Heredity is Chapter 8 of CBSE Class 10 Science. This chapter explains how traits are passed from parents to offspring, the rules of inheritance discovered by Gregor Mendel, how sex is determined in humans, and the basics of evolution. Understanding heredity helps explain why children resemble their parents yet are not identical to them.
This chapter carries 5–8 marks in board exams. Mendel’s experiments, monohybrid and dihybrid crosses, sex determination, and the difference between acquired and inherited traits are the most frequently tested topics.
Key Concepts
1. Heredity and Variation
Heredity is the transmission of characters (traits) from parents to offspring. Variation is the difference in characters between individuals of the same species.
- Offspring inherit traits from both parents (in sexual reproduction)
- No two offspring are exactly alike — variations arise due to different combinations of parental genes
- Inherited traits are coded in DNA, organised into units called genes
Inherited Traits vs Acquired Traits
| Inherited Traits | Acquired Traits |
|---|---|
| Present from birth, coded in DNA | Developed during lifetime due to environment or practice |
| Can be passed to offspring | Cannot be passed to offspring |
| Example: eye colour, blood group, attached/free earlobes | Example: muscular body from exercise, knowledge from studying, scars |
Key point: Only changes in the DNA of reproductive cells (germ cells) can be inherited. Changes in body cells (somatic cells) — like a scar or a learned skill — are NOT passed to the next generation.
2. Mendel’s Experiments
Gregor Johann Mendel (1822–1884), an Austrian monk, is known as the “Father of Genetics”. He performed breeding experiments on garden pea plants (Pisum sativum) and discovered the fundamental laws of inheritance.
Why Pea Plants?
- Short generation time (grows and reproduces quickly)
- Many easily distinguishable contrasting traits
- Self-pollinating (but can be cross-pollinated manually)
- Large number of offspring produced
Seven Contrasting Traits Mendel Studied
| Trait | Dominant | Recessive |
|---|---|---|
| Seed shape | Round | Wrinkled |
| Seed colour | Yellow | Green |
| Flower colour | Violet | White |
| Pod shape | Inflated | Constricted |
| Pod colour | Green | Yellow |
| Flower position | Axial | Terminal |
| Plant height | Tall | Dwarf |
3. Monohybrid Cross (One Trait at a Time)
Mendel crossed a pure tall plant (TT) with a pure dwarf plant (tt).
F₁ generation (First filial generation):
- ALL plants were tall
- This means the tall trait is dominant and the dwarf trait is recessive
- F₁ plants are genotype Tt (heterozygous) — they carry both alleles but only tall appears
F₂ generation (F₁ × F₁ cross — Tt × Tt):
| T | t | |
|---|---|---|
| T | TT (Tall) | Tt (Tall) |
| t | Tt (Tall) | tt (Dwarf) |
Results:
- Phenotypic ratio: 3 Tall : 1 Dwarf (3:1)
- Genotypic ratio: 1 TT : 2 Tt : 1 tt (1:2:1)
- The dwarf trait reappeared in F₂ — it was hidden (not lost) in F₁
Key Terms
| Term | Meaning |
|---|---|
| Gene | Unit of inheritance; a segment of DNA that codes for a specific protein/trait |
| Allele | Different forms of the same gene (e.g., T for tall, t for dwarf) |
| Dominant | Allele that expresses itself even in heterozygous condition (e.g., T) |
| Recessive | Allele that is masked in heterozygous condition; shows only in homozygous state (e.g., tt) |
| Homozygous | Both alleles same (TT or tt) |
| Heterozygous | Alleles different (Tt) — also called hybrid |
| Genotype | Genetic makeup (e.g., TT, Tt, tt) |
| Phenotype | Physical appearance (e.g., tall or dwarf) |
4. Dihybrid Cross (Two Traits at a Time)
Mendel crossed pea plants with round yellow seeds (RRYY) × wrinkled green seeds (rryy).
F₁ generation: All plants had round yellow seeds (RrYy) — both dominant traits appeared.
F₂ generation (F₁ × F₁): Four types of offspring appeared:
| Phenotype | Ratio |
|---|---|
| Round Yellow | 9 |
| Round Green | 3 |
| Wrinkled Yellow | 3 |
| Wrinkled Green | 1 |
F₂ phenotypic ratio: 9:3:3:1
Key conclusion: The two traits (seed shape and seed colour) are inherited independently of each other. This is Mendel’s Law of Independent Assortment.
New combinations appeared (round green, wrinkled yellow) that were not present in either parent — showing that genes for different traits are sorted independently during gamete formation.
5. How Do Traits Get Expressed?
The pathway from gene to trait:
- A gene (segment of DNA) provides the instruction
- The gene codes for a specific protein (enzyme or structural protein)
- The protein controls a specific biochemical process
- The process determines the trait (observable characteristic)
Example: The gene for plant height codes for an enzyme that produces a growth hormone. If the gene is functional (T), the plant grows tall. If the gene is altered (t), less hormone is produced, and the plant is dwarf.
Important: Both parents contribute one copy of each gene. So every individual has two alleles for each trait — one from the father, one from the mother.
- Humans have 23 pairs of chromosomes (46 total) — 23 from each parent
- Each chromosome carries many genes
6. Sex Determination
Sex Determination in Humans
Humans have 23 pairs of chromosomes — 22 pairs are autosomes (same in males and females) and 1 pair is sex chromosomes.
- Females: XX (two X chromosomes)
- Males: XY (one X and one Y chromosome)
How sex is determined:
- All eggs from the mother carry one X chromosome
- Sperm from the father carry either X or Y
- If X-sperm fertilises the egg → XX → Girl
- If Y-sperm fertilises the egg → XY → Boy
| X (from mother) | X (from mother) | |
|---|---|---|
| X (from father) | XX (Girl) | XX (Girl) |
| Y (from father) | XY (Boy) | XY (Boy) |
Key point: The sex of the child is determined by the father’s sperm (whether it carries X or Y), not the mother. There is a 50:50 chance of having a boy or girl.
7. Evolution — Brief Overview
Accumulation of Variations
Sexual reproduction creates variations in each generation. Over thousands of generations, these variations accumulate. If a variation gives a survival advantage in a particular environment, organisms with that variation reproduce more — this is natural selection (proposed by Charles Darwin).
Speciation
When populations of a species become geographically isolated (separated by a mountain, river, etc.), they cannot interbreed. Over time, they accumulate different variations and eventually become so different that they form new species — this is called speciation.
Evolution vs Progress
Evolution is NOT progress from “lower” to “higher” organisms. It is simply the accumulation of changes that help organisms survive in their specific environment. Bacteria are not “less evolved” than humans — they are perfectly adapted to their niche.
Important Definitions
| Term | Definition |
|---|---|
| Heredity | Transmission of traits from parents to offspring |
| Variation | Differences in traits among individuals of the same species |
| Gene | Unit of inheritance; segment of DNA coding for a protein/trait |
| Allele | Alternative forms of the same gene (e.g., T and t) |
| Dominant trait | Trait that appears in heterozygous condition |
| Recessive trait | Trait that is masked in heterozygous condition; appears only in homozygous state |
| Genotype | Genetic constitution of an organism (e.g., Tt) |
| Phenotype | Observable characteristic/physical appearance (e.g., tall) |
| Homozygous | Having two identical alleles for a trait (TT or tt) |
| Heterozygous | Having two different alleles for a trait (Tt) |
| Monohybrid cross | Cross between organisms considering only one contrasting trait |
| Dihybrid cross | Cross between organisms considering two contrasting traits simultaneously |
| Sex chromosomes | Chromosomes that determine the sex of an individual (X and Y in humans) |
| Natural selection | Process where organisms with advantageous traits survive and reproduce more |
| Speciation | Formation of new species due to geographical isolation and accumulated variations |
Solved Examples (NCERT-Based)
Example 1
A man with blood group A marries a woman with blood group O. Can they have a child with blood group O?
Answer: Yes, it is possible. Blood group A can be genotype IAIA (homozygous) or IAi (heterozygous). Blood group O is always ii. If the father is IAi (heterozygous A), then the cross IAi × ii gives: IAi (blood group A) and ii (blood group O) in a 1:1 ratio. So yes, they can have a child with blood group O if the father is heterozygous.
Example 2
In a monohybrid cross between tall (Tt) and dwarf (tt) pea plants, what will be the phenotypic ratio of the offspring?
Answer: Cross: Tt × tt. Using a Punnett square: Tt, Tt, tt, tt → 2 Tall (Tt) : 2 Dwarf (tt) → 1:1 ratio. This is a test cross — used to determine if a tall plant is TT or Tt.
Example 3
How is the sex of the child determined in humans? Is it correct to blame the mother for giving birth to a girl?
Answer: No, it is absolutely incorrect to blame the mother. The sex of a child depends entirely on which type of sperm (from the father) fertilises the egg. The mother always contributes an X chromosome. If the father’s sperm carrying an X chromosome fertilises the egg, the child is XX (girl). If the Y-bearing sperm fertilises the egg, the child is XY (boy). The probability is 50:50, and the mother has no role in determining sex.
Example 4
If a round, yellow-seeded pea plant (RrYy) is crossed with a wrinkled, green-seeded pea plant (rryy), what will be the ratio of offspring phenotypes?
Answer: The gametes of RrYy are: RY, Ry, rY, ry. The gametes of rryy are: ry only. The offspring will be: RrYy (round yellow), Rryy (round green), rrYy (wrinkled yellow), rryy (wrinkled green) — in equal proportions. Phenotypic ratio: 1:1:1:1.
Important Questions for Board Exams
1-Mark Questions
- What are genes?
- Name the plant Mendel used for his experiments.
- What is the phenotypic ratio of a monohybrid cross in F₂ generation?
- Who determines the sex of a child — the father or the mother?
- What is an allele?
2-Mark Questions
- Distinguish between dominant and recessive traits with an example.
- What is the difference between genotype and phenotype?
- Why did Mendel choose pea plants for his experiments? Give two reasons.
- What are inherited traits? Give two examples.
- Differentiate between homozygous and heterozygous organisms.
3-Mark Questions
- Explain Mendel’s monohybrid cross with the help of a Punnett square. State the phenotypic and genotypic ratios in the F₂ generation.
- How is sex determined in human beings? Explain with a diagram.
- Explain the Law of Independent Assortment using a dihybrid cross.
- Distinguish between acquired traits and inherited traits. Why are acquired traits not passed to the next generation?
- How do traits get expressed from genes? Explain the pathway.
5-Mark Questions
- Describe Mendel’s dihybrid cross experiment. Show the F₂ generation results with a checkerboard (Punnett square). What law did Mendel deduce from this experiment?
- What is heredity? Explain with the help of Mendel’s monohybrid cross how traits are inherited from one generation to the next.
Quick Revision Points
- Heredity = passing traits from parents to offspring; coded in genes (DNA segments)
- Acquired traits (scars, muscles from exercise) cannot be inherited — only DNA changes in germ cells are inheritable
- Mendel used pea plants; studied 7 contrasting traits
- Monohybrid cross F₂ ratio: 3:1 (phenotypic), 1:2:1 (genotypic)
- Dominant trait masks recessive in heterozygous condition (Tt appears tall)
- Dihybrid cross F₂ ratio: 9:3:3:1 → Law of Independent Assortment
- Gene → Protein → Biochemical process → Trait (expression pathway)
- Sex determination: XX = Female, XY = Male; father’s sperm decides sex (X or Y)
- 50:50 chance of boy or girl — mother is NOT responsible for child’s sex
- Humans: 23 pairs chromosomes (22 autosomes + 1 pair sex chromosomes)
- Evolution = accumulation of variations + natural selection; NOT “lower” to “higher”
- Speciation = new species from geographically isolated populations
Previous Chapter: Chapter 7 — How do Organisms Reproduce?
Next Chapter: Chapter 9 — Light: Reflection and Refraction
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