Why do children resemble their parents while possessing their own distinct individual features? How did life evolve from simple single-celled organisms into the vast diversity of plants, animals, and humans seen today? Heredity and Evolution forms Chapter 9 of the CBSE Class 10 Biology curriculum.
This comprehensive guide covers Mendel’s pea plant experiments, monohybrid and dihybrid crosses, human sex determination mechanisms, acquired vs inherited traits, factors driving speciation, and evolutionary evidence through homologous organs, analogous organs, and fossils.
- 1. Accumulation of Variation & Concept of Heredity
- 2. Mendel's Experiments with Garden Pea Plants (*Pisum sativum*)
- 3. Monohybrid Cross & Ratios (3:1 and 1:2:1)
- 4. Dihybrid Cross & Law of Independent Assortment (9:3:3:1)
- 5. Sex Determination Mechanism in Humans
- 6. Acquired Traits vs Inherited Traits
- 7. Speciation & Factors Driving Evolution
- 8. Evidence of Evolution: Homologous, Analogous Organs & Fossils
- 9. Solved Board Exam Questions
- 10. Frequently Asked Questions (FAQ)
1. Accumulation of Variation & Concept of Heredity
Heredity refers to the transmission of genetically determined characters (traits) from parents to offspring across generations.
Accumulation of Variation:
While asexual reproduction creates minimal variation due to small DNA copying inaccuracies, sexual reproduction generates immense variation due to the combination of two distinct parental genomes during fertilization. These small variations accumulate over successive generations, serving as the raw material for Evolution and natural selection.
2. Mendel's Experiments with Garden Pea Plants (*Pisum sativum*)
Gregor Johann Mendel ("Father of Genetics") conducted pioneer plant hybridization experiments on the Garden Pea plant (*Pisum sativum*).
2. Possesses distinct, easily observable contrasting traits (e.g., Tall vs Short stem, Round vs Wrinkled seeds, Yellow vs Green seeds).
3. Flowers are naturally self-pollinating, but cross-pollination can be performed manually with ease.
4. Produces a large number of offspring seeds in a single generation.
3. Monohybrid Cross & Ratios (3:1 and 1:2:1)
A Monohybrid Cross involves a cross between two pea plants considering a single pair of contrasting traits (e.g., Stem Height: Pure Tall TT × Pure Dwarf tt).
• Gametes: T from tall parent, t from dwarf parent
• First Filial Generation (F₁): All plants are Tt (Heterozygous Tall). Why dwarf trait disappeared? Because Tallness (T) is the Dominant trait that masks the Recessive dwarf trait (t).
• Self-Pollination of F₁ (Tt × Tt): Yields F₂ generation with genotypes TT, Tt, Tt, tt.
• F₂ Phenotypic Ratio = 3 Tall : 1 Dwarf (3 : 1)
• F₂ Genotypic Ratio = 1 TT : 2 Tt : 1 tt (1 : 2 : 1)
2. Law of Segregation (Purity of Gametes): During gamete formation, the two alleles of a trait separate/segregate so that each gamete receives only one allele with equal probability.
4. Dihybrid Cross & Law of Independent Assortment (9:3:3:1)
A Dihybrid Cross considers two pairs of contrasting traits simultaneously (e.g., Seed Shape & Color: Round Yellow RRYY × Wrinkled Green rryy).
• F₂ Generation (RrYy × RrYy) Phenotypic Ratio:
9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green (9 : 3 : 3 : 1)
Law of Independent Assortment: When two pairs of traits are combined in a hybrid, the segregation of one pair of characters is completely independent of the other pair during gamete formation. This produces new combination traits in F₂!
5. Sex Determination Mechanism in Humans
In human beings, sex is determined purely genetically at the moment of fertilization.
• Females: Have a perfect pair of XX sex chromosomes. All female eggs carry an X chromosome.
• Males: Have a mismatched pair of XY sex chromosomes. 50% sperms carry an X chromosome and 50% sperms carry a Y chromosome.
Genetic Cross:
• Egg (X) + Sperm (X) → XX (Female Child / Girl)
• Egg (X) + Sperm (Y) → XY (Male Child / Boy)
Conclusion: The sex of the child is determined strictly by the father (whether the fertilizing sperm carries X or Y), with a 50% (1:1) mathematical probability for every pregnancy!
6. Acquired Traits vs Inherited Traits
| Feature | Acquired Traits | Inherited Traits |
|---|---|---|
| Definition & Cause | Develop in an individual during lifetime due to lifestyle, environmental influences, or injury. | Passed from parents to progeny via genes in DNA. |
| Cellular Location | Changes occur ONLY in non-reproductive Somatic cells (body cells). | Changes exist in the DNA of Germ cells (gametes: sperm/egg). |
| Heritability | CANNOT be inherited by next generation. | Inherited by offspring generation after generation. |
| Role in Evolution | Do NOT direct evolutionary speciation. | Drive evolutionary changes and speciation. |
| Real-Life Examples | Muscles built by weightlifting, scar from injury, learning to ride a bicycle, loss of weight due to starvation. | Eye color, blood group, hair texture, height, attached vs free earlobes. |
7. Speciation & Factors Driving Evolution
Speciation is the evolutionary process by which one existing species splits into two or more distinct new species that can no longer interbreed.
2. Genetic Drift: Random changes in gene frequency in small populations due to chance events, regardless of adaptive value.
3. Natural Selection: Environment selects individuals with advantageous variations, giving them higher survival and reproduction rates (Darwin's theory).
4. Reproductive Isolation: Over time, accumulated genetic changes prevent members of the two sub-populations from interbreeding successfully.
8. Evidence of Evolution: Homologous, Analogous Organs & Fossils
| Evidence Type | Anatomical Definition & Explanation | Classic Examples |
|---|---|---|
| Homologous Organs | Organs having the same basic structural design and origin, but modified to perform different functions in different species. Demonstrates Divergent Evolution (common ancestor!). | Forelimbs of a Frog (hopping), Lizard (crawling), Bird (flying), Bat (flying), and Human (grasping). All have same arrangement of bones (humerus, radius, ulna, carpals)! |
| Analogous Organs | Organs having different basic structure and origin, but modified to perform similar functions due to similar ecological niches. Demonstrates Convergent Evolution. | Wing of a Bat (skin fold stretched between fingers) vs Wing of a Bird (feathered arm) vs Wing of an Insect (chitinous membrane). |
| Fossils | Preserved remains, imprints, or traces of ancient organisms that lived millions of years ago in Earth's crust. Age determined via Relative Depth or Carbon-14 Radiometric Dating. | Archaeopteryx fossil (connecting link showing birds evolved from reptiles — has teeth/tail like reptile and feathers like bird!). |
Artificial Selection (Evolution by Human Selection):
Humans cultivated wild cabbage over 2,000 years and selected different traits, giving rise to distinct vegetables: Cabbage (short distance between leaves), Broccoli (arrested flower development), Cauliflower (sterile flowers), Kohlrabi (swollen stem), and Kale (large leaves).
9. Solved Board Exam Questions
(a) F₁ Progeny: All plants are Gg (Green stemmed), because Green (G) is dominant over Brown (g).
(b) F₂ Progeny (Gg × Gg): Genotypes = 1 GG (Green), 2 Gg (Green), 1 gg (Brown). Total 4 parts.
Percentage of brown stemmed (gg) = (1/4) × 100 = 25%.
(c) Ratio of GG to Gg in F₂: 1 GG : 2 Gg = 1 : 2.
All eggs produced by mothers carry an X chromosome. Sperms produced by fathers carry either an X chromosome (50%) or a Y chromosome (50%).
• Egg (X) + Sperm (X) → XX (Girl)
• Egg (X) + Sperm (Y) → XY (Boy)
Since the mother always contributes an X chromosome, the sex of the child depends entirely on whether the fertilizing sperm from the father carries an X or Y chromosome.
(2) Analogous Organs: Different anatomical origin and structure, but perform similar functions due to similar environmental adaptation. Example: Wings of a bat (skin fold) and wings of a butterfly (chitinous membrane).
Example: A mouse whose tail is cut off surgically will still produce offspring with full tails, because cutting the tail did not change the DNA of its reproductive germ cells.
• Selection for short distance between leaves → Cabbage
• Selection for arrested flower development → Broccoli
• Selection for sterile flowers → Cauliflower
• Selection for swollen stem → Kohlrabi
• Selection for larger leaves → Kale.
Explore Related CBSE Class 9 & 10 Science Guides
10. Frequently Asked Questions (FAQ)
A monohybrid cross is a genetic cross considering a single pair of contrasting traits (e.g., TT x tt).
In the F2 generation:
• Phenotypic Ratio: 3 : 1 (3 Tall : 1 Dwarf)
• Genotypic Ratio: 1 : 2 : 1 (1 TT : 2 Tt : 1 tt).
A dihybrid cross considers two pairs of contrasting traits simultaneously (e.g., Seed shape & color: RRYY x rryy).
The F2 generation phenotypic ratio is 9 : 3 : 3 : 1 (9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green).
Humans have 23 pairs of chromosomes. Females carry XX sex chromosomes, while males carry XY. All female eggs carry an X chromosome. Males produce two types of sperms: 50% carrying X and 50% carrying Y.
If an X-sperm fertilizes the egg, an XX female child is born. If a Y-sperm fertilizes the egg, an XY male child is born. The father's sperm determines the child's sex.
Homologous Organs: Have same basic structure/origin but perform different functions (e.g., forelimb of human vs forelimb of bird). Evidence of common ancestry.
Analogous Organs: Have different origin/structure but perform similar functions (e.g., wing of a bat vs wing of an insect).
Fossils are preserved remains, impressions, or traces of ancient organisms that lived millions of years ago. They provide direct historical evidence of evolution by showing transitional forms (like Archaeopteryx, which bridges reptiles and birds) and demonstrating how species evolved progressively over geological time.
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