Heredity and Evolution - Class 10 CBSE Complete Biology Notes

Heredity and Evolution - Class 10 CBSE Biology Chapter 4

Heredity and Evolution form the foundation of understanding how traits pass from one generation to another and how species change over time. This comprehensive guide covers all essential concepts from the CBSE Class 10 curriculum, including Mendel's groundbreaking experiments, mechanisms of inheritance, and the evolutionary processes that have shaped life on Earth.

Introduction to Heredity

Heredity is the biological process through which characteristics are transmitted genetically from parents to offspring. This phenomenon explains why children resemble their parents and why certain traits run in families.

Genetics is the scientific study of heredity and variations. The term was first used by W. Bateson in 1905. Gregor Johann Mendel is regarded as the 'Father of Genetics' for his pioneering work on inheritance patterns in pea plants.

What is Variation?

Variation refers to the differences in characters or traits among individuals of a species. These differences arise due to:

• Errors in DNA copying during reproduction
• Sexual reproduction (mixing of genetic material from two parents)
• Environmental factors

Importance of Variations:

• Support organisms in the struggle for existence
• Enable adaptation to changing environments
• Form the basis of heredity
• Provide raw materials for evolution and development of new species
• Help in improving races of important plants and animals

Differences: Heredity vs Evolution

Insight: Heredity ensures that traits are passed from one generation to the next, while evolution ensures that species can adapt and change over time in response to environmental pressures.

Gregor Mendel: Father of Genetics

Gregor Johann Mendel (1822-1884) was an Austrian monk who conducted groundbreaking experiments on pea plants that laid the foundation for modern genetics.

Why Did Mendel Choose Pea Plants?

Mendel selected garden pea (Pisum sativum) for his experiments because:

1. Sexual reproduction - They have distinct male and female gametes
2. Seven pairs of contrasting characters - Easy to identify and track
3. Short generation time - Many generations can be studied quickly
4. Easy cultivation - Simple to grow, maintain, and handle
5. High seed production - Each plant produces many seeds
6. Pure breeding - Available in pure lines with consistent traits
7. Easy cross-pollination - Flowers can be emasculated and artificially pollinated

Mendel's Seven Contrasting Traits

Observation: All ratios approximate 3:1, confirming Mendel's principle of dominance and segregation.

Mendel's Experiments and Laws

Mendel's Experimental Technique

1. Selection of pure parent plants (producing similar traits in every generation)
2. Production of F₁ generation by cross-breeding (hybridization)
3. Raising F₂ and subsequent generations by self-fertilization of hybrids
4. Emasculation - Removal of anthers before maturity to prevent self-fertilization
5. Covering flowers to avoid foreign pollen entry
6. Controlled pollination - Transfer of desired pollen to stigma of emasculated flower

Monohybrid Cross

A monohybrid cross involves the study of inheritance of one pair of contrasting traits.

Example: Cross between Tall (TT) and Dwarf (tt) pea plants

Parental Generation (P):

• Male: Tall plant (TT)
• Female: Dwarf plant (tt)

Gametes:

• From TT: All T
• From tt: All t

F₁ Generation:

• All offspring: Tt (Heterozygous tall)
• Phenotype: 100% Tall
• Genotype: 100% Tt

F₁ Self-pollination (Tt × Tt):

F₂ Generation Results:

• Genotypic ratio: 1 TT : 2 Tt : 1 tt (1:2:1)
• Phenotypic ratio: 3 Tall : 1 Dwarf (3:1)

Conclusion: The recessive trait (dwarfness) that disappeared in F₁ reappeared in F₂ generation, proving that traits don't blend but remain separate.

Dihybrid Cross

A dihybrid cross involves the study of inheritance of two pairs of contrasting traits simultaneously.

Example: Cross between Round Yellow (RRYY) and Wrinkled Green (rryy) seeds

Parental Generation:

• Male: Round, Yellow seeds (RRYY)
• Female: Wrinkled, Green seeds (rryy)

Gametes:

• From RRYY: All RY
• From rryy: All ry

F₁ Generation:

• All offspring: RrYy (Round, Yellow)
• All seeds appear round and yellow (both dominant traits expressed)

F₁ Self-pollination (RrYy × RrYy):

Each parent can produce four types of gametes: RY, Ry, rY, ry

Punnett Square for Dihybrid Cross:

F₂ Generation Results:

• Phenotypic ratio: 9:3:3:1
  • 9 Round Yellow
  • 3 Round Green
  • 3 Wrinkled Yellow
  • 1 Wrinkled Green
• Genotypic ratio: 1:2:2:4:1:2:1:2:1

Key Observation: The dihybrid ratio of 9:3:3:1 indicates that the two pairs of traits are inherited independently of each other.

Mendel's Laws of Inheritance (Mendelism)

1. Principle of Paired Factors

Each trait of an individual is determined by two factors (now called genes). The alternative forms of a gene are called alleles.

2. Principle of Dominance

When two contrasting alleles are present together, only one (dominant) expresses itself while the other (recessive) remains hidden.

Example: In Tt, T (tall) is dominant over t (dwarf), so the plant appears tall.

3. Law of Segregation (Law of Purity of Gametes)

• A pair of contrasting factors (alleles) remains together without blending
• They separate (segregate) during gamete formation
• Each gamete receives only one factor of a pair
• During fertilization, factors unite randomly to form the next generation

Example: A Tt plant produces two types of gametes: 50% carrying T and 50% carrying t.

4. Law of Independent Assortment

When two or more pairs of traits are considered together:

• Each pair of alleles segregates independently
• The distribution of one pair doesn't affect the distribution of another pair
• This creates new combinations in offspring

Example: In RrYy, the segregation of R/r is independent of Y/y segregation, producing four types of gametes: RY, Ry, rY, ry in equal proportions.

Important Genetic Terms

Essential Vocabulary for Understanding Heredity

Understanding Dominance

Dominant traits can be expressed in:

• Homozygous condition (TT)
• Heterozygous condition (Tt)

Recessive traits are expressed only in:

• Homozygous condition (tt)

Types of Variation

Variations are essential for evolution and can be classified in multiple ways:

A. Based on Cells Affected

1. Somatic Variations

• Definition: Variations affecting body (somatic) cells
• Inheritance: Not inheritable (die with the individual)
• Causes:
  • Environmental factors (climate, food, lifestyle)
  • Use and disuse of organs
  • Conscious efforts (bodybuilding, learning)
• Examples:
  • Increased muscle mass from exercise
  • Tanned skin from sun exposure
  • Acquired skills like playing piano

2. Germinal Variations

• Definition: Variations affecting germ cells (reproductive cells)
• Inheritance: Inheritable (passed to offspring)
• Causes:
  • Mutations in DNA
  • Recombination during sexual reproduction
• Examples:
  • Eye color
  • Blood group
  • Height (genetic component)

B. Based on Origin

1. Genetic Variations (Inheritable)

• Result from changes in DNA
• Passed from parents to offspring through gametes
• Provide material for evolution
• Examples:
  • Different seed colors in pea plants
  • Blood groups in humans
  • Skin color variations

2. Environmental Variations (Non-inheritable)

• Result from environmental influences
• Not passed to next generation
• Examples:
  • Weight loss due to starvation
  • Scars from injuries
  • Language learned

C. Based on Expression

1. Continuous Variations

• Show a range of intermediate forms
• Controlled by multiple genes (polygenic)
• Examples:
  • Human height (short to very tall)
  • Skin color (light to dark)
  • Intelligence

2. Discontinuous Variations

• Show distinct categories without intermediates
• Usually controlled by single genes
• Examples:
  • ABO blood groups (A, B, AB, O)
  • Attached vs. free earlobes
  • Ability to roll tongue

Exceptions to Mendelism

1. Incomplete Dominance

In incomplete dominance, neither allele is completely dominant. The heterozygous condition produces an intermediate phenotype.

Example: Flower Color in Snapdragon (Antirrhinum majus)

Cross:

• Red flower (RR) × White flower (rr)

F₁ Generation:

• All Pink flowers (Rr)
• The red color is incompletely dominant

F₁ Self-pollination (Rr × Rr):

F₂ Ratio:

• Phenotypic ratio: 1 Red : 2 Pink : 1 White (1:2:1)
• Genotypic ratio: 1 RR : 2 Rr : 1 rr (1:2:1)

Note: Phenotypic ratio equals genotypic ratio in incomplete dominance.

2. Co-dominance

In co-dominance, both alleles express themselves equally and simultaneously in the heterozygous condition.

Example: ABO Blood Group System

Genetic Basis:

• Controlled by gene I (Isoagglutinin)
• Three alleles: I^A, I^B, I^O
• I^A and I^B are co-dominant
• Both I^A and I^B are dominant over I^O

Blood Group Genotypes:

Example Cross: I^AI^B × I^BI^O

Offspring possibilities:

• I^AI^B (Blood group AB) - 25%
• I^AI^O (Blood group A) - 25%
• I^BI^B (Blood group B) - 25%
• I^BI^O (Blood group B) - 25%

Result: Children can have blood groups AB, A, or B

3. Multiple Allelism

When a gene exists in more than two allelic forms in a population, it's called multiple allelism.

ABO blood group is the classic example:

• Three alleles (I^A, I^B, I^O) control one trait
• An individual can have only two alleles (one on each chromosome)
• The population has multiple allelic forms

DNA Structure and Inheritance

Structure of DNA (Double Helix Model)

Proposed by James Watson, Francis Crick, and Maurice Wilkins in 1953

Components of DNA:

1. Deoxyribose Sugar (C₅H₁₀O₄)
2. Phosphate Group
3. Nitrogenous Bases:
   • Purines: Adenine (A), Guanine (G)
   • Pyrimidines: Cytosine (C), Thymine (T)

Key Features:

• Two polynucleotide chains running in opposite (antiparallel) directions
• Chains have 5' and 3' ends
• Base Pairing Rules (Chargaff's Rules):
  • Adenine pairs with Thymine (A=T) - 2 hydrogen bonds
  • Guanine pairs with Cytosine (G≡C) - 3 hydrogen bonds
• Chains twisted to form double helix
• Distance between base pairs: 0.34 nm
• Distance per helical turn: 3.4 nm (contains 10 base pairs)
• Diameter of DNA molecule: 2 nm

Significance:

• DNA carries genetic information in the sequence of bases
• Base pairing ensures accurate replication
• The double helix structure protects genetic information

Sex Determination

Sex Determination in Humans

Chromosomal Basis:

• Humans have 46 chromosomes (23 pairs)
• 22 pairs are autosomes (non-sex chromosomes)
• 1 pair is sex chromosomes

Sex Chromosomes:

• Females: XX (homogametic - produce only one type of gamete)
• Males: XY (heterogametic - produce two types of gametes)

Mechanism of Sex Determination

Female Parent (XX):

• All eggs contain: 22 + X

Male Parent (XY):

• 50% sperm contain: 22 + X
• 50% sperm contain: 22 + Y

Fertilization:

Result:

• 50% probability of male child
• 50% probability of female child
• Sex is determined by the father's sperm

Important Note: The sex of a child depends on whether the sperm carrying X or Y chromosome fertilizes the egg. The mother only contributes X chromosome.

Evolution and Natural Selection

What is Evolution?

Evolution is the gradual change in the forms of life from simple to complex over generations, resulting in the diversity of life we see today.

The word 'evolution' comes from the Latin word 'evolvere' meaning 'to unroll' or 'unfold'.

Theories of Origin of Life

Chemical Evolution (Most Accepted Theory)

Concept: Life originated from inorganic molecules through a progressive series of chemical reactions.

Miller-Urey Experiment (1953):

Setup:

• Simulated primitive Earth conditions
• Gas mixture: CH₄ (methane), NH₃ (ammonia), H₂ (hydrogen), H₂O (water vapor)
• Ratio: 2:2:1 plus water vapor
• Temperature: Just below 100°C
• Energy source: Electric sparks (simulating lightning)

Result:

• After one week, 15% carbon converted to organic compounds
• Produced: Amino acids, simple sugars, purines, pyrimidines

Significance: Proved that organic molecules can form from inorganic substances under appropriate conditions.

Darwin's Theory of Natural Selection

Charles Darwin (1809-1882) proposed the Theory of Natural Selection in his book "The Origin of Species" (1859).

Key Principles:

1. Overproduction: Organisms produce more offspring than can survive
2. Struggle for Existence: Competition for limited resources
3. Variation: Individuals in a population show variations
4. Survival of the Fittest: Individuals with favorable variations survive
5. Natural Selection: Nature selects the best-adapted organisms
6. Inheritance: Favorable variations are passed to offspring
7. Speciation: Over time, populations diverge to form new species

Example: Evolution of Giraffe's Long Neck

Darwin's Explanation:

• Ancestral giraffes had varying neck lengths
• Giraffes with longer necks could reach higher leaves
• Better-fed longer-necked giraffes survived and reproduced more
• Over generations, average neck length increased
• Eventually, only long-necked giraffes remained

How Natural Selection Leads to Speciation

Understanding Speciation Through Examples

Case Study: The Beetle Population

Consider a population of 12 red beetles living in green bushes, reproducing sexually and preyed upon by crows.

Scenario 1: Natural Selection

1. Variation arises: Some offspring are green (due to genetic mutation)
2. Differential survival:
   • Green beetles are camouflaged and not eaten
   • Red beetles are visible and eaten by crows
3. Reproductive success: Green beetles survive to reproduce
4. Result: Over generations, green beetles dominate the population

Mechanism: Natural selection (crows acting as selective agent)

Scenario 2: Genetic Drift

1. New variation: Blue beetles appear
2. Random event: Elephant tramples the bushes
3. Chance survival: By luck, mostly blue beetles survive
4. Result: Blue beetles become common

Mechanism: Genetic drift (random chance, not adaptation)

Scenario 3: Environmental Change

1. Environment changes: Plant disease reduces leaf material
2. Effect: All beetles become smaller due to food scarcity
3. Recovery: When plants recover, beetles return to normal size
4. Result: No genetic change occurred

Mechanism: Acquired trait (not inherited)

Factors Leading to Speciation

1. Genetic Variation

• Mutations in DNA
• Sexual reproduction creating new combinations
• Provides raw material for evolution

2. Natural Selection

• Environment selects favorable traits
• Increases frequency of beneficial alleles
• Adapts population to environment

3. Genetic Drift

• Random changes in gene frequency
• More significant in small populations
• Can fix even neutral or harmful traits by chance

4. Geographical Isolation

• Physical barriers separate populations
• Prevents gene flow between groups
• Allopatric speciation

Example:

• Beetle population spread across a mountain
• River forms, dividing population into two
• Different selection pressures on each side
• Over time, two species evolve

5. Reproductive Isolation

• Populations can no longer interbreed
• Due to behavioral, temporal, or mechanical barriers
• Sympatric speciation (without geographical separation)

Example:

• Green female beetles prefer green males
• Reproductive barrier prevents mating with red males
• Two populations diverge even in same area

Complete Process of Speciation

1. Initial Population: Single interbreeding group
2. Variation Arises: Through mutation and recombination
3. Isolation: Geographical or reproductive barriers
4. Differential Selection: Different environments favor different traits
5. Genetic Divergence: Gene pools become increasingly different
6. Reproductive Incompatibility: Groups can no longer interbreed
7. New Species Formed: Two distinct species now exist

Time Scale: Speciation typically requires thousands to millions of years

Evidence of Evolution

1. Homologous Organs

Definition: Organs with similar basic structure and origin but different external appearance and functions.

Examples:

All follow pentadactyl (five-fingered) structure:

• Humerus
• Radius and Ulna
• Carpals
• Metacarpals
• Phalanges (digits)

Plant Examples:

• Thorn of Bougainvillea (modified branch for protection)
• Tendril of Passiflora (modified branch for support)

Significance:

• Indicates common ancestry
• Shows divergent evolution (same structure, different functions)
• Proves organisms evolved from common ancestor

2. Analogous Organs

Definition: Organs with different basic structure and origin but similar external appearance and functions.

Examples:

Significance:

• Shows convergent evolution (different structures, same function)
• Adaptation to similar environments
• Does not indicate common ancestry

3. Vestigial Organs

Definition: Reduced, non-functional organs in some organisms that correspond to fully developed, functional organs in related organisms.

Human Vestigial Organs (approximately 180):

Significance:

• Proves organisms have changed over time
• Shows evolutionary history
• Indicates structural changes during evolution

4. Fossils

Definition: Remains, traces, or impressions of dead organisms preserved in rocks.

Study of Fossils: Palaeontology

How Fossils Form (Fossilization):

1. Organism dies and settles at lake/sea bottom
2. Covered by sediment (sand, silt, mud)
3. Soft parts decay, hard parts (bones, shells) remain
4. More sediment accumulates over time
5. Sediments compress into sedimentary rock
6. Minerals replace organic material
7. Geological processes bring fossils to surface

Dating Fossils:

A. Relative Dating:

• Fossils closer to surface are more recent
• Deeper fossils are older
• Based on rock layer position

B. Absolute Dating (Radioactive Dating):

• Carbon-14 dating: For recent fossils (up to 50,000 years)
• Uranium-Lead dating: For very old fossils (millions of years)
• Potassium-Argon dating: For volcanic rocks

Important Fossil Examples:

Archaeopteryx (Lizard-Bird):

• Lived 180 million years ago
• Reptilian features: Teeth, long tail, clawed fingers, solid bones
• Bird features: Feathers, wings, beak
• Significance: Connecting link between reptiles and birds

Significance of Fossils:

• Direct evidence of evolution
• Shows progression from simple to complex life
• Helps establish evolutionary relationships
• Provides timeline of life on Earth

5. Embryological Evidence

Observation: Early embryos of vertebrates (fish, salamander, tortoise, chick, rabbit, human) look remarkably similar.

Common Features in Early Embryos:

• Gill pouches
• Tail
• Similar body structure
• Comparable developmental stages

Haeckel's Biogenetic Law:"Ontogeny recapitulates phylogeny"

• Ontogeny = Individual development
• Phylogeny = Evolutionary history
• Meaning: An organism repeats its evolutionary history during embryonic development

Significance:

• Shows common ancestry of vertebrates
• Indicates evolutionary relationships
• Demonstrates gradual changes in body plans

Stages of Evolution

Evolution of Complex Organs

Evolution creates complex structures through gradual, step-by-step changes over many generations.

Example 1: Evolution of the Eye

Stage 1: Light-sensitive cells (Planaria)

• Rudimentary eye spots
• Can only detect light and dark

Stage 2: Cup-shaped eye (Some molluscs)

• Can detect direction of light
• Simple image formation

Stage 3: Pinhole camera eye (Nautilus)

• Better image formation
• No lens

Stage 4: Eye with lens (Fish, mammals)

• Focused images
• Better vision

Stage 5: Complex eye (Birds, primates)

• Color vision
• High resolution
• Advanced processing

Key Point: Each stage provided survival advantage, even if not perfect.

Example 2: Evolution of Wings and Feathers

Stage 1: Insulation

• Feathers first evolved in dinosaurs for warmth
• Not used for flying

Stage 2: Display

• Feathers used for attracting mates
• Still not for flight

Stage 3: Gliding

• Small dinosaurs used feathered forelimbs to glide
• Beginning of flight

Stage 4: Active Flight

• Birds adapted feathers for powered flight
• Wings became primary locomotion

Important Concept:Exaptation

• A feature that evolved for one function later adapted for another
• Feathers: Insulation → Flight
• Wings: Gliding → Powered flight

Human Evolution

Classification of Humans

• Phylum: Chordata
• Class: Mammalia
• Order: Primates
• Family: Hominidae
• Genus: Homo
• Species: sapiens

Evolutionary Lineage of Humans

Common Ancestor: Anthropoid mammals (shared with apes)

Important Note: Humans did NOT evolve from chimpanzees. Both humans and chimpanzees evolved from a common ancestor that lived millions of years ago.

Major Stages in Human Evolution

Characteristics of Human Evolution

Progressive Changes:

1. Brain size increase: 400 cc → 1350 cc
2. Upright posture: Bipedal locomotion
3. Tool use: Simple stones → complex tools
4. Language development: Communication skills
5. Social organization: Family units → civilizations
6. Reduced body hair: Less coverage over time
7. Flatter face: Protruding jaw → flat face
8. Opposable thumb: Better grasping ability

Key Innovation - Bipedalism:

• Freed hands for tool use
• Better vision over tall grass
• More efficient locomotion
• Allowed brain expansion

Artificial Selection

Definition: Process by which humans select organisms with desired traits and breed them to improve those characteristics.

Difference from Natural Selection:

Examples of Artificial Selection

Example 1: Wild Cabbage Evolution

Ancestral Plant: Wild cabbage (Brassica oleracea)

Selection for Different Parts:

Process:

• Same species, different selection pressures
• Human-directed breeding
• Shows rapid morphological changes

Example 2: Cattle Breeding

Goal: High milk-yielding cows

Process:

1. Monitor milk yield of many cows
2. Select highest-yielding cows
3. Breed their calves together
4. Repeat for multiple generations
5. Result: High milk-yielding breed

Modern Applications:

• Crop improvement (high yield, disease resistance)
• Livestock breeding (meat quality, egg production)
• Pet breeding (specific appearance, behavior)
• Plant ornamentals (flower color, size)

Evolution vs. Progress

Important Concept: Evolution does NOT mean "progress" toward a goal.

Common Misconceptions

• Wrong: Evolution is a ladder with humans at the top v/s Correct: Evolution is a branching tree with many endpoints
• Wrong: Species evolve to become "better" v/s Correct: Species evolve to become better adapted to their environment
• Wrong: Evolution has a direction or purpose v/s Correct: Evolution is random changes + natural selection

The Evolutionary Tree Concept

• No species is "higher" or "lower"
• All living species are equally evolved
• Bacteria are as "modern" as humans
• Each species is adapted to its niche

Example:

• Bacteria survived for 3.5 billion years
• They thrive in extreme environments
• They are perfectly adapted to their lifestyle
• In terms of survival, they are highly "successful"

Important Formulas

Important NCERT Points for Class 10 Heredity and Evolution

Quick Revision Points

Heredity Basics

1. Heredity is the transmission of genetic characteristics from parents to offspring
2. Genetics is the study of heredity and variations
3. Gregor Mendel is the Father of Genetics
4. Pea plant (Pisum sativum) was used by Mendel
5. Mendel studied 7 pairs of contrasting traits

Mendel's Laws

6. Law of Dominance: One trait masks another
7. Law of Segregation: Alleles separate during gamete formation
8. Law of Independent Assortment: Traits are inherited independently

Important Ratios

9. Monohybrid F₂ ratio: 3:1 (phenotypic), 1:2:1 (genotypic)
10. Dihybrid F₂ ratio: 9:3:3:1 (phenotypic)
11. Test cross ratio: 1:1
12. Incomplete dominance ratio: 1:2:1

Genetic Terms

13. Gene: Unit of heredity
14. Allele: Alternative form of a gene
15. Dominant allele: Expresses in heterozygous condition
16. Recessive allele: Expresses only in homozygous condition
17. Genotype: Genetic makeup
18. Phenotype: Physical expression
19. Homozygous: Identical alleles (TT or tt)
20. Heterozygous: Different alleles (Tt)

DNA and Chromosomes

21. DNA carries genetic information
22. DNA has double helix structure
23. Watson and Crick proposed DNA structure (1953)
24. Four nitrogenous bases: A, T, G, C
25. Base pairing: A-T (2 bonds), G-C (3 bonds)
26. Chromosomes carry genes

Sex Determination

27. Humans have 46 chromosomes (23 pairs)
28. 22 pairs are autosomes, 1 pair is sex chromosomes
29. Females: XX (homogametic)
30. Males: XY (heterogametic)
31. Father determines sex of child
32. 50% chance of male or female child

Variations

33. Variation: Differences among individuals
34. Variations arise from DNA copying errors and sexual reproduction
35. Somatic variations: Not inherited
36. Germinal variations: Inherited
37. Variations provide raw material for evolution

Evolution

38. Evolution: Gradual change from simple to complex forms
39. Darwin proposed Theory of Natural Selection
40. Book: "Origin of Species" (1859)
41. Natural selection: Survival of the fittest
42. Favorable variations are selected by nature

Evidence of Evolution

43. Fossils: Preserved remains of ancient organisms
44. Palaeontology: Study of fossils
45. Homologous organs: Same structure, different functions (divergent evolution)
46. Analogous organs: Different structure, same function (convergent evolution)
47. Vestigial organs: Reduced, non-functional organs
48. Archaeopteryx: Connecting link between reptiles and birds

Speciation

49. Speciation: Formation of new species
50. Geographical isolation leads to speciation
51. Reproductive isolation prevents interbreeding
52. Genetic drift: Random changes in gene frequency

Human Evolution

53. Humans belong to genus Homo, species sapiens
54. Humans evolved from anthropoid ancestors
55. Ramapithecus: Earliest man-like primate (found in India)
56. Homo habilis: First tool user
57. Homo erectus: First to migrate from Africa
58. Modern humans: Homo sapiens sapiens

Artificial Selection

59. Humans select traits for breeding
60. Wild cabbage evolved into cabbage, broccoli, cauliflower
61. Faster than natural selection
62. Used in agriculture and animal husbandry

Tips for CBSE Class 10 Exam Preparation

High-Weightage Topics

• Mendel's experiments and laws
• Monohybrid and dihybrid crosses
• Sex determination
• Evolution and natural selection
• Evidence of evolution (homologous, analogous, vestigial organs)
• Human evolution timeline

Common Question Types

1. Definitions: Heredity, genetics, variation, evolution, speciation
2. Diagram-based: Monohybrid cross, dihybrid cross, DNA structure, sex determination
3. Differentiate: Homologous vs analogous, dominant vs recessive, acquired vs inherited
4. Examples: Seven traits of pea plant, vestigial organs, homologous organs
5. Reasoning: Why sex is determined by father, importance of variations
6. Application: Blood group inheritance, genetic problems

Important Diagrams to Practice

• Monohybrid cross (TT × tt)
• Dihybrid cross (RRYY × rryy)
• Sex determination in humans
• DNA double helix structure
• Homologous organs (forelimbs of vertebrates)
• Human evolution timeline

Study Strategy

1. Understand concepts before memorizing
2. Practice Punnett squares for genetic crosses
3. Learn all definitions precisely
4. Draw and label diagrams multiple times
5. Solve NCERT exercises thoroughly
6. Revise ratios (3:1, 9:3:3:1, 1:1)
7. Make short notes of key points
8. Practice previous year questions

Conclusion

Heredity and Evolution are fundamental concepts that explain how traits pass through generations and how species change over time. Mendel's pioneering work laid the foundation for genetics, revealing the mathematical patterns of inheritance. Understanding these principles helps us appreciate the diversity of life and our own evolutionary journey.