Board exam focus — Heredity and Evolution (CBSE & HBSE)

Focuses on how traits are passed from parents to offspring and the mechanisms of evolutionary change. CBSE emphasizes Mendel's laws and sex determination; HBSE focuses on acquired vs inherited traits and basic evolutionary concepts.

Accumulation of Variation & Heredity

Inheritance from the previous generation provides both a common basic body design and subtle changes (variations) for the next generation.

1. Variation in Reproduction

• Asexual: Minor variations due to small inaccuracies in DNA copying.
• Sexual: Greater variation due to the combination of two different individuals' DNA.

2. Heredity and Mendel's Laws

Gregor Mendel, the 'Father of Genetics', used pea plants (Pisum sativum) for his experiments.

• Monohybrid Cross: Crossing plants with one pair of contrasting characters (e.g., Tall vs Short).
• F1 Generation: All plants were Tall.
• F2 Generation: Tall to Short ratio was 3:1.
• Dihybrid Cross: Crossing plants with two pairs of contrasting characters (e.g., Round-Green vs Wrinkled-Yellow).
• Ratio: 9:3:3:1.

Dominant vs Recessive

Traits that appear in the F1 generation are Dominant (e.g., Tallness), while those that are hidden are Recessive (e.g., Dwarfness).

Sex Determination

The mechanism by which the sex of an individual is determined.

Human Sex Determination

Humans have 23 pairs of chromosomes. 22 pairs are autosomes (same in both), and 1 pair is the Sex Chromosome.

• Females: Have a perfect pair of XX chromosomes.
• Males: Have a mismatched pair, XY (Y is smaller).

The Logic:

• An egg (from mother) always carries an X chromosome.
• A sperm (from father) can carry either an X or a Y chromosome.
• XX combination: Girl child.
• XY combination: Boy child.
• Conclusion: The sex of the child is determined by the father's genetic contribution.

Evolution and Speciation

Evolution is the gradual change in the characteristics of a population over generations.

1. Acquired vs Inherited Traits

• Acquired: Changes in non-reproductive tissues (somatic cells) due to environmental factors (e.g., weight loss, learning a skill). These are not passed to the next generation.
• Inherited: Changes in the DNA of reproductive cells (germ cells). These are passed to the next generation.

2. Speciation

The process of formation of new species from existing ones.

• Factors: Genetic drift, Natural selection, and Geographical isolation (e.g., mountains or rivers separating populations, leading to reproductive isolation).

Tracing Evolutionary Relationships

We can trace relationships by studying various evidences:

1. Homologous Organs

Organs with similar basic structure/origin but different functions. (Suggests Common Ancestry).

• Example: Forelimbs of a human, bird, and lizard.

2. Analogous Organs

Organs with different basic structure/origin but similar functions. (Suggests Adaptation to similar environment).

• Example: Wings of a bird and wings of a bat.

3. Fossils

Preserved remains or impressions of organisms that lived in the past. They provide a time-line of evolution (e.g., Archaeopteryx as a link between reptiles and birds).

4. Human Evolution

All humans belong to a single species: Homo sapiens. Our earliest ancestors are traced back to Africa.

Frequently asked questions

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Do these notes follow CBSE and HBSE?

Yes. The Heredity and Evolution notes are NCERT-aligned and include guidance for both CBSE and Haryana Board (HBSE), with important questions and MCQs for revision.

What does the Heredity and Evolution chapter cover?

Concept explanations, key formulas and definitions, fully solved examples and board-pattern practice questions for Heredity and Evolution.