Notes Class 10 Science Chapter 8 Heredity

Notes For All Chapters Science Class 10 CBSE

1. Basic Concept of Heredity

Heredity = Transmission of traits from parents to offspring.

Reproduction produces new individuals that:

• Look similar to parents.
• Show some differences/variations.

Variation = Small changes in DNA or traits that make organisms different.

Importance of heredity:

• Maintains species characteristics.
• Produces variations → essential for evolution.
• Helps organisms survive in changing environment.

2. Accumulation of Variations During Reproduction

Each generation inherits basic body design + subtle differences.

Variations accumulate in successive generations.

Asexual reproduction:

• Variations are very small.
• Example: One bacterium divides → daughter cells almost identical.
• Differences only due to DNA copying errors.

Sexual reproduction:

• Produces much greater diversity.
• Involves mixing of DNA from two parents.

Survival chances of variations:

• Not equal for all.
• Environment decides which variations are useful.
• Example: Heat-resistant bacteria survive during heat waves.

Variations selected by environment = natural selection → leads to evolution.

3. Heredity – Rules of Inheritance

Offspring look similar but not identical.

Traits are inherited according to certain rules.

Each parent contributes equal genetic material.

Each child has two versions of every trait (one from mother, one from father).

Whether a trait is visible depends on whether it is dominant or recessive.

4. Inherited Traits

Trait = Characteristic feature passed from parents to children.

Examples:

• Human beings have two eyes, nose, ears → inherited basic design.
• Shape of earlobe (free/attached) → shows variation within humans.

Inheritance can be studied by comparing traits in children and their parents.

5. Mendel’s Contribution (Experiments with Pea Plants)

Gregor Johann Mendel (1822–1884) → Father of Genetics.

Studied pea plants because:

• Easy to grow.
• Many distinct contrasting traits (tall/short, round/wrinkled seeds, etc.).
• Produce large number of seeds.
• Self-pollinating but can also be cross-pollinated.

Mendel’s Observations

1. Monohybrid Cross (one trait at a time):

• Tall × Short pea plants.
• F1 → All tall (tallness is dominant).
• F2 → Ratio 3 Tall : 1 Short.
• Inference: Traits controlled by “factors” (genes), present in pairs.

2. Dihybrid Cross (two traits at a time):

• Example: Tall round seeds × Short wrinkled seeds.
• F1 → All tall round (dominant traits).
• F2 → New combinations appear (Tall wrinkled, Short round).
• Inference: Traits are independently inherited.

Mendel’s Laws

• Law of Dominance → One trait dominates over the other.
• Law of Segregation → Two factors separate during gamete formation.
• Law of Independent Assortment → Traits assort independently.

6. Dominant and Recessive Traits

Dominant trait → Expressed even if only one copy is present (e.g., Tallness, Round seed).

Recessive trait → Expressed only if both copies are recessive (e.g., Shortness, Wrinkled seed).

Examples:

• TT = Tall (pure dominant).
• Tt = Tall (dominant masks recessive).
• tt = Short (recessive).

7. Expression of Traits

Gene = Segment of DNA that codes for a protein.

Proteins → control cell functions & traits.

Example:

• Gene for height → produces enzyme → controls hormone production.
• Efficient enzyme → more hormone → Tall plant.
• Inefficient enzyme → less hormone → Short plant.

Therefore, traits = controlled by proteins, which are made by genes.

8. Role of Chromosomes in Inheritance

DNA is packed into chromosomes.

Each organism has two sets of chromosomes (one from each parent).

During gamete formation (meiosis):

• Only one set of chromosomes goes into each gamete.

Fertilization:

• Male gamete + Female gamete = normal chromosome number restored.
• Ensures stability of DNA across generations.

9. Sex Determination

Different organisms use different methods:

• Reptiles → temperature of eggs decides sex.
• Snails → can change sex.
• Humans → sex genetically determined.

In Humans

Total chromosomes = 46 (23 pairs).

22 pairs = autosomes.

1 pair = sex chromosomes.

• Female = XX.
• Male = XY.

All children inherit X from mother.

From father:

• X → Daughter (XX).
• Y → Son (XY).

Father decides the sex of child.