1. Introduction

• ★Reproduction and Variation:

  Reproduction is the biological process by which living organisms produce new individuals similar to themselves. However, no two individuals are exactly alike because of subtle differences called variations, which occur naturally during the process of reproduction.

• ★Asexual versus Sexual Reproduction:

  In asexual reproduction, a single parent produces offspring that are genetically almost identical to itself. The only source of variation in such cases is the occasional error during DNA replication, for example, in bacteria undergoing binary fission. In contrast, sexual reproduction involves two parents contributing genetic material to the offspring. This results in a much higher degree of variation, as each offspring receives a unique combination of genes from both parents, which provides the raw material for evolution to act upon.

• ★Natural Selection:

  Not all variations are equally beneficial in a given environment. The environment "selects" traits that provide a survival or reproductive advantage. For instance, bacteria that develop a heat-resistant mutation are more likely to survive a sudden heatwave, while their non-resistant counterparts perish, illustrating how beneficial variations are naturally preserved.

2. Heredity

• ★Definition:

  Heredity is the biological process through which traits and characteristics are transmitted from parents to offspring. This occurs through genetic material encoded in DNA, which carries the instructions for the development, growth, and functioning of the organism.

• ★Inherited Traits:

  Traits in an offspring are influenced by both the mother’s and father’s DNA because each parent contributes equally to the genetic makeup of the child. Each characteristic is determined by two copies of a gene, one from each parent, known as alleles. The interaction between these alleles determines the trait expressed in the offspring.

• ★Genes and Chromosomes:

  Genes are specific segments of DNA that contain instructions to produce proteins, which ultimately determine physical traits. Genes are organized on chromosomes, which are long, thread-like structures found in the nucleus of cells. Most body cells contain pairs of chromosomes — one inherited from the mother and one from the father — ensuring that offspring inherit a combination of traits from both parents. Sex cells, or gametes, carry only one set of chromosomes so that normal chromosome number is restored after fertilization.

2.1 Mendel’s Laws of Inheritance

Gregor Mendel, an Austrian monk, used pea plants to discover clear and predictable patterns of inheritance. His careful experiments revealed how traits are passed from one generation to another, forming the basis of classical genetics.

Monohybrid Cross (One Trait)

• ★Experiment: Mendel crossed tall pea plants (TT) with short plants (tt) to study the inheritance of height.
• ★F1 Generation: All offspring were tall (Tt), showing that tallness is dominant over shortness.
• ★F2 Generation: When the F1 plants were self-pollinated, the progeny exhibited a phenotypic ratio of 3 tall:1 short. The genotypic ratio was 1 TT : 2 Tt : 1 tt.
• Interpretation: Traits are inherited as separate discrete units (genes). A dominant allele expresses itself even if only one copy is present, whereas a recessive allele requires two copies to be expressed.

Dihybrid Cross (Two Traits)

• ★Experiment: Mendel crossed plants differing in two traits simultaneously: height (Tall/Short) and seed shape (Round/Wrinkled), i.e., TTRR × ttrr.
• ★F1 Generation: All offspring showed the dominant traits (Tall & Round).
• ★F2 Generation: Self-pollination of F1 offspring resulted in a mix of parental and new trait combinations, confirming that each trait segregates independently.
• Key Conclusion: Different traits are inherited independently, as explained by Mendel’s law of independent assortment, provided the genes are located on different chromosomes.

2.2 Mechanism of Heredity

• ★Genes control traits through proteins: DNA carries instructions to make specific proteins, which determine the organism’s characteristics. For example, genes controlling height may produce enzymes that regulate growth hormones. If the gene is altered, the enzyme may work less effectively, resulting in a shorter plant.
• ★Chromosome organization: Genes are located on chromosomes, which are paired in most body cells. Gametes carry only one set of chromosomes so that fertilization restores the full set, maintaining genetic continuity across generations.

2.3 Sex Determination

• ★Environmental sex determination: In some reptiles, the temperature at which eggs develop determines the sex of the offspring. Some snails can even change sex depending on environmental conditions.
• ★Genetic sex determination in humans: Humans have 23 pairs of chromosomes. The first 22 pairs are autosomes and are identical in males and females. The 23rd pair are sex chromosomes: females have XX, and males have XY. An egg always contributes an X chromosome, while the sperm may contribute X or Y. If sperm contributes X → child is female (XX), if Y → child is male (XY). Therefore, the father’s sperm determines the sex of the child.

3. Evolution

• ★Definition: Evolution is the gradual process through which populations of organisms change over successive generations, resulting in the appearance of new traits and species adapted to their environment.
• ★Mechanisms of Evolution: Variation provides the raw material. Natural selection, genetic drift, mutation, and gene flow modify the genetic composition of populations over time, leading to evolutionary change.

3.1 Mechanisms of Evolution

• ★Natural Selection: Traits that provide a survival advantage are more likely to be passed on. For example, green beetles in green bushes are less visible to predators than red beetles, so green beetles survive and reproduce more.
• ★Genetic Drift: Random events can change allele frequencies in small populations. For instance, if a storm randomly kills most beetles, the survivors’ traits determine the next generation by chance rather than advantage.
• ★Mutation: Random changes in DNA can introduce new traits. Some mutations are beneficial and can become common in a population if they improve survival.
• ★Gene Flow: Movement of individuals between populations brings new genes and alters gene frequencies, influencing evolution.
• ★Acquired Traits: Traits acquired during life, such as muscle development from exercise, are not inherited because they do not affect the DNA in gametes.

3.2 Speciation

• ★Definition: Speciation is the process through which one species splits into two or more distinct species that cannot interbreed under natural conditions.
• ★Geographic isolation: A physical barrier, like a river or mountain, splits a population. Over time, different mutations, selection pressures, and genetic drift cause the two populations to diverge genetically. Eventually, they become reproductively isolated and are classified as separate species.

4. Evolution and Classification

• ★Classification reflects evolution: Species are grouped based on shared characteristics and evolutionary relationships. The more features two species share, the more recently they share a common ancestor.
• ★Homologous organs: Structures with the same basic anatomy but different functions indicate common ancestry, for example, the forelimbs of humans, whales, bats, and frogs.
• ★Analogous organs: Structures with similar functions but different origins indicate adaptation to similar environments, such as the wings of birds and bats.
• ★Fossils and Molecular Phylogeny: Fossils provide historical evidence of extinct species and transitional forms. DNA comparisons reveal evolutionary relationships by showing how closely related species are.

4.1 Evolution by Stages

• ★Complex organs evolve gradually: Organs like the eye evolved incrementally over millions of years. Each intermediate stage provided a functional advantage, allowing gradual adaptation.
• For instance, simple light-sensitive cells in planaria evolved into more complex eyes in insects and vertebrates, improving survival at each stage.
• ★Feathers: Initially evolved for warmth and insulation in dinosaurs and were later adapted for flight in birds.

4.2 Artificial Selection

• ★Definition: Humans deliberately select plants or animals with desirable traits and breed them, guiding their evolution.
• Wild cabbage has been selectively bred into cabbage (large leaves), broccoli (arrested flower growth), cauliflower (dense flowers), kale (large leaves), and kohlrabi (swollen stems).

5. Human Evolution

• ★All humans belong to the same species: Modern humans are Homo sapiens. There is no biological basis for dividing humans into separate "races" because genetic differences are very small.
• ★Origin and Migration: The earliest Homo sapiens originated in Africa. Humans migrated across Asia, Europe, and eventually the entire globe, adapting to different climates and ecosystems over thousands of years.

6. Exam Tips & Summary