproduction.
Chromosomes and DNA from Class 9.
Variation in populations.

What You Will Learn (Core Objectives)

Explain Gregor Mendel's choice of Garden Pea (Pisum sativum) and seven pairs of contrasting traits.
Analyze Mendel's Monohybrid Cross: Law of Dominance and Law of Segregation ($3 : 1$ phenotypic, $1 : 2 : 1$ genotypic ratio).
Analyze Mendel's Dihybrid Cross: Law of Independent Assortment ($9 : 3 : 3 : 1$ phenotypic ratio).
Distinguish between Dominant and Recessive alleles and Genotype vs Phenotype.
Explain Sex Determination in human beings through XX and XY sex chromosomes.

Chapter Roadmap & Progression

1 1. Mendel's Monohybrid Cross ($3 :...

2 2. Dihybrid Cross ($9 : 3 : 3 : 1$)

3 3. Genetic Sex Determination in Hum...

Complete Concept Guide (100% Curriculum Coverage)

1. Mendel's Monohybrid Cross ($3 : 1$)

Mendel crossed pure tall ($TT$) and pure dwarf ($tt$) pea plants. In the $F_1$ generation, all offspring were Tall ($Tt$) because the tall trait is Dominant. Self-pollinating $F_1$ yielded the $F_2$ generation with:
• Phenotypic Ratio: $\mathbf{3\text{ Tall} : 1\text{ Dwarf}}$
• Genotypic Ratio: $\mathbf{1\text{ TT} : 2\text{ Tt} : 1\text{ tt}}$

2. Dihybrid Cross ($9 : 3 : 3 : 1$)

Crossing round-yellow seeds ($RRYY$) with wrinkled-green seeds ($rryy$) produced a novel mix in $F_2$: Round-Yellow (9), Round-Green (3), Wrinkled-Yellow (3), Wrinkled-Green (1), proving the Law of Independent Assortment!

3. Genetic Sex Determination in Humans

Human somatic cells have 23 pairs of chromosomes: 22 pairs of autosomes and 1 pair of sex chromosomes. Females have identical XX chromosomes; males have XY chromosomes.
• If an egg ($X$) is fertilized by an $X$-bearing sperm → Baby Girl (XX).
• If an egg ($X$) is fertilized by a $Y$-bearing sperm → Baby Boy (XY).
Scientific Truth: Genetic sex is determined strictly by the father's sperm, never by the mother!

Visual Learning & Conceptual Map

Heredity Master Matrix

Conceptual framework, core mechanisms, and analytical relationships

Academic Architecture

1. Mendel's Monohybrid Cross ($3 : 1$) • 2. Dihybrid Cross ($9 : 3 : 3 : 1$)

Chapter Summary & 10 Key Takeaways

Takeaway 1

Gregor Mendel: Father of Genetics who discovered laws of inheritance using pea plants.

Takeaway 2

Dominant vs Recessive: Dominant traits express in heterozygous state ($Tt$); recessive need homozygous ($tt$).

Takeaway 3

Monohybrid Ratio: $3 : 1$ phenotypic; $1 : 2 : 1$ genotypic.

Takeaway 4

Dihybrid Ratio: $9 : 3 : 3 : 1$ demonstrating independent assortment.

Takeaway 5

Sex Determination: 50% statistical chance; determined by father's X or Y sperm.

Check Your Understanding (Diagnostic Practice Questions)

Diagnostic questions testing core conceptual clarity. Answers are hidden initially — solve each problem first, then click to reveal the step-by-step verified solution.

Why did Gregor Mendel choose the garden pea plant (Pisum sativum) for his genetic experiments?

Reveal Answer & Explanation

Answer: Pea plants have clear distinct contrasting traits, short lifespans, produce large numbers of seeds, and can easily be cross-pollinated or self-pollinated.
Distinct contrasting traits and short life.

A cross between a tall pea plant (TT) and a short pea plant (tt) resulted in progeny that were all tall. Why?

Reveal Answer & Explanation

Answer: Because the allele for tallness ($T$) is dominant over the allele for shortness ($t$); in the heterozygous $Tt$ state, only the dominant trait expresses.
Tallness is the dominant trait.

Explain how the sex of a child is determined genetically in human beings.

Reveal Answer & Explanation

Answer: Females produce eggs carrying only X chromosomes. Males produce sperm carrying either an X or a Y chromosome. Fertilization by an X-sperm produces a girl (XX); fertilization by a Y-sperm produces a boy (XY).
Determined by X or Y sperm from father.

What is the genotypic ratio of a monohybrid cross in the $F_2$ generation?

Reveal Answer & Explanation

Answer: $1 : 2 : 1$ ($1\text{ TT} : 2\text{ Tt} : 1\text{ tt}$).
1:2:1.

Can an acquired trait (such as weight loss due to starvation) be inherited by offspring? Explain.

Reveal Answer & Explanation

Answer: No, acquired traits do not alter the DNA sequences in reproductive germ cells (gametes); only genetic alterations in germ cells can be passed to future generations.
Does not alter germ cell DNA.

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