Heredity & Evolution — 50 Q&A

Heredity is the transmission of characteristics from parents to offspring through genes.

Variation refers to differences among individuals of a species. Types: continuous (e.g., height) and discontinuous (e.g., blood groups); also genetic vs environmental variation.

A gene is a unit of heredity made of DNA; alleles are different forms of a gene. Genes are located on chromosomes (linear DNA-protein structures in nucleus).

Mendel showed inheritance follows patterns: law of dominance (dominant traits mask recessive in F1) and law of segregation (alleles separate during gamete formation).

Genotype = genetic constitution (e.g., TT, Tt, tt). Phenotype = observable trait (e.g., tall or short plant).

Monohybrid cross studies inheritance of one trait (e.g., T × t). Mendel observed F2 phenotypic ratio 3:1 (dominant:recessive).

Dihybrid cross studies two traits simultaneously. Mendel's law of independent assortment states alleles of different genes assort independently — typical F2 ratio 9:3:3:1.

Dominance: one allele masks another. Codominance: both alleles expressed (AB blood group). Incomplete dominance: intermediate phenotype (red × white → pink in some plants).

Humans: XY system (males XY, females XX). Bees: haplodiploidy — males from unfertilised haploid eggs, females from fertilised diploid eggs.

Mutation = change in DNA sequence. Types: point, chromosomal, insertion/deletion. Effects: harmful, neutral, or beneficial. Example: sickle-cell trait confers malaria resistance (heterozygote advantage).

Evolution is change in inherited characteristics of biological populations over successive generations — often summarised as descent with modification.

Lamarck proposed use & disuse and inheritance of acquired traits (e.g., giraffes stretching necks). It is incorrect because acquired changes are not generally passed genetically to offspring.

Natural selection: individuals with favourable variations survive & reproduce more, changing population over time (e.g., industrial melanism in peppered moths).

Selection pressures are environmental factors that influence survival (predation, climate, disease). Human-driven example: artificial selection in domestication of dogs/crops.

Speciation is formation of new species, often due to reproductive isolation (e.g., geographic separation) leading to independent evolution and inability to interbreed.

Fossils (imprints, petrified remains) show past life forms and transitional fossils (e.g., Archaeopteryx) indicate evolutionary links between groups over geological time.

Homologous organs (common origin, different function) e.g., forelimbs of human, whale, bat. Analogous organs (similar function, different origin) e.g., wings of insects and birds (convergent evolution).

Adaptive radiation is rapid diversification of a single lineage into many species adapted to different niches — e.g., Darwin's finches in Galápagos islands.

DNA is the hereditary material: double helix of nucleotides (A, T, G, C) with complementary base pairing (A–T, G–C) encoding genetic information.

Diploid (2n) = two sets of chromosomes (body cells); haploid (n) = one set (gametes). Meiosis produces haploid gametes; mitosis maintains diploid status.

Crossing-over is exchange of chromosome segments between homologous chromosomes during meiosis (chiasma), generating genetic recombination and increasing variation.

A pedigree chart traces inheritance of a trait across generations in a family using standard symbols (squares = male, circles = female, shaded = affected).

Sickle-cell anaemia is caused by point mutation in β-globin gene causing abnormal hemoglobin; homozygotes show disease, heterozygotes have some resistance to malaria.

Bacteria with random mutations allowing survival in antibiotics reproduce. Repeated antibiotic use selects resistant strains — an example of natural selection driven by human pressure.

Peas had easily observed traits, true-breeding varieties, short generation times and controllable crosses — ideal to identify inheritance patterns and formulate laws.

Vestigial organs are reduced structures with little/no function, e.g., human appendix, whale pelvic bones — evidence of evolutionary ancestry.

Molecular clock uses mutation rates in DNA to estimate divergence times between species, assuming roughly constant accumulation of neutral mutations.

Artificial selection is human-directed breeding for desired traits (e.g., crops, dog breeds). Unlike natural selection, humans choose which individuals breed, not the environment.

Neutral theory suggests most molecular changes are selectively neutral and fixed by genetic drift rather than positive selection; useful for molecular clock ideas.

Fossils form by burial and mineralization (permineralization), casts or impressions. Dating uses relative methods (strata) and absolute methods like radiometric dating (radioactive decay).

Gene pool = total set of alleles in a population. It determines genetic diversity; evolution is change in allele frequencies within the gene pool over time.

Isolation types: temporal (breeding at different times), behavioural (different mating rituals), mechanical (incompatible anatomy), ecological (different habitats) — all reduce gene flow and promote speciation.

Microevolution = small-scale changes in allele frequencies within populations. Macroevolution = large-scale patterns including speciation and major taxonomic changes over long time.

Cultural evolution involves transmission of behaviours/knowledge socially (language, technology) and operates faster than genetic evolution; it is not encoded in DNA.

Human evolution traces hominids: early bipedal Australopithecus → Homo erectus (tool use, dispersal) → Homo sapiens with larger brain, complex tools and culture. Key traits: bipedalism, larger brain, language.

Gene flow = transfer of alleles between populations via migration; it reduces genetic differences and can introduce new variation into a population.

Fitness = reproductive success of an individual relative to others; higher fitness alleles increase in frequency under natural selection. It is context-dependent on environment.

Convergent evolution is independent evolution of similar features in unrelated groups due to similar environmental pressures (analogous structures).

Gradualism: slow, steady evolutionary change. Punctuated equilibrium: long periods of stasis interrupted by short rapid speciation events — both patterns observed in fossil record.

Closely related species have more similar DNA/protein sequences (e.g., human and chimpanzee genomes). Molecular comparisons provide quantitative evidence for relatedness.

Evolutionary principles inform medicine (antibiotic resistance), agriculture (crop breeding), conservation (biodiversity management) and understanding life's history.

• Mendelian laws explain inheritance patterns; genes & alleles on chromosomes determine traits.
• Variation (genetic + environmental) is essential for evolution.
• Mutations, recombination and selection change allele frequencies.
• Evidence for evolution: fossils, comparative anatomy, embryology, molecular biology.