Heredity is the transmission of characteristics from parents to offspring through genes. Evolution is the gradual change in the inherited characteristics of biological populations over successive generations.
Key Concepts
Gene: A segment of DNA that codes for a specific trait. Genes are located on chromosomes found in the nucleus of every cell.
Alleles: Different forms of the same gene. An organism typically inherits two alleles for each gene — one from each parent.
Dominant allele: An allele whose trait is expressed even when only one copy is present (represented by a capital letter, e.g., T).
Recessive allele: An allele whose trait is expressed only when two copies are present (represented by a lowercase letter, e.g., t).
Genotype: The genetic makeup of an organism (e.g., TT, Tt, or tt).
Phenotype: The observable physical trait resulting from the genotype (e.g., tall or short).
Homozygous: An organism with two identical alleles (TT or tt).
Heterozygous: An organism with two different alleles (Tt).
- Gregor Mendel (the Father of Genetics) conducted experiments on pea plants, discovering the laws of inheritance:
- Law of Dominance: In a cross between two pure-breeding varieties, only one trait (dominant) appears in the F1 generation.
- Law of Segregation: The two alleles for each trait separate during gamete formation so each gamete carries only one allele.
Acquired traits (gained during an organism's lifetime, e.g., a body builder's muscles) are NOT passed to offspring. Only inherited traits encoded in genes are passed on.
Evolution: Changes in allele frequencies in a population over time. Charles Darwin proposed the theory of Natural Selection — organisms with favourable traits survive and reproduce more successfully, passing those traits to offspring.
Speciation: The formation of new species from existing ones, often caused by geographic isolation, genetic drift, or natural selection.
Homologous organs: Structures with similar origin but different functions (e.g., human arm, whale flipper, bat wing) — evidence of common ancestry.
Analogous organs: Structures with different origins but similar functions (e.g., wings of birds and insects) — result of convergent evolution.
Fossils: Preserved remains or impressions of ancient organisms; provide direct evidence of evolution.
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Mendel crossed pure tall (TT) pea plants with pure short (tt) plants. All F1 offspring were tall (Tt) because T is dominant. When F1 plants were self-pollinated, the F2 ratio was 3 tall : 1 short (TT, Tt, Tt, tt). This 3:1 ratio is the classic Mendelian ratio.
A cross between two heterozygous tall plants (Tt x Tt). Using a Punnett square: TT (25%), Tt (50%), tt (25%). Phenotype ratio = 3 tall : 1 short. Genotype ratio = 1 TT : 2 Tt : 1 tt.
A plant with round seeds (RR) is crossed with one having wrinkled seeds (rr). F1 plants are all Rr (round). F1 x F1 gives F2 ratio 3 round : 1 wrinkled. This shows that 'R' (round) is dominant over 'r' (wrinkled).
In humans, tongue rolling (T) is dominant over non-rolling (t). Two heterozygous parents (Tt x Tt) have four children. On average, 3 will be rollers and 1 will not. However, actual offspring may differ due to chance — probability, not certainty.
Acquired trait example: A blacksmith's son does not inherit muscular forearms developed by his father through work. Only traits coded in DNA are heritable.
Homologous organs — the forelimbs of a human, horse, whale, and bat all have the same basic bone structure (humerus, radius, ulna, carpals) yet perform different functions. This indicates evolution from a common ancestor.
Natural selection in peppered moths (Biston betularia) — before industrialisation, light-coloured moths survived better on light-coloured trees; after soot darkened trees, dark moths survived better. Over time, the population shifted towards darker moths.
Key Relationships
- F1 generation from pure-breeding cross: all dominant phenotype
- F2 generation from F1 self-cross: 3 dominant : 1 recessive (phenotype ratio)
- Genotype ratio in F2: 1 : 2 : 1
Common mistakes
Students often confuse genotype with phenotype — remember genotype is the gene combination (TT, Tt, tt) while phenotype is what you see (tall/short). Also, do not confuse homologous (same origin, different function) with analogous (different origin, same function).
Summary
Mendel's laws explain how traits are inherited through dominant and recessive alleles. Evolution occurs through natural selection acting on inherited variation. Fossils, homologous organs, and molecular evidence all support the theory of evolution.