Class 10 Science Chapter 8 Heredity Notes
🔹 Introduction
- Every living organism produces offspring through reproduction.
- The new individuals resemble their parents, but they are not identical — small differences always exist.
- These differences are called variations.
- The reason behind similarities and differences among organisms is the process called heredity.
Heredity → Transmission of characters or traits from parents to offspring.
Variation → Differences in characters among individuals of the same species.
🧩 Why is Heredity Important?
- Heredity ensures that offspring receive basic body design (species identity).
- It also introduces slight modifications, which cause variation in future generations.
- These variations make each individual unique.
🔹 8.1 Accumulation of Variation During Reproduction
1. Inheritance and Variation
- Each new generation receives a set of instructions (genes) from its parents.
- These genes determine the basic body design and also carry small differences.
- When the next generation reproduces, it passes on both:
- the inherited traits, and
- the newly created variations.
Thus, variations keep accumulating with each generation.
2. Variation in Asexual Reproduction
- Only one parent is involved.
- Offspring are produced by simple cell division (mitosis) or binary fission.
- All offspring are genetically almost identical to the parent (clones).
- Only minor variations occur due to small DNA copying errors during cell division.
Example: When one bacterium divides into two, and then four — all are very similar.
Minor DNA copying errors cause minute differences, but diversity remains low.
3. Variation in Sexual Reproduction
- Two parents contribute equal genetic material (half from male, half from female).
- During fertilisation, DNA from both parents mixes in new combinations.
- This produces greater diversity in offspring.
- Hence, sexual reproduction introduces more variation than asexual reproduction.
4. Effect of Variation on Survival
- Not all variations are equally useful.
- Some may provide advantages for survival in specific environments.
| Example | Variation | Advantage |
|---|---|---|
| Bacteria | Heat-resistant form | Survive during heat waves |
| Moths | Colour variation | Camouflage from predators |
| Humans | Blood group/ear lobe differences | Genetic diversity |
- Natural selection acts on these variations.
- Variants better suited to the environment survive and reproduce — forming the basis of evolution.
5. Key Idea (Summary of 8.1)
| Concept | Explanation |
|---|---|
| Variation | Differences among individuals of a species |
| Source of Variation | Errors in DNA copying, sexual reproduction, environmental influence |
| Asexual Reproduction | Produces less variation |
| Sexual Reproduction | Produces more variation |
| Importance of Variation | Helps organisms adapt, survive, and evolve |
🧠 Remember
- Variation is essential for evolution.
- Reproduction ensures continuity of species with diversity.
- Environmental factors select the best-adapted variations for survival.
🔹 8.2 Heredity
Definition:
Heredity is the process by which traits and characteristics are passed from parents to offspring through genes.
- The main outcome of reproduction is that offspring resemble their parents.
- However, they are not exact copies — small differences (variations) are always present.
- These similarities and differences follow certain rules of heredity.
Why Study Heredity?
- To understand how traits are transmitted from one generation to another.
- To study why children resemble their parents but are still not identical.
- To learn how variations accumulate, leading to diversity and evolution.
🔹 8.2.1 Inherited Traits
1. What is a Trait?
A trait is a specific characteristic or feature of an organism that can be inherited from its parents.
Examples of traits:
- Eye colour
- Height
- Hair type
- Shape of earlobes
- Tongue rolling ability
2. Inherited Traits
Traits that are passed from parents to offspring through genes are called inherited traits.
- They are controlled by genetic material (DNA) present in cells.
- They do not depend on environmental factors or learning.
- These traits form the basis of heredity.
| Examples | Explanation |
|---|---|
| Earlobe type | Free or attached (inherited from parents) |
| Tongue rolling | Ability or inability is genetic |
| Hair colour | Determined by genes, not by dye |
| Blood group | Inherited (A, B, AB, O) |
3. Acquired Traits (for comparison)
Traits that develop during an individual’s lifetime due to environment, use/disuse of organs, or experience — not inherited.
| Inherited Trait | Acquired Trait |
|---|---|
| Present at birth | Developed after birth |
| Passed through genes | Not passed through genes |
| Example: Eye colour | Example: Muscle development by exercise |
| Found in all generations | Lost after one generation |
4. Human Example – Earlobes
- Free earlobes: Hang freely from the head.
- Attached earlobes: Attached directly to the side of the head.
- These are two variants of a single trait controlled by genes.
🧩 Activity 8.1: Observe & Record
- Observe the earlobes of all students in your class.
- Make two lists:
- Students with free earlobes
- Students with attached earlobes
- Calculate the percentage of each type.
- Compare the earlobe types of students and their parents.
Conclusion:
The pattern shows that earlobe type is inherited, not developed by habit or environment.
5. Why Offspring Resemble Parents but are not Identical
- Each offspring receives genetic material from both parents (half from each).
- Therefore, the new combination of genes creates a unique individual.
- Common basic design = species identity
- Minor differences = individual uniqueness
6. Key Points to Remember
| Concept | Explanation |
|---|---|
| Heredity | Transmission of characters from parents to offspring |
| Trait | Observable character (height, colour, etc.) |
| Inherited Trait | Passed through genes (genetic) |
| Acquired Trait | Developed due to environment, not inherited |
| Genes | Basic units of heredity present on chromosomes |
🧠 Think & Answer
- Why do all members of the same species look similar but not identical?
→ Because they share basic body design (species trait) but differ in small inherited variations. - Can an acquired trait be inherited?
→ No. Acquired traits (like learning to ride a cycle) do not affect the genes in reproductive cells. - What ensures that traits are inherited?
→ The DNA and genes transmitted during reproduction.
🔹 Introduction to Mendel’s Work
👨🔬 Gregor Johann Mendel (1822–1884)
- Known as the Father of Genetics.
- Born in Austria; studied pea plants (Pisum sativum) in his monastery garden.
- First person to use mathematical logic and statistics to study heredity.
- His results led to the formulation of Mendel’s Laws of Inheritance.
🔹 Why Pea Plants?
Mendel chose pea plants because they:
- Have many contrasting characters (like tall/short, round/wrinkled, etc.).
- Are easy to cultivate and grow quickly.
- Can be self-pollinated or cross-pollinated easily.
- Produce a large number of offspring for accurate statistical study.
🔹 Mendel’s Experiments
Mendel studied seven pairs of contrasting traits in pea plants:
| Character | Dominant Trait | Recessive Trait |
|---|---|---|
| 1. Stem height | Tall | Short |
| 2. Flower colour | Violet | White |
| 3. Flower position | Axial | Terminal |
| 4. Pod colour | Green | Yellow |
| 5. Pod shape | Inflated | Constricted |
| 6. Seed colour | Yellow | Green |
| 7. Seed shape | Round | Wrinkled |
🔹 A. Monohybrid Cross (Inheritance of One Trait)
Experiment
- Crossed a tall plant (TT) with a short plant (tt).
Results
| Generation | Cross | Offspring | Observation |
|---|---|---|---|
| P (Parent) | TT × tt | – | Pure tall and pure short |
| F₁ (First Filial) | – | All Tall (Tt) | Only tall trait appeared |
| F₂ (Second Filial) | Tt × Tt | 3 Tall : 1 Short | Reappearance of short trait |
Explanation
- Every trait is controlled by two factors (alleles).
- These alleles separate during gamete formation and recombine during fertilisation.
- In F₁, both tall (T) and short (t) factors are present, but only tall expresses itself.
Key Terms
| Term | Meaning |
|---|---|
| Dominant trait | Expressed in F₁ generation (e.g., Tallness) |
| Recessive trait | Hidden in F₁ but appears in F₂ (e.g., Shortness) |
| Genotype | Genetic makeup (e.g., TT, Tt, tt) |
| Phenotype | Observable character (e.g., Tall, Short) |
Genotypic and Phenotypic Ratios (F₂ Generation)
| Type | Ratio |
|---|---|
| Genotypic Ratio | 1 TT : 2 Tt : 1 tt |
| Phenotypic Ratio | 3 Tall : 1 Short |
🧠 Observation & Conclusion
- Each trait is controlled by a pair of alleles.
- During gamete formation, the alleles segregate (separate).
- Out of the two alleles, one may dominate the other.
- Hence, F₁ shows only the dominant trait.
- F₂ shows both traits in a 3:1 ratio.
🔹 B. Dihybrid Cross (Inheritance of Two Traits)
Experiment
- Mendel crossed:
- Round, Yellow seeds (RRYY) with
- Wrinkled, Green seeds (rryy)
F₁ Generation
- All plants had Round, Yellow seeds (RrYy).
→ Round and Yellow = Dominant traits.
F₂ Generation (Self-pollination of F₁ plants)
- Four types of combinations appeared:
| Traits Combination | Phenotype | Ratio |
|---|---|---|
| Round, Yellow | Both dominant traits | 9 |
| Round, Green | Round (dominant) + Green (recessive) | 3 |
| Wrinkled, Yellow | Wrinkled (recessive) + Yellow (dominant) | 3 |
| Wrinkled, Green | Both recessive traits | 1 |
➡ Phenotypic Ratio = 9 : 3 : 3 : 1
Conclusion – Law of Independent Assortment
- Each pair of alleles is inherited independently of the other.
- Example: Seed shape (round/wrinkled) and seed colour (yellow/green) are inherited separately, not linked.
- Hence, new combinations (like round-green or wrinkled-yellow) appear in offspring.
🔹 C. Mendel’s Laws of Inheritance
| Law | Statement | Explanation |
|---|---|---|
| 1. Law of Dominance | In a pair of contrasting traits, only one (dominant) is expressed in the hybrid. | Example: Tt → only tall (T) is expressed. |
| 2. Law of Segregation (Purity of Gametes) | The two alleles for a trait separate during gamete formation and recombine during fertilisation. | Explains 3:1 ratio in F₂ generation. |
| 3. Law of Independent Assortment | Genes for different traits are inherited independently. | Explains 9:3:3:1 ratio in dihybrid cross. |
🧩 Activity / Question
In Mendel’s F₂ generation (tall × short plants), what will be the expected genotypic ratio?
→ 1 TT : 2 Tt : 1 tt
In pea plants, which traits are dominant — tallness and round seeds, or shortness and wrinkled seeds?
→ Tallness and round seeds are dominant traits.
🔹 Key Takeaways
- Mendel discovered hereditary factors (now called genes).
- Traits are transmitted from parents to offspring in predictable ratios.
- Dominant and recessive traits determine the appearance of offspring.
- Independent inheritance produces new combinations of traits.
🔹 1. Understanding the Mechanism of Heredity
- Heredity is controlled by DNA present in the cell’s nucleus.
- DNA contains all the information required to build and operate the body.
- The specific segments of DNA that carry information for a particular trait are called genes.
Gene: A unit of heredity that controls the expression of a specific characteristic or trait.
It is a segment of DNA that provides information for the formation of a protein.
🔹 2. Genes and Proteins
- Genes work by controlling the synthesis of proteins in the cell.
- Proteins, especially enzymes and hormones, control various biological functions.
- These proteins determine the observable traits (phenotypes).
🧩 Example – Plant Height
Let’s understand this with an example of tall and short pea plants:
| Condition | Gene Function | Result |
|---|---|---|
| Gene produces an efficient enzyme | More growth hormone is formed | Tall plant |
| Gene produces a less efficient enzyme | Less growth hormone is formed | Short plant |
✅ Hence, the gene indirectly controls the height of the plant by affecting hormone production.
🔹 3. How Genes are Inherited
- Both parents contribute one set of genes each to the offspring.
- Therefore, each offspring has two copies of each gene — one from the mother and one from the father.
- These two copies may be:
- Same (homozygous) → TT or tt
- Different (heterozygous) → Tt
What Happens During Reproduction?
- In sexual reproduction, special cells called germ cells (sperms and eggs) are formed.
- Germ cells contain only one set of chromosomes (half the normal number).
- This ensures that when male and female gametes fuse during fertilisation, the chromosome number is restored in the offspring.
🔹 4. Role of Chromosomes
Chromosomes are thread-like structures present in the nucleus of each cell that carry genes.
- In most human cells: 46 chromosomes (arranged in 23 pairs).
- Each pair consists of:
- One chromosome from the father (paternal)
- One chromosome from the mother (maternal)
Hence, genes are always inherited in pairs.
🧩 Chromosome Behavior During Gamete Formation
- Each gamete (sperm or egg) gets only one chromosome from each pair.
- It may be of maternal or paternal origin — chosen randomly.
- When two gametes fuse, the zygote restores the normal diploid (2n) chromosome number.
- This process maintains genetic stability generation after generation.
🔹 5. Summary Table
| Concept | Explanation |
|---|---|
| DNA | Deoxyribonucleic Acid – carries hereditary information |
| Gene | Segment of DNA controlling a specific trait |
| Protein | Molecule produced under gene control; decides expression of trait |
| Chromosome | Structure made of DNA carrying many genes |
| Gamete | Reproductive cell having half number of chromosomes |
| Zygote | Fertilised cell restoring full set of chromosomes |
| Homozygous | Both genes identical (TT or tt) |
| Heterozygous | Genes different (Tt) |
🧠 Understanding Example
- Suppose both parents contribute gene for height:
- Father → T (tall)
- Mother → t (short)
- Offspring genotype = Tt → phenotype = Tall
- Because T (tall) is dominant, the plant appears tall even though one “short” gene is present.
🔹 6. Important Concepts
- Both parents contribute equally to the genetic makeup of offspring.
- Each trait is controlled by two versions (alleles) of a gene.
- Dominant alleles mask recessive alleles in heterozygous condition.
- Gametes carry only one set of genes — ensures balanced inheritance.
- The structure of chromosomes ensures stability of DNA across generations.
🧠 Think & Answer
- What is the function of a gene?
→ To carry information for the synthesis of specific proteins controlling traits. - Why do germ cells have half the chromosome number?
→ So that after fertilisation, the offspring has the correct total chromosome number. - If both parents contribute equally to DNA, how can traits differ?
→ Because the combination of genes (dominant/recessive) can produce different visible outcomes.
🔹 8.2.4 Sex Determination
1. Need to Know
- In sexually reproducing species, the two sexes must differ so that male and female gametes can fuse.
- But how does an organism become male or female?
- The process that decides whether a new individual will be boy or girl, male or female, is called sex determination.
2. Different Methods in Nature
| Organism Type | Basis of Sex Determination | Example |
|---|---|---|
| Environmental | Temperature of egg incubation | Some reptiles (turtles, alligators) |
| Behavioral/Changeable | Individuals can change sex | Snails |
| Genetic | Determined by sex chromosomes | Humans, birds, many animals |
3. Human Sex Determination
- Humans have 46 chromosomes (23 pairs) in each cell.
- 22 pairs → Autosomes
- 1 pair → Sex chromosomes
| Sex | Chromosome pair |
|---|---|
| Female | XX |
| Male | XY |
4. How It Happens
- Mother (XX) → produces only X type eggs.
- Father (XY) → produces two types of sperms:
- half carry X,
- half carry Y.
When fertilisation occurs:
| Sperm Type | Egg (X) + Sperm | Child Sex |
|---|---|---|
| X bearing | X + X = XX | Girl |
| Y bearing | X + Y = XY | Boy |
➡️ Therefore, father’s sperm decides the sex of the child, not the mother.
5. Summary of Sex Determination in Humans
- Sex is determined genetically, not environmentally.
- Each child inherits X chromosome from the mother.
- The father contributes either X or Y, which decides the child’s sex.
- Thus, chance of boy or girl = 50% each.
🔹 Chapter Summary
| Key Idea | Main Point |
|---|---|
| Variation | Differences among individuals produced during reproduction. |
| Importance of Variation | Enables better adaptation and survival → basis of evolution. |
| Heredity | Transmission of traits from parents to offspring through genes. |
| Genes & DNA | Genes are DNA segments that control protein formation and traits. |
| Mendel’s Work | Discovered laws of inheritance through pea plant experiments. |
| Law of Dominance | Only one trait from a pair (dominant) is expressed. |
| Law of Segregation | Two alleles separate during gamete formation. |
| Law of Independent Assortment | Traits of different genes are inherited independently. |
| Chromosomes | Carry genes; ensure genetic stability. |
| Sex Determination (Humans) | XX = female, XY = male; father decides child’s sex. |
🔹 Key Terms
| Term | Definition |
|---|---|
| Heredity | Transmission of traits from parents to offspring. |
| Variation | Differences among organisms of the same species. |
| Gene | Segment of DNA that controls a particular trait. |
| DNA | Genetic material carrying hereditary information. |
| Chromosome | Thread-like structure made of DNA and proteins. |
| Alleles | Alternative forms of a gene for a trait (e.g. T & t). |
| Dominant Trait | Expressed trait in heterozygous condition. |
| Recessive Trait | Hidden trait in heterozygous condition. |
| Genotype | Genetic composition (e.g. TT, Tt, tt). |
| Phenotype | Observable appearance (Tall or Short). |
| Gametes | Reproductive cells carrying half the chromosome number. |
| Zygote | Fertilised cell formed after fusion of gametes. |
| Sex Chromosomes | Chromosomes responsible for determining sex (XX, XY). |
🔹 NCERT Textbook-Style Practice Questions
- How do Mendel’s experiments show that traits may be dominant or recessive?
→ Because in the F₁ generation only the dominant trait appears, while the recessive one is hidden but reappears in F₂. - How do Mendel’s experiments prove that traits are inherited independently?
→ In the dihybrid cross, new combinations of traits (round-green and wrinkled-yellow) appeared in a 9 : 3 : 3 : 1 ratio. - A man with blood group A marries a woman with blood group O. Their daughter has blood group O. Is A dominant or recessive?
→ Blood group A is dominant; the father must be heterozygous (AO). - How is equal genetic contribution of male and female ensured in offspring?
→ Each parent produces gametes with half the chromosomes; fusion restores the full number. - How is sex determined in humans?
→ By the type of sperm (X or Y) that fertilises the ovum; X→girl, Y→boy.
🧬 Chapter 8: Heredity – Notes Summary
🔹 Introduction
- Reproduction produces new individuals that are similar but not identical to their parents.
- This similarity and variation is due to heredity.
- Heredity = The transmission of traits (characters) from parents to offspring.
🔹 8.1 Accumulation of Variation During Reproduction
1. Variation
- The differences among individuals of a species.
- Even in asexual reproduction, small variations occur due to inaccuracies in DNA copying.
- In sexual reproduction, variations are much greater because of mixing of DNA from two parents.
2. Importance of Variation
- Variations help species adapt to changing environments.
- Some variations may offer advantages for survival.
- Example: Heat-resistant bacteria survive better in hot conditions.
- Hence, variation is the basis of evolution.
🔹 8.2 Heredity
Definition:
Heredity is the process by which traits are passed from parents to offspring through genes.
🔹 8.2.1 Inherited Traits
1. Inherited Trait
- A characteristic that an organism gets from its parents.
- Example: Earlobe type (attached or free), eye colour, height, etc.
2. Activity Example:
- Observe students’ earlobes (free or attached).
- Compare with their parents.
- Such patterns help in understanding inheritance rules.
🔹 8.2.2 Rules for the Inheritance of Traits – Mendel’s Experiments
Gregor Johann Mendel (1822–1884)
- Known as Father of Genetics.
- Conducted experiments on pea plants (Pisum sativum).
- Studied inheritance of traits like:
- Tall/short plants
- Round/wrinkled seeds
- Violet/white flowers
- Used mathematical analysis of results → proposed Laws of Inheritance.
Mendel’s Experiments
(a) Monohybrid Cross (One trait)
- Crossed tall (TT) and short (tt) pea plants.
| Generation | Crossed plants | Result |
|---|---|---|
| Parent (P) | TT × tt | All tall (F₁ generation) |
| F₁ self-crossed | Tt × Tt | 3 tall : 1 short (F₂ generation) |
- Inference:
- Each trait is controlled by two factors (now called genes).
- One gene from each parent.
- Dominant trait (T) masks recessive trait (t).
Dominant and Recessive Traits
- Dominant: Expressed in the F₁ generation (Tall – T)
- Recessive: Hidden in F₁ but reappears in F₂ (Short – t)
(b) Dihybrid Cross (Two traits)
- Crossed pea plants having:
- Round, yellow seeds (RRYY)
- Wrinkled, green seeds (rryy)
| Generation | Result |
|---|---|
| F₁ | All round, yellow (RrYy) – both traits dominant |
| F₂ | 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green |
- Inference: Traits are inherited independently (Law of Independent Assortment).
Mendel’s Laws of Inheritance
- Law of Dominance – In a pair of contrasting traits, only one (dominant) is expressed.
- Law of Segregation – Two factors (alleles) separate during gamete formation and recombine at fertilization.
- Law of Independent Assortment – Different traits are inherited independently.
🔹 8.2.3 How do Traits get Expressed?
1. Role of Genes
- Gene: A segment of DNA that provides information for a protein.
- Proteins control cell functions and traits.
2. Example – Plant Height
- Gene controls an enzyme that helps make growth hormone.
- Efficient enzyme → more hormone → Tall plant
- Less efficient enzyme → less hormone → Short plant
3. Chromosomes
- DNA is organized into chromosomes.
- Each cell has two sets of chromosomes (one from each parent).
- Germ cells (sperms/eggs) have one set → combine during fertilization → restore two sets in offspring.
🔹 8.2.4 Sex Determination
1. Different Methods in Different Species
- Reptiles: Temperature of egg incubation decides sex.
- Snails: Can change sex – not genetically fixed.
- Humans: Sex is genetically determined.
2. Human Chromosomes
- Humans have 23 pairs (46) chromosomes.
- 22 pairs – autosomes.
- 1 pair – sex chromosomes.
| Sex | Chromosome pair |
|---|---|
| Female | XX |
| Male | XY |
3. Sex Determination in Humans
- Mother always contributes X chromosome.
- Father contributes X or Y.
- X from father → Girl (XX)
- Y from father → Boy (XY)
- Hence, father determines the sex of the child.
🧩 Summary (Key Points)
- Variations occur during reproduction; some are inherited.
- Variations can help in survival and evolution.
- In sexually reproducing organisms:
- Each trait is controlled by a pair of genes.
- Dominant gene expresses, recessive is hidden.
- Independent inheritance leads to new combinations of traits.
- Sex determination varies among species.
- In humans, sex depends on whether sperm contributes X or Y chromosome.
📝 Important Terms
| Term | Definition |
|---|---|
| Heredity | Transmission of traits from parents to offspring |
| Variation | Differences among individuals of a species |
| Gene | Unit of heredity; a segment of DNA |
| Dominant Trait | Expressed trait in heterozygous condition |
| Recessive Trait | Hidden trait in heterozygous condition |
| Chromosome | Thread-like structure carrying genes |
| Alleles | Alternative forms of a gene |
| Genotype | Genetic makeup (TT, Tt, tt) |
| Phenotype | Observable character (Tall or Short) |
| Monohybrid cross | Cross involving one trait |
| Dihybrid cross | Cross involving two traits |
| Gametes | Reproductive cells having one set of genes |
🧠 NCERT Textbook Questions (For Practice)
- If a trait A exists in 10% of a population of an asexually reproducing species and a trait B in 60%, which trait arose earlier?
→ Trait B, as it has spread to a larger proportion. - How does variation promote survival?
→ It enables some individuals to survive environmental changes. - How is sex determined in human beings?
→ By the type of sperm (X or Y chromosome) fertilizing the ovum. - How do Mendel’s experiments show dominance and independent inheritance?
→ Monohybrid cross shows dominance, dihybrid cross shows independent assortment.