Part III · Common Biology · Chapter Twelve
Genetics and Evolution
Expect 8–12 questions: Mendelian ratios and modifications (incomplete dominance, epistasis), sex-linked disorders (haemophilia, colour-blindness), DNA structure and replication (Watson-Crick, Meselson-Stahl), the genetic code and lac operon, Hardy-Weinberg equilibrium problems, Darwin & Modern Synthesis, speciation types, and HP-specific items (Himalayan endemics as allopatric speciation examples, CSIR-IHBT tea-genome work, UHF Solan apple breeding).
Read · 90 min
Revise · 20 min
MCQs · 30
Syllabus Coverage
Mendel's laws • Modifications of Mendelian ratios • Linkage and recombination • Sex determination and sex-linked inheritance • Mutations and human genetic disorders • DNA structure and replication • Transcription, translation and the genetic code • Gene regulation (lac operon, trp operon) • Theories of evolution • Hardy-Weinberg equilibrium • Evidence for evolution and speciation.
Mendel 1865 (published) · Sutton-Boveri chromosome theory 1902 · Bateson-Punnett linkage 1906 · Morgan Drosophila 1910 (Nobel 1933) · Griffith transformation 1928 · Avery-MacLeod-McCarty DNA = transforming principle 1944 · Hershey-Chase phage experiment 1952 · Watson-Crick double helix 1953 (Nobel 1962, with Wilkins) · Meselson-Stahl semiconservative replication 1958 · Jacob-Monod operon model 1961 (Nobel 1965) · Nirenberg-Khorana-Holley genetic code 1968 (Nobel) · Greider-Blackburn telomerase 1985 (Nobel 2009)
12.1 Mendel and the Laws of Inheritance
Gregor Johann Mendel (1822–1884), an Augustinian friar at Brno (Brünn), published Versuche über Pflanzenhybriden (Experiments on Plant Hybridisation) in 1865 — a paper ignored for 35 years until rediscovered simultaneously by De Vries, Correns and Tschermak in 1900. His subject was the garden pea Pisum sativum: cheap, rapid (annual), self-fertile (controls crosses), and available in true-breeding lines. He chose seven pairs of contrasting traits, each controlled by a single gene on a different chromosome — a lucky choice that avoided the complications of linkage and pleiotropy.
Key Terms — Gene, Allele, Genotype, Phenotype
Gene: a heritable unit of information at a specific chromosomal locus that determines a trait. Allele: one of two or more alternative forms of a gene. Genotype: the allelic constitution (e.g., Tt). Phenotype: the observable expression of the genotype in a given environment. Homozygous: both alleles identical (TT or tt). Heterozygous: alleles different (Tt). True-breeding: homozygous — self-fertilisation yields unchanged offspring.
| Trait | Dominant (symbol) | Recessive (symbol) | F₂ ratio |
|---|---|---|---|
| Seed shape | Round (R) | Wrinkled (r) | 5474 : 1850 ≈ 2.96 : 1 |
| Seed colour (cotyledon) | Yellow (Y) | Green (y) | 6022 : 2001 ≈ 3.01 : 1 |
| Seed coat colour | Grey/coloured (G) | White (g) | 705 : 224 ≈ 3.15 : 1 |
| Pod shape | Inflated (V) | Constricted (v) | 882 : 299 ≈ 2.95 : 1 |
| Pod colour | Green (I) | Yellow (i) | 428 : 152 ≈ 2.82 : 1 |
| Flower position | Axial (A) | Terminal (a) | 651 : 207 ≈ 3.14 : 1 |
| Stem height | Tall (T) | Dwarf (t) | 787 : 277 ≈ 2.84 : 1 |
12.1.1 The Three Laws
Law 1 — Law of Dominance. When two individuals homozygous for contrasting alleles are crossed, the hybrid (F₁) resembles only one parent — the dominant. The recessive allele is present but masked. Strictly, this is an observation rather than a universally applicable law (incomplete dominance violates it), but Mendel used it to explain the uniform F₁.
Law 2 — Law of Segregation (Purity of Gametes). The two alleles of any gene separate (segregate) at meiosis so that each gamete carries only one allele. The alleles do not blend; they recombine faithfully in each generation. This is the most fundamental Mendelian law and holds even when dominance is absent. Basis: homologous chromosome separation in meiosis I.
Law 3 — Law of Independent Assortment. Allele pairs at different loci segregate independently of one another. This produces new combinations (recombinants) in the F₂ of a dihybrid cross. Caveat: applies only when genes are on different chromosomes (or far apart on the same chromosome). Linked genes violate this law.
Gene & Allele
A gene is the locus (physical address). An allele is a version of that gene. “Tall” and “dwarf” are alleles of the height gene. You can have many alleles per gene in a population (multiple allelism) but only two per diploid individual.
Genotype & Phenotype
The genotype is the allele pair: Tt, TT, or tt. The phenotype is the observable result: tall or dwarf. Two different genotypes (TT and Tt) can produce the same phenotype (tall) when dominance is complete. Environment can modify phenotype without changing genotype (norm of reaction).
12.1.2 Monohybrid Cross and Ratios
Cross TT (tall) × tt (dwarf): F₁ all Tt (tall). Intercross F₁ × F₁: F₂ genotypic ratio TT : Tt : tt = 1 : 2 : 1; phenotypic ratio tall : dwarf = 3 : 1.
12.1.3 Dihybrid Cross and Independent Assortment
Cross TTYY × ttyy: F₁ all TtYy (tall, yellow). Intercross F₁ produces four gamete types in equal proportions (TY, Ty, tY, ty). F₂ phenotypic ratio: 9 Round-Yellow : 3 Round-Green : 3 Wrinkled-Yellow : 1 Wrinkled-Green. Total 16 combinations in 4×4 grid.
12.1.4 Test Cross
To determine whether a dominant-phenotype individual is homozygous (TT) or heterozygous (Tt), cross it with the homozygous recessive (tt). If all offspring are dominant phenotype → parent was TT. If ratio is 1 dominant : 1 recessive → parent was Tt. This is the test cross (back cross to recessive).
Worked Example — Monohybrid test cross interpretation
A tall pea plant is test-crossed with a dwarf plant. Among 80 offspring, 43 are tall and 37 are dwarf.
Step 1: Dwarf is recessive (tt), so the test-cross parent contributes only t gametes.
Step 2: The 43 tall : 37 dwarf ratio approximates 1:1, so the tall parent is Tt (heterozygous).
Step 3: Cross: Tt × tt → gametes T, t × t → offspring Tt (tall) and tt (dwarf) in 1:1 ratio. Answer: heterozygous tall.
Mnemonic — Punnett Square Steps
Genotypes first → Gametes listed on axes → Combine in cells → Count phenotypes. Remember: "GG CC" — Genotypes-Gametes, Count-Classes.
For 3:1 ratio, remember “three-quarter dominant, quarter recessive” — applies only to simple dominance, single locus.
12.2 Beyond Mendel — Incomplete Dominance, Codominance, Lethality, Multiple Alleles, Pleiotropy, Polygenic Inheritance
Mendel's model of complete dominance is the simplest inheritance pattern. Many genes show deviations in which the heterozygote is distinguishable from both homozygotes, or where one gene influences many traits, or where many genes together shape one quantitative trait.
12.2.1 Incomplete Dominance
Neither allele is fully dominant; the heterozygote is phenotypically intermediate. Classic example: flower colour in Mirabilis jalapa (four-o'clock). Cross red (R¹R¹) × white (R²R²): F₁ all pink (R¹R²). F₁ × F₁: F₂ ratio 1 red : 2 pink : 1 white. Key point: phenotypic ratio = genotypic ratio 1:2:1 — unlike full dominance where phenotypic 3:1 ≠ genotypic 1:2:1.
Incomplete Dominance
Heterozygote = blend of parents. Both alleles expressed but neither fully. Phenotype is intermediate. Example: red + white = pink in Mirabilis; snapdragon (Antirrhinum). F₂ ratio 1:2:1 (phenotypic = genotypic).
Codominance
Heterozygote shows BOTH parental phenotypes simultaneously — no blending. Example: ABO blood group IₐIₛ = type AB (both A and B antigens present). Roan coat in cattle (red + white hairs). F₂ still 1:2:1 genotypically, but all three distinct phenotypes are visible.
12.2.2 Multiple Alleles and ABO Blood Groups
A gene may have more than two allelic forms in a population, though any diploid individual carries at most two. The ABO blood group gene (I locus) has three alleles: Iₐ, Iₛ, i. Iₐ and Iₛ are codominant; both are dominant over i. This gives six genotypes producing four blood groups (A, B, AB, O). Other examples: rabbit coat colour (Castle's series with four alleles: C+, cch, ch, c); histocompatibility (HLA) genes (hundreds of alleles).
| Blood group (phenotype) | Genotype(s) | Antigen on RBC | Antibody in plasma |
|---|---|---|---|
| A | IₐIₐ or Iₐi | A | anti-B |
| B | IₛIₛ or Iₛi | B | anti-A |
| AB | IₐIₛ | A and B | neither |
| O | ii | neither | anti-A and anti-B |
12.2.3 Lethal Alleles
Some alleles cause death when homozygous, distorting expected ratios. Yellow coat colour in mice: the Ay allele is dominant for coat colour but homozygous lethal (impairs yolk-sac nutrition). Cross AyA × AyA would give 1 AyAy (lethal) : 2 AyA (yellow) : 1 AA (agouti). Surviving ratio: 2 yellow : 1 agouti — an apparent 2:1 ratio is the hallmark of a dominant lethal when homozygous.
12.2.4 Pleiotropy
A single gene affecting multiple, seemingly unrelated traits. Sickle-cell anaemia is the premier example: one mutation (Glu→Val at position 6 of β-globin) causes haemolytic anaemia, pain crises, splenomegaly, susceptibility to infection, retinal damage, strokes — dozens of phenotypic consequences from one allele. Marfan syndrome (fibrillin-1 gene): arachnodactyly, tall stature, aortic aneurysm, lens dislocation — again, one gene, multiple phenotypes.
12.2.5 Polygenic Inheritance
A single trait controlled by multiple genes, each contributing a small additive effect. The trait shows continuous variation (bell-shaped distribution) rather than discrete classes. Classic example: Nilsson-Ehle (1909) on wheat (Triticum aestivum) kernel colour — two independently assorting loci (A, B) with additive alleles (A¹, A², B¹, B²). F₁ A¹A²B¹B² (intermediate red). F₂ 5-class distribution 1:4:6:4:1. Human examples: skin colour, height, IQ — all polygenic + environmental.
12.2.6 Epistasis
Two or more loci interact such that alleles at one locus mask the expression of alleles at another locus. This modifies the standard 9:3:3:1 dihybrid ratio. Key epistasis types for exams:
| Type | Modified ratio | Mechanism | Example |
|---|---|---|---|
| Dominant epistasis | 12 : 3 : 1 | Dominant allele at locus A masks B | Squash fruit colour |
| Duplicate dominant | 15 : 1 | Dominant at A OR B produces same phenotype | Capsella bursa-pastoris leaf shape |
| Recessive epistasis | 9 : 3 : 4 | Recessive homozygote at locus B masks A | Labrador coat (bb = yellow regardless of A) |
| Complementary | 9 : 7 | Both dominant alleles needed for phenotype | Flower colour in Lathyrus odoratus |
| Dominant+recessive | 13 : 3 | Dominant at A OR recessive homozygote at B masks phenotype | Frizzled poultry feathers |
| Duplicate recessive | 9 : 6 : 1 | Both loci contribute additively | Grain size in oats |
Mnemonic — Epistasis Ratios
"Dominant 12-3-1, Complementary 9-7, Recessive 9-3-4, Duplicate 15-1."
Remember: all sum to 16. When you see “9:7” think Lathyrus sweet pea complementary genes. When you see “9:3:4” think Labrador retriever coat. When you see “12:3:1” think dominant epistasis (squash).
12.3 Linkage, Recombination & Genetic Mapping
Mendel's Law of Independent Assortment holds only for genes on different chromosomes. Genes on the same chromosome tend to be inherited together — they are linked. Linkage was first demonstrated by Bateson and Punnett (1906) in sweet pea (Lathyrus odoratus): flower colour and pollen shape did not assort independently. Thomas Hunt Morgan (1910) established the chromosomal basis of linkage using Drosophila melanogaster, for which he received the Nobel Prize in 1933.
Linkage Group
All genes located on the same chromosome form one linkage group. The number of linkage groups equals the haploid chromosome number of the organism. Drosophila has 4 linkage groups (n=4); humans have 23; pea has 7 — which is why Mendel's 7 traits happened to sort independently (one per chromosome).
12.3.1 Coupling and Repulsion
When two dominant alleles (AB) enter from the same parent — AB/ab — they are in coupling (cis) phase. When dominant alleles entered from different parents — Ab/aB — they are in repulsion (trans) phase. Morgan noted that linked genes in coupling tend to stay together, but crossing over can exchange segments between homologs, creating recombinant gametes (Ab and aB in coupling, or AB and ab in repulsion).
12.3.2 Recombination Frequency and Centimorgans
The frequency of crossing over between two loci is proportional to the physical distance between them. Recombination frequency (RF) = (number of recombinant offspring) / (total offspring) × 100%.
One centimorgan (cM) = 1% recombination frequency = 1 map unit (mu). Maximum observable RF is 50% (independent assortment — so far apart they appear unlinked). Genes within ~50 cM show measurable linkage.
Worked Example — Two-point Cross & Map Distance
Drosophila cross: AB/ab female (coupling) × ab/ab male (test cross). Offspring: 460 AB, 450 ab (parental types), 40 Ab, 50 aB (recombinant types). Total = 1000.
RF = (40 + 50) / 1000 × 100 = 9 cM. Genes A and B are 9 map units apart on the same chromosome.
Map: A ——9 cM—— B
12.3.3 Three-Point Cross and Gene Order
A three-point cross uses three linked markers simultaneously. Parental classes are most frequent; double-crossover classes (involving two crossover events in the same tetrad interval) are least frequent. The least-frequent class identifies the middle gene. Map distances from three-point data are additive and more accurate than two-point data, since they account for double crossovers that are invisible in two-point analysis.
12.3.4 Chromosomal Theory of Inheritance
Sutton (1902) and Boveri (1902), working independently (often cited together), noted that chromosome behaviour at meiosis parallels the behaviour of Mendel's factors: chromosomes occur in pairs, segregate to different gametes, and different pairs assort independently. Walter Sutton also noted that the number of traits far exceeds the number of chromosomes — hence multiple traits per chromosome, requiring linkage. Morgan's work on Drosophila confirmed gene-chromosome co-inheritance experimentally.
Mendelian
Autosomal, single-gene, complete dominance. F₂ ratios 3:1 or 9:3:3:1. Segregation and independent assortment. Predicts discrete phenotypic classes. Examples: seed shape, ABO (partially), height in pea.
Non-Mendelian
Departures: incomplete dominance, codominance, multiple alleles, epistasis, linkage, sex-linked, maternal (cytoplasmic) inheritance, imprinting, polygenic traits. Ratios deviate from 3:1 or 9:3:3:1. Often requires additional information (sex, pedigree structure) to interpret.
12.4 Sex Determination & Sex-Linked Inheritance
Sex determination is the mechanism by which the sex of an organism is established. Genetic, chromosomal, and environmental mechanisms all exist in different organisms.
XX-XY (male heterogametic)
Mammals (including humans) and many plants. Female = XX (homogametic); male = XY (heterogametic). The SRY gene on Y chromosome triggers testis development. All eggs carry X; sperm carry X or Y — father determines offspring sex.
Also: Drosophila XX-XY but sex determined by X:autosome ratio (not Y). Y is needed for male fertility, not sex determination in flies.
Other Systems
XX-XO (grasshopper Melanoplus, Drosophila X:A ratio): female = 2X; male = X (no Y). Males produce two types of sperm (X-bearing and lacking sex chromosome).
ZW-ZZ (birds, snakes, butterflies): female heterogametic. Female = ZW; male = ZZ. Z is the sex chromosome analogous to X.
Haplo-diploidy (honey bees Apis mellifera): unfertilised eggs (haploid) → drones (males); fertilised eggs (diploid) → workers/queen (females).
Temperature-dependent (some reptiles): sex ratio depends on incubation temperature — no sex chromosomes. Alligator: >34°C → male; <30°C → female.
| System | Female | Male | Heterogametic sex | Examples |
|---|---|---|---|---|
| XX-XY | XX | XY | Male | Humans, cattle, Drosophila (Y needed for fertility) |
| XX-XO | XX | X (no Y) | Male | Grasshoppers, some bugs |
| ZW-ZZ | ZW | ZZ | Female | Birds, snakes, moths, butterflies |
| Haplo-diploidy | 2n (diploid) | n (haploid) | Male (haploid) | Honey bees, wasps, ants |
| Environmental (TSD) | Temperature-set | Temperature-set | None | Alligators, some turtles, lizards |
12.4.1 Sex-Linked Inheritance
Genes located on sex chromosomes are sex-linked. X-linked genes are more common because the X chromosome is larger and carries many non-sex-determination genes. Males (XY) have only one X — they are hemizygous for X-linked genes, so even a single recessive allele is expressed (no masking by a second copy). This explains why X-linked recessive disorders are more common in males.
12.4.2 Red-Green Colour Blindness
Caused by mutations in OPN1LW (red photopsin) or OPN1MW (green photopsin) genes on the X chromosome. X-linked recessive. Affected males: XcY (where Xc = colour-blind allele). Carrier females: XXc (normal vision, 50% of sons will be colour-blind). Affected females: XcXc (rare, since father must be colour-blind and mother must carry allele).
Prevalence: 8% of males, <1% of females (matches hemizygosity hypothesis).
12.4.3 Haemophilia
Haemophilia A: deficiency of clotting Factor VIII; X-linked recessive gene at Xq28. Haemophilia B: deficiency of Factor IX (Christmas disease); also X-linked recessive. Both manifest primarily in males. The pedigree of the European royal families through Queen Victoria is the textbook example: Victoria was a carrier (XHXh), and the mutation was transmitted to her descendants including Tsarevitch Alexei Romanov (Haemophilia B, confirmed by sequencing in 2009).
Worked Example — Sex-Linked Inheritance Cross
A carrier woman (XHXh) marries a normal man (XHY). What are the expected proportions of offspring?
Gametes from mother: XH (50%) and Xh (50%). Gametes from father: XH (50%) and Y (50%).
Offspring: XHXH (normal female, 25%), XHXh (carrier female, 25%), XHY (normal male, 25%), XhY (haemophilic male, 25%).
Answer: 1/2 of sons will have haemophilia; 1/2 of daughters will be carriers. No daughter will be haemophilic (she would need a haemophilic father).
12.4.4 Y-linked (Holandric) Inheritance
Genes on the non-recombining region of the Y chromosome are transmitted exclusively from father to all sons. Classic example: hairy ear pinna (hypertrichosis pinnae auris) — once cited as Y-linked, though recent evidence suggests partial pseudoautosomal involvement. The SRY gene itself is holandric. Y-linked genes do not appear in daughters; all sons of an affected father are affected.
12.5 Mutations & Pedigree Analysis
A mutation is a heritable change in the nucleotide sequence of DNA (gene mutation / point mutation) or in chromosome structure or number (chromosomal mutation). Mutations are the ultimate source of all genetic variation and are the raw material for evolution.
12.5.1 Types of Gene (Point) Mutations
Substitution: one nucleotide replaced by another. If codon meaning changes: missense (different amino acid — e.g., sickle-cell HbS: GAG→GTG, Glu→Val) or nonsense (premature stop codon). If codon meaning unchanged: silent/synonymous.
Insertion/Deletion (indels): addition or removal of nucleotides. If not a multiple of three, causes a frameshift — all downstream codons are misread. Frameshift mutations are generally more damaging than substitutions.
Repeat expansion: trinucleotide repeats expand beyond a threshold. Huntington disease: CAG repeat in HTT gene; >36 repeats = pathological. Shows anticipation (repeats expand across generations, earlier/worse onset in offspring).
12.5.2 Chromosomal Mutations
Structural changes: deletion (loss of segment), duplication, inversion (segment reverses orientation), translocation (segment moves to different chromosome). Cri-du-chat syndrome: deletion of 5p (short arm of chromosome 5); characteristic cat-like cry in infants.
Numerical changes: Aneuploidy (abnormal number of individual chromosomes) or polyploidy (entire genome sets multiplied).
| Disorder | Type | Karyotype / Mutation | Clinical features | Pattern |
|---|---|---|---|---|
| Down syndrome | Trisomy 21 | 47 + extra chr. 21 (95% non-disjunction) | Intellectual disability, flat face, Brushfield spots, heart defects | Chromosomal |
| Klinefelter syndrome | 47 XXY | Extra X in male | Tall, sterile male, gynecomastia, reduced testosterone | Chromosomal (sex-chr.) |
| Turner syndrome | 45 X (XO) | Missing X in female | Short stature, webbed neck, ovarian dysgenesis, infertile female, normal intelligence | Chromosomal (sex-chr.) |
| Patau syndrome | Trisomy 13 | 47 + extra chr. 13 | Severe defects; mostly fatal in infancy | Chromosomal |
| Edwards syndrome | Trisomy 18 | 47 + extra chr. 18 | Clenched fists, rocker-bottom feet; 90% die by age 1 | Chromosomal |
| Sickle-cell anaemia | Point mutation | HbS: β-globin Glu6Val (A→T) | Haemolytic anaemia, pain crises, splenomegaly; partial malaria protection in heterozygotes | Autosomal recessive |
| Thalassaemia | Various mutations | α- or β-globin deletions/point mutations | Anaemia; β-thal major: severe (Cooley's anaemia); α-thal: silent to fatal hydrops | Autosomal recessive |
| PKU | Point mutation | PAH gene (chr. 12q); phenylalanine hydroxylase deficient | Intellectual disability if untreated; dietary Phe restriction is curative | Autosomal recessive |
| Cystic fibrosis | ΔF508 deletion (most common) | CFTR gene (chr. 7q); 3 bp deletion → ΔF508 | Thick mucus in lungs, GI tract; chronic infections; reduced fertility | Autosomal recessive |
| Huntington disease | CAG repeat expansion | HTT gene (chr. 4p); >36 CAG repeats | Involuntary movements, psychiatric, dementia; onset 30–50 yr; anticipation | Autosomal dominant |
| Haemophilia A | Factor VIII deficiency | F8 gene (Xq28) mutations | Prolonged bleeding; haemarthroses | X-linked recessive |
| Colour blindness | Opsin gene mutations | OPN1LW/MW (Xq28) | Inability to distinguish red from green; most common in males (8%) | X-linked recessive |
12.5.3 Pedigree Analysis
A pedigree is a diagram showing the inheritance of a trait through generations of a family. Symbols: circle = female; square = male; filled = affected; horizontal line = mating; vertical + horizontal = offspring. Roman numerals for generations; Arabic numerals for individuals.
Rules for identifying inheritance pattern:
- Autosomal dominant: affected in every generation (vertical transmission); both sexes affected equally; affected × unaffected can produce unaffected.
- Autosomal recessive: can skip generations; unaffected parents produce affected offspring (both carriers); consanguinity increases risk.
- X-linked recessive: more males affected; carrier females appear normal; affected father cannot transmit to sons (he passes Y to sons); all daughters of affected father are carriers.
- X-linked dominant: affected father passes to ALL daughters, NO sons; more females affected than males.
- Y-linked: only males affected; all sons of affected father affected; no female ever affected.
Worked Example — Pedigree interpretation
Pedigree: Generation I: unaffected male × unaffected female. Generation II: one affected male, two unaffected females, one unaffected male. Generation III: one affected male (son of unaffected F-II).
Analysis: (1) Affected males in II and III; unaffected parents in I produce affected son → recessive. (2) If X-linked recessive: I female must be carrier; II affected male = XhY; II unaffected females may be carriers; III affected male from unaffected mother → mother must be carrier. Consistent. (3) If autosomal recessive: both I parents are carriers (Aa × Aa). Both interpretations need testing with more family data.
Key clue: If NO daughters are affected across multiple generations despite many females, prefer X-linked recessive.
12.6 The Molecular Basis of Inheritance — DNA Structure & Replication
The chemical nature of the genetic material was established in three landmark experiments. Griffith (1928) showed that a “transforming principle” from heat-killed virulent Streptococcus pneumoniae (smooth, capsulated) could convert non-virulent rough cells to virulent. Avery, MacLeod and McCarty (1944) purified the transforming principle and showed it was DNA (not protein). Hershey and Chase (1952) used bacteriophage T2 labelled with radioactive ₊S (protein) and ₊P (DNA): only ₊P entered bacteria and directed phage production, confirming DNA as the hereditary molecule.
12.6.1 DNA Structure — Watson-Crick Model (1953)
James Watson and Francis Crick, using X-ray diffraction data from Rosalind Franklin and Maurice Wilkins, proposed the double-helix model on 25 April 1953 (Nobel Prize 1962 to Watson, Crick, Wilkins). Key features:
- Antiparallel double helix: two strands running in opposite directions (5'→3' and 3'→5').
- B-form (physiological): right-handed helix; 10 bp per turn; pitch 3.4 nm; diameter 2 nm.
- Complementary base pairing: A=T (two hydrogen bonds); G≡C (three hydrogen bonds). Chargaff's rules: [A]=[T] and [G]=[C] in any DNA.
- Major groove (wider, 22Å wide) and minor groove (12Å wide) — protein-binding sites.
- Sugar-phosphate backbone on outside; bases stacked in interior via hydrophobic stacking interactions.
- Other forms: A-DNA (dehydrated, right-handed, 11 bp/turn, shorter/wider); Z-DNA (left-handed, 12 bp/turn, seen at high salt).
DNA
Deoxyribose sugar (2' no OH). Bases: A, T, G, C. Usually double-stranded. Highly stable; stores genetic information long-term. Found mainly in nucleus (also mitochondria, chloroplasts). Uracil absent.
RNA
Ribose sugar (2' OH present). Bases: A, U, G, C (U replaces T). Usually single-stranded (can form secondary structures: hairpins, clover-leaf in tRNA). Less stable than DNA. Multiple types: mRNA, tRNA, rRNA, snRNA, miRNA, siRNA. Thymine absent.
12.6.2 DNA Replication — Semiconservative Model
Meselson and Stahl (1958) proved that replication is semiconservative: each daughter duplex contains one parental strand and one newly synthesised strand. Method: E. coli grown in ¹⁵N medium (heavy nitrogen), then transferred to ¹⁴N; after one generation, CsCl density-gradient centrifugation showed all DNA at intermediate density; after two generations, equal amounts of intermediate and light DNA appeared — consistent only with semiconservative replication.
Key enzymes in replication: Helicase unwinds double helix at origin (uses ATP). Topoisomerase II (gyrase in bacteria) relieves positive supercoiling ahead of the fork. SSB proteins (single-strand binding) stabilise unwound strands. Primase (an RNA polymerase) synthesises short RNA primers (∼10 nt) providing a 3'-OH for extension. DNA Polymerase III (prokaryotes) / Pol δ/ε (eukaryotes) extends from primer in 5'→3' direction only. DNA Pol I removes RNA primers and fills gaps (5'→3' exonuclease + 5'→3' polymerase activities). Ligase seals the nick between Okazaki fragments using ATP (eukaryotes) or NAD⁺ (bacteria).
Telomeres and telomerase: Eukaryotic chromosome ends (telomeres) contain repetitive sequences (TTAGGG in humans). Each round of replication shortens telomeres because primers at the 5' end of the lagging strand cannot be replaced. Telomerase (discovered by Greider & Blackburn, 1985; Nobel 2009) is a ribonucleoprotein that uses its built-in RNA template to extend telomeric DNA, preventing shortening in germline and stem cells.
12.7 Transcription, Translation & the Genetic Code
The Central Dogma of Molecular Biology (Crick, 1958) states: DNA → RNA → Protein. DNA is transcribed to mRNA; mRNA is translated to protein. Reverse transcription (RNA→DNA) occurs in retroviruses. Direct translation of DNA or protein→DNA does not occur naturally.
12.7.1 Transcription
In bacteria: RNA polymerase (single type, α₂ββ'ω core + σ factor) binds the promoter (consensus −10 TATAAT and −35 TTGACA boxes). The σ factor helps recognize the promoter; after initiation, σ dissociates and the core enzyme elongates. Termination: Rho-independent (hairpin in nascent RNA + poly-U) or Rho-dependent (Rho helicase unwinds RNA:DNA hybrid).
In eukaryotes: Three RNA polymerases: Pol I (rRNA 28S, 18S, 5.8S), Pol II (mRNA and most snRNA), Pol III (tRNA, 5S rRNA). Pol II requires general transcription factors (TBP binds TATA box ~25 bp upstream; TFIIA,B,D,E,F,H assemble pre-initiation complex). Enhancers and silencers (distal regulatory elements) contact promoter via DNA looping.
Pre-mRNA processing (eukaryotes only):
- 5' capping: addition of 7-methylguanosine cap immediately after initiation; protects mRNA from exonucleases; aids ribosome binding.
- 3' polyadenylation: cleavage at poly-A signal (AATAAA) ~10–30 nt upstream; ∼200 A residues added by poly-A polymerase; aids mRNA stability and nuclear export.
- Splicing: introns (intervening sequences) removed; exons (expressed sequences) joined by the spliceosome (U1, U2, U4, U5, U6 snRNPs). Branch-point A attacks 5' splice site (lariat intermediate) → exons joined. Some introns are self-splicing ribozymes (Group I & II). This is the basis of alternative splicing — one gene can produce multiple proteins (human α-tropomyosin: 18+ isoforms).
Introns
Intervening sequences present in eukaryotic pre-mRNA (and many eukaryotic genes). Removed by the spliceosome. Not present in mature mRNA. Not translated. May have regulatory roles (splicing, miRNA source). Absent in most prokaryotes (exception: some archaeal genes).
Exons
Expressed sequences that remain in the mature mRNA and are translated. Exons are joined by splicing. A gene's coding sequence is the concatenation of all exons. Alternative splicing allows different exon combinations → protein diversity. Exons are found in both prokaryotes and eukaryotes (though the term is primarily eukaryotic).
12.7.2 The Genetic Code
The genetic code maps codons (triplets of mRNA nucleotides) to amino acids. Established by Nirenberg and Matthei (1961) (poly-U → poly-Phe, first codon), and fully cracked by Nirenberg, Khorana, and Holley (Nobel Prize 1968).
Properties of the genetic code:
- Triplet: three nucleotides per codon; 4³ = 64 codons possible.
- Non-overlapping: each nucleotide belongs to one codon only.
- Comma-less: no punctuation between codons; reading is continuous.
- Degenerate (redundant): 61 sense codons encode 20 amino acids → most amino acids have multiple codons. Third codon position shows most degeneracy (wobble position, Crick 1966).
- Universal: same code in almost all organisms (minor exceptions: some mitochondria, ciliates like Tetrahymena use UAA/UAG as Gln).
- Unambiguous: each codon specifies exactly one amino acid (the reverse is not true).
- Start codon: AUG (codes for Met in eukaryotes; fMet in prokaryotes) — also sets the reading frame.
- Stop codons: UAA (“ochre”), UAG (“amber”), UGA (“opal/umber”) — no amino acid, recognised by release factors.
| Codon(s) | Amino acid | Notes |
|---|---|---|
| AUG | Methionine (Met, M) | Start codon; sets reading frame |
| UAA, UAG, UGA | Stop (termination) | Ochre, Amber, Opal |
| UUU, UUC | Phenylalanine (Phe, F) | First codon cracked (Nirenberg 1961) |
| GGU, GGC, GGA, GGG | Glycine (Gly, G) | Fully degenerate box (all 4 code Gly) |
| UGG | Tryptophan (Trp, W) | Only codon for Trp; no degeneracy |
| AUU, AUC, AUA | Isoleucine (Ile, I) | Three codons, no 4th |
| GAG, GAA | Glutamic acid (Glu, E) | GAG→GUG sickle-cell mutation |
| GUG, GUU, GUC, GUA | Valine (Val, V) | GUG also alternative start |
| CGN (all 6) | Arginine (Arg, R) | Most codons: 6 |
12.7.3 Translation
Ribosomes: Prokaryotic 70S (30S + 50S subunits); eukaryotic 80S (40S + 60S). Mitochondrial and chloroplast ribosomes are 70S (prokaryote-like). Three tRNA binding sites: A-site (aminoacyl-tRNA enters), P-site (peptidyl-tRNA holds growing chain), E-site (exit site for deacylated tRNA).
Initiation: Small subunit binds mRNA. In bacteria, the Shine-Dalgarno sequence (purine-rich, ~5–10 nt upstream of AUG) base-pairs with 16S rRNA to position the start codon. In eukaryotes, the ribosome scans from the 5' cap until it encounters the first AUG in a favourable Kozak sequence (GCCAUGG). Initiator tRNA (fMet-tRNA in prokaryotes; Met-tRNA in eukaryotes) occupies the P-site.
Elongation: (1) Aminoacyl-tRNA delivered to A-site by EF-Tu (with GTP). (2) Peptidyl transferase (catalytic RNA of 23S/28S rRNA — a ribozyme) forms a peptide bond, transferring the growing chain from P-site tRNA to the amino acid on A-site tRNA. (3) Translocation: ribosome moves one codon in 3' direction (EF-G/GTP). Empty tRNA moves to E-site and exits.
Termination: Stop codon in A-site is recognised by release factor (RF1, RF2 in bacteria; eRF1 in eukaryotes). Peptidyl transferase catalyses hydrolysis, releasing the polypeptide. Ribosome dissociates; mRNA is released.
Polyribosomes (polysomes): Multiple ribosomes translate the same mRNA simultaneously, increasing protein output.
Transcription
DNA → RNA. Enzyme: RNA polymerase. Template: one DNA strand (non-coding / antisense). Product: mRNA (also tRNA, rRNA). Occurs in nucleus (eukaryotes) or at the nucleoid (prokaryotes). No amino acids involved. Antiparallel synthesis; product complementary to template strand.
Translation
mRNA → Protein. Machinery: ribosome, tRNA, aminoacyl-tRNA synthetases. Template: mRNA codons. Product: polypeptide chain. Occurs at ribosomes (cytoplasm; RER in secretory proteins). Direction: 5' to 3' on mRNA; N-terminus to C-terminus in protein. tRNA is the adaptor between codon and amino acid.
Mnemonic — Base Pairing & Codons
AT GC in DNA: “All Tigers Grow Claws”. In RNA replace T with U: A pairs with U.
Stop codons: UAA = “U Are Away”; UAG = “U Are Gone”; UGA = “U Go Away”. Start codon AUG = “AUGust begins protein synthesis.”
12.8 Gene Expression & Regulation — Lac Operon, Trp Operon, Eukaryotic Regulation
Cells do not express all genes all the time. Gene regulation allows organisms to respond to environmental signals, differentiate into cell types, and conserve energy. Jacob and Monod (1961), working in Escherichia coli, proposed the operon model — the first molecular-level explanation of gene regulation (Nobel Prize 1965).
12.8.1 The Lac Operon (Negative Inducible)
The lac operon controls lactose metabolism in E. coli. It is negatively regulated (a repressor normally turns it off) and inducible (lactose switches it on). Components:
- lacI gene: constitutively expressed; encodes the Lac repressor protein.
- Promoter (P): RNA polymerase binding site.
- Operator (O): DNA-binding site for the repressor (overlaps with promoter).
- lacZ: β-galactosidase (cleaves lactose into glucose + galactose; also converts lactose to allolactose).
- lacY: lactose permease (transports lactose into cell).
- lacA: transacetylase (minor role).
12.8.2 Catabolite Repression and the CAP Site
Even when lactose is present, the lac operon is only weakly transcribed if glucose is also present (glucose preference). When glucose is absent, cAMP levels rise; cAMP binds the catabolite activator protein (CAP), and the CAP-cAMP complex binds a site upstream of the promoter, stimulating RNA polymerase binding (positive regulation). This is why lac operon is diauxic: bacteria consume glucose first, then lactose.
12.8.3 The Trp Operon (Negative Repressible)
The trp operon encodes enzymes for tryptophan biosynthesis. It is negatively regulated and repressible (an end-product turns it off). The trp repressor protein (TrpR) is inactive (aporepressor) by itself. When tryptophan levels are high, Trp acts as a co-repressor: Trp + aporepressor → active repressor → binds operator → blocks transcription. When Trp is low, repressor is inactive → operon is ON. This is feedback repression. The trp operon also uses attenuation (leader peptide encoding two Trp codons — coupling of transcription and translation)
| Feature | Lac operon | Trp operon |
|---|---|---|
| Pathway | Catabolic (lactose breakdown) | Anabolic (Trp biosynthesis) |
| Regulatory type | Negative inducible | Negative repressible |
| Signal molecule | Allolactose (inducer) — inactivates repressor | Tryptophan (co-repressor) — activates repressor |
| Default state | OFF (repressor active when no lactose) | ON (repressor inactive when no Trp) |
| Nobel Prize | Jacob & Monod 1965 | — (no separate Nobel; part of operon work) |
12.8.4 Gene Regulation in Eukaryotes
Eukaryotic gene regulation is far more complex than prokaryotic operons. Key levels:
- Chromatin remodelling: nucleosome repositioning (SWI/SNF complex) exposes or occludes DNA. Histone acetylation (HAT = open, active) and deacetylation (HDAC = closed, inactive).
- DNA methylation: CpG methylation (by DNMT enzymes) correlates with gene silencing; epigenetic inheritance.
- Transcription factors: activators bind enhancers (thousands of bp from promoter) and contact the general transcription machinery via co-activators (mediator complex). Repressors compete or block.
- Post-transcriptional: mRNA stability (AU-rich elements in 3'-UTR), alternative splicing, RNA editing.
- RNA interference (RNAi): short dsRNA processed to siRNA or miRNA by Dicer; RISC complex directs cleavage or translational repression of complementary mRNA (Andrew Fire & Craig Mello — Nobel 2006).
- Post-translational: phosphorylation, ubiquitylation, glycosylation alter protein activity or stability.
12.9 Theories of Evolution — Lamarck, Darwin, Modern Synthesis
Life on Earth has been changing since its origin ∼3.8–4.0 billion years ago. Multiple theories have been proposed to explain the mechanism of evolutionary change.
Lamarck (1809)
Use and disuse: organs used more develop; unused atrophy. Inheritance of acquired characters: modifications during an organism's life are passed to offspring. Example: giraffe stretches neck, passes longer neck to offspring. Philosophie Zoologique (1809). Refuted by Weismann (mouse tail-cutting experiment; germplasm theory). Correct insight: organisms change in response to environment. Wrong: acquired changes not inherited (except epigenetics, a modern nuance).
Darwin (1859)
Natural selection on heritable variation. On the Origin of Species by Means of Natural Selection (1859). Voyage of HMS Beagle (1831–36), Galápagos finches. Key observations: overproduction of offspring; limited resources (Malthus); heritable variation exists; those better suited survive and reproduce more. Co-proposed with Alfred Russel Wallace (1858 paper). Does not require acquired inheritance or directed variation.
Modern Synthesis (Neo-Darwinism, 1930s–50s): R.A. Fisher, J.B.S. Haldane, Sewall Wright (population genetics); Theodosius Dobzhansky (Genetics and the Origin of Species, 1937); Ernst Mayr (speciation); G.G. Simpson (palaeontology). Merges Darwinian natural selection with Mendelian genetics and population genetics.
Mnemonic — Lamarck vs Darwin
Lamarck: Learned traits inherited. Use and disuse. Giraffe Learned to stretch.
Darwin: Designed by nature. Differential survival of the Daringly fit. Does not require intent.
Weismann's Watertight germplasm separates body (soma) from germ cells — acquired body changes cannot reach the DNA in germ cells. Killed Lamarck.
12.9.1 Origin of Life
Oparin (1924) and Haldane (1929) independently proposed that life arose from inorganic molecules in a primitive reducing atmosphere (CH₄, NH₃, H₂O, H₂) — the “primordial soup” hypothesis. Stanley Miller and Harold Urey (1953) tested this experimentally: sparking the simulated early-Earth atmosphere produced amino acids, sugars, and nitrogenous bases — demonstrating that organic molecules can form abiotically (abiogenesis).
RNA World hypothesis (Walter Gilbert, 1986): RNA may have been the first molecule to both store genetic information and catalyse reactions (ribozymes, demonstrated by Thomas Cech and Sidney Altman — Nobel Chemistry 1989). RNA could have self-replicated before DNA or proteins existed. Later, DNA (more stable) took over information storage; proteins (more versatile) took over catalysis.
Age of Earth: ∼4.5 billion years (radiometric dating). Oldest microfossils: ∼3.5 billion years (Apex Chert, Australia). First eukaryotes: ∼1.5 billion years. First multicellular: Ediacaran biota ∼600 Mya. Cambrian explosion: ∼540 Mya — rapid diversification of animal phyla.
12.9.2 Darwin’s Natural Selection — Mechanism
Directional selection: favours one extreme of phenotypic range; shifts mean of population (e.g., industrial melanism in Biston betularia; antibiotic resistance). Stabilising selection: favours intermediate phenotypes; reduces variance; common in stable environments (e.g., birth weight in humans — very small and very large babies have lower survival). Disruptive selection: favours both extremes simultaneously; increases variance; can lead to speciation (e.g., beak size in African seedcracker finch Pyrenestes ostrinus).
12.10 Hardy-Weinberg Equilibrium & Population Genetics
The Hardy-Weinberg principle (1908, independently proposed by G.H. Hardy, a mathematician, and W. Weinberg, a physician) states that allele and genotype frequencies in an ideal population remain constant from generation to generation in the absence of evolutionary forces.
12.10.1 The Equations
For a gene with two alleles, A (frequency p) and a (frequency q):
- Allele frequency: p + q = 1
- Genotype frequency: p² + 2pq + q² = 1
- p² = frequency of AA; 2pq = frequency of Aa; q² = frequency of aa
Worked Example — Hardy-Weinberg Problem
In a population, 1 in 10,000 individuals show albinism (autosomal recessive). Calculate allele frequencies and the frequency of carriers.
Step 1: q² = 1/10,000 = 0.0001 → q = √0.0001 = 0.01 (frequency of recessive allele a).
Step 2: p = 1 − q = 1 − 0.01 = 0.99 (frequency of dominant allele A).
Step 3: Carrier frequency (Aa) = 2pq = 2 × 0.99 × 0.01 = 0.0198 ≈ 1 in 50.
Answer: About 1 in 50 individuals are carriers of the albinism allele, even though only 1 in 10,000 are albinos. This illustrates why recessive disorders persist in populations even when rare.
12.10.2 Conditions for Hardy-Weinberg Equilibrium
The HWE requires five idealisations. Any departure means evolution is occurring:
| Condition | Violation → Evolutionary force | Effect on population |
|---|---|---|
| Large (infinite) population size | Small population → Genetic drift | Random changes in allele frequencies (bottleneck, founder effect) |
| Random mating (panmixia) | Non-random mating (e.g., assortative, inbreeding) | Changes genotype frequencies; increases homozygosity (inbreeding) |
| No mutation | Mutation → new alleles | Introduces new variation; slow but essential source |
| No migration (gene flow) | Gene flow (immigration/emigration) | Introduces or removes alleles; homogenises populations |
| No natural selection | Selection differential fitness | Changes allele frequencies in direction of fitness advantage |
12.10.3 Forces of Evolution
Mutation: ultimate source of all genetic variation. Typically rare (10−⁵–10−⁶ per locus per generation); random with respect to fitness. Provides raw material for evolution.
Genetic drift: random sampling error in small populations. Bottleneck effect: population crash followed by recovery from few survivors — loses allele diversity (e.g., cheetah, elephant seal). Founder effect: small group colonises new area; carries only a subset of source population's alleles (e.g., Amish Ellis-Van Creveld syndrome; Pingelap colour blindness island).
Gene flow (migration): transfer of alleles between populations via migration. Reduces genetic differentiation between populations; can introduce adaptive alleles (e.g., HIV resistance allele CCR5-Δ32 spreading).
Natural selection: differential reproductive success based on phenotype. Changes allele frequencies in an adaptive direction. The only force that systematically increases adaptation. Types: directional, stabilising, disruptive, sexual selection.
Sexual selection: Darwin's second mechanism. Within-sex competition or mate choice favours traits that increase mating success even if they reduce survival (peacock tail, deer antlers). Can drive rapid divergence.
12.11 Evidence for Evolution & Speciation
12.11.1 Evidence for Evolution
1. Fossil record: preserved remains or traces of organisms in sedimentary rock. Transitional fossils link ancestral and derived groups: Archaeopteryx (reptile + bird features); Tiktaalik (fish + tetrapod); Pakicetus (terrestrial ancestor of whales). Geologic time scale established by stratigraphy and radiometric dating (Pb/U, K/Ar, C-14). Australopithecus afarensis (“Lucy”, 3.2 Mya — bipedal ape); H. habilis (∼2.4 Mya, first stone tools); H. erectus (∼1.9 Mya, fire, left Africa); H. neanderthalensis (∼400 kya, Europe/Asia; coexisted with H. sapiens); H. sapiens (∼300 kya, Africa).
2. Comparative anatomy:
- Homologous organs — same basic structure, different function; evidence of common ancestry (divergent evolution). Human arm, bat wing, whale flipper, horse forelimb all share humerus-radius-ulna-carpals-phalanges.
- Analogous organs — different structure, same function; evidence of convergent evolution. Bird wing vs insect wing vs bat wing (all fly, but entirely different structural origins). Eye of vertebrates vs cephalopods (octopus) — same function, independent origins.
- Vestigial organs — reduced, non-functional remnants of structures useful in ancestors. Human appendix, ear muscles, coccyx (remnant tail vertebrae); whale pelvic bones (landlubber ancestor); python hindlimb rudiments; Orobanche scale leaves (no chlorophyll).
Homologous
Same structure (anatomy), different function. Arise from same embryological origin; share common ancestor. Evidence of divergent evolution. Examples: forelimb of human (grasping), bat (flying), whale (swimming), horse (running). All derived from same tetrapod pentadactyl limb. Confirm evolutionary relationship.
Analogous
Different structure, same function. No common ancestry for that structure; evolved independently. Evidence of convergent evolution (similar selection pressures → similar solutions). Examples: wings of insects, birds, bats (all fly, very different anatomy). Thorn (stem modification) vs spine (leaf modification) — analogy of defence. Do NOT confirm evolutionary relationship.
3. Embryological evidence: Karl Ernst von Baer's observation that early embryos of very different vertebrates (fish, amphibian, reptile, bird, human) are nearly indistinguishable — all have gill slits (pharyngeal arches) and tails at some stage. Haeckel's biogenetic law (“ontogeny recapitulates phylogeny”) is an overstatement, but embryological similarities do reflect shared ancestry.
4. Molecular evidence: DNA and protein sequences reflect evolutionary relationships more precisely than morphology. Cytochrome c amino-acid sequence is nearly identical in humans and chimpanzees (1 difference in 104 amino acids); differs more from yeast (45 differences). Molecular clocks (neutral substitution rate) allow timing of divergence. Conserved gene sequences (Hox genes, ribosomal RNA) demonstrate universal common ancestry.
5. Biogeographical evidence: distribution of organisms reflects evolutionary history and continental drift. Darwin's Galápagos finches (14 species from one mainland ancestor). Marsupials in Australia (geographic isolation after separation from Gondwana). Identical fossils on now-separated continents support plate tectonics + evolution.
12.11.2 Speciation
A species (biological species concept, Ernst Mayr 1942) = a group of actually or potentially interbreeding populations that are reproductively isolated from other such groups. Speciation is the process by which one species splits into two or more reproductively isolated groups.
Reproductive isolating mechanisms:
- Pre-zygotic: habitat isolation, temporal isolation (different breeding seasons), behavioural isolation (courtship differences), mechanical isolation (incompatible genitalia/flowers), gametic incompatibility.
- Post-zygotic: hybrid inviability (embryo dies), hybrid sterility (mule = horse × donkey is sterile), hybrid breakdown (F₂ hybrids fail).
Allopatric Speciation
Geographic separation first, then divergence. A physical barrier (mountain range, river, ocean) divides a population → isolated subpopulations accumulate genetic differences independently → become reproductively isolated even if barrier removed. Most common mode in animals. Classic: Galápagos finches; Darwin's finches (14 spp. from mainland ancestor). HP angle: Western Himalayan endemics like Aconitum heterophyllum (vatsnabh), Picrorhiza kurroa (kutki) — isolated in high-altitude valleys, diverged from related Tibetan/central Asian ancestors. Podophyllum hexandrum (Himalayan mayapple) vs eastern P. peltatum (American mayapple) — vicariance by Tethys Sea.
Sympatric Speciation
No geographic barrier; new species arises within the geographic range of the parent species. Mechanisms: (1) Polyploidy (most common in plants) — doubling of chromosome set creates instant reproductive isolation. Allopolyploidy: hybridisation between two species + polyploidisation. Brassica napus (canola) = allotetraploid of B. oleracea and B. rapa. Wheat (Triticum) = allohexaploid (6 genomes from 3 ancestors). (2) Ecological specialisation within same area — cichlids of African Rift lakes (800+ species from one ancestor in <15,000 years). Apple maggot fly (Rhagoletis pomonella) shifted from hawthorn to apple — sympatric speciation in progress.
Phyletic Gradualism
Darwin's original view: evolutionary change is slow, continuous, and gradual over long time periods. Transitions are smooth; fossil record incompleteness explains gaps. Change accumulates uniformly within lineages.
Punctuated Equilibrium
Proposed by Niles Eldredge and Stephen Jay Gould (1972): species show long periods of stasis (little change) punctuated by rapid bursts of change at speciation events. Fossil gaps are real, not artefactual. Most morphological change happens during speciation (small peripheral populations), not gradually within established species.
12.12 Quick-Reference Tables
| Modified ratio | Type of epistasis | Classic example |
|---|---|---|
| 9 : 3 : 3 : 1 | No epistasis (normal dihybrid) | Seed shape × seed colour in pea |
| 12 : 3 : 1 | Dominant epistasis (A dominant epistatic to B) | Squash fruit colour; poultry feather |
| 9 : 7 | Complementary / duplicate recessive | Flower colour in Lathyrus odoratus |
| 9 : 3 : 4 | Recessive epistasis (bb epistatic to A_) | Labrador coat colour |
| 15 : 1 | Duplicate dominant (either A or B produces trait) | Capsella bursa-pastoris leaf shape |
| 13 : 3 | Dominant + recessive epistasis | Frizzled feather in poultry |
| 9 : 6 : 1 | Additive duplicate (each contributes equally) | Grain colour in oats |
| System | Heterogametic sex | Organisms | Key gene |
|---|---|---|---|
| XX-XY | Male (XY) | Mammals, most insects except Drosophila (X:A ratio) | SRY (Y-chromosome, mammals) |
| XX-XO | Male (X only) | Grasshoppers (Melanoplus), some bugs | X:autosome ratio |
| ZW-ZZ | Female (ZW) | Birds, snakes, moths, butterflies, some fish | DMRT1 on Z (birds) |
| Haplo-diploidy | Male (haploid) | Hymenoptera (bees, wasps, ants) | Ploidy level |
| TSD (temp-dependent) | None (environmental) | Alligators, some turtles, lizards | Temperature-regulated hormonal cascade |
| Event | Approximate age | Evidence |
|---|---|---|
| Formation of Earth | 4.5 Bya | Radiometric dating of oldest minerals |
| First life (prokaryotes) | 3.8–4.0 Bya | Stromatolites, carbon isotope signals |
| Oldest microfossils | 3.5 Bya | Apex Chert, W. Australia |
| Cyanobacteria (oxygenic photosynthesis) | 2.7 Bya | Stromatolites; BIF (banded iron formation) |
| Great Oxidation Event | 2.4 Bya | Atmospheric O₂ rise |
| First eukaryotes | 1.5–2.0 Bya | Fossil cell with nucleus |
| First multicellular | 0.6 Bya (Ediacaran) | Ediacaran biota fossils |
| Cambrian explosion | 541 Mya | Rapid diversification of animal phyla |
| First land plants | 470 Mya | Spore fossils |
| First tetrapods | 375 Mya | Tiktaalik |
| Mass extinction (P-T) | 252 Mya | 96% marine species extinct |
| First dinosaurs/mammals | 228–225 Mya | Triassic fossils |
| K-Pg extinction | 66 Mya | Chicxulub impact + volcanic activity |
| Australopithecus afarensis (Lucy) | 3.2 Mya | Ethiopia fossils |
| Homo sapiens | 300 kya | Jebel Irhoud, Morocco |
Quick Recap
- Mendel's three laws: Dominance (F₁ uniform), Segregation (alleles separate at meiosis, purity of gametes), Independent Assortment (different loci; violated by linkage). F₂ monohybrid 3:1; dihybrid 9:3:3:1.
- Incomplete dominance (1:2:1 phenotypic = genotypic, e.g. Mirabilis pink); codominance (heterozygote shows both, e.g. ABO blood group AB); multiple alleles (ABO: Iₐ, Iₛ, i); lethal alleles (yellow mouse 2:1).
- Epistasis modifies 9:3:3:1 → 12:3:1 (dominant), 9:7 (complementary), 9:3:4 (recessive), 15:1 (duplicate dominant); all ratios sum to 16.
- Linkage groups = haploid chromosome number; RF (%) = cM = map units; maximum 50 cM; Bateson-Punnett 1906; Morgan 1910 Drosophila.
- Sex determination: XX-XY (male heterogametic, mammals); ZW-ZZ (female heterogametic, birds); XX-XO (grasshoppers); haplo-diploidy (bees). SRY gene on Y triggers male development.
- X-linked recessive disorders more common in males (hemizygous): haemophilia A (Factor VIII, Xq28), colour blindness (OPN1LW/MW, Xq28), DMD. Y-linked (holandric): hairy ears, SRY.
- Chromosomal disorders: Down syndrome (trisomy 21, 47+21); Klinefelter (47 XXY, sterile male); Turner (45 X, sterile female); cri-du-chat (5p deletion).
- Single-gene: sickle-cell (HbS, Glu→Val, A→T mutation, autosomal recessive, pleiotropy); cystic fibrosis (ΔF508 CFTR); Huntington (CAG expansion, autosomal dominant, anticipation).
- Griffith 1928 (transformation); Avery 1944 (DNA = transforming principle); Hershey-Chase 1952 (³2;P vs ³5;S confirms DNA). Watson-Crick 1953: antiparallel double helix, B-form (10 bp/turn, 3.4 nm, 2 nm diameter), A=T (2 H-bonds), G≡C (3 H-bonds).
- Replication is semiconservative (Meselson-Stahl 1958, ¹⁵N/¹⁴N). Helicase unwinds; primase lays RNA primer; Pol III extends; leading strand continuous; lagging strand = Okazaki fragments; Pol I removes primer; ligase seals. Telomerase prevents end-shortening.
- Genetic code: triplet, non-overlapping, comma-less, degenerate (61 sense + 3 stop), nearly universal. AUG = start; UAA/UAG/UGA = stop. Wobble at 3rd position. Nirenberg-Khorana-Holley 1968 Nobel.
- Eukaryotic pre-mRNA processing: 5' 7-methylguanosine cap, 3' poly-A tail, spliceosome removes introns (lariat intermediate). Alternative splicing → protein diversity.
- Lac operon: negative inducible; repressor blocks operator when no lactose; allolactose inactivates repressor → lacZ (β-gal), lacY (permease), lacA expressed. CAP-cAMP = positive regulation under low glucose. Trp operon: negative repressible; Trp = co-repressor.
- Lamarck (1809) — use/disuse + inheritance of acquired characters; refuted by Weismann. Darwin (1859) — natural selection on heritable variation (On the Origin of Species). Modern Synthesis (1930s–50s): Mendelism + Darwinism + population genetics (Fisher, Haldane, Wright, Dobzhansky, Mayr).
- Hardy-Weinberg: p + q = 1; p² + 2pq + q² = 1. Five conditions (large population, random mating, no mutation, migration, selection). Departures = evolution. Genetic drift (bottleneck, founder), mutation, gene flow, natural selection (directional, stabilising, disruptive).
- Evidence for evolution: fossils (Archaeopteryx, Tiktaalik, Australopithecus); homologous (same anatomy, different function → divergent evolution) vs analogous (different anatomy, same function → convergent evolution); vestigial; embryological; molecular (cytochrome c); biogeography.
Chapter 12 Cheatsheet
Mendelian Ratios
- Monohybrid F₂: 3:1 phenotype; 1:2:1 genotype
- Dihybrid F₂: 9:3:3:1 (16 combinations)
- Test cross: 1:1 (if heterozygous)
- Incomplete dominance: 1:2:1 (phenotype = genotype)
- Codominance: 1:2:1 (all three phenotypes visible)
- Lethal allele: 2:1 (yellow mouse)
DNA & Replication Key Facts
- B-DNA: 10 bp/turn, 3.4 nm pitch, 2 nm dia.
- A=T (2 H-bonds); G≡C (3 H-bonds)
- Meselson-Stahl 1958 → semiconservative
- Leading strand: continuous; Lagging: Okazaki fragments
- Telomerase: Nobel 2009 (Greider, Blackburn, Szostak)
- Prokaryote: Pol III main; Pol I removes primers
Genetic Code
- AUG = start (Met); UAA/UAG/UGA = stop
- 61 sense codons, 3 stop codons
- Degenerate: 20 aa, 61 codons (wobble at 3rd)
- Universal (few exceptions: mitochondria, ciliates)
- 70S ribosome (prokaryote); 80S (eukaryote)
- Peptidyl transferase = ribozyme (rRNA)
Lac Operon Essentials
- Negative inducible; lacI encodes repressor
- No lactose → repressor on operator → OFF
- Allolactose inactivates repressor → ON
- CAP-cAMP: positive regulation (low glucose)
- lacZ = β-galactosidase; lacY = permease
- Trp operon = negative repressible (Trp = co-repressor)
Hardy-Weinberg & Evolution Forces
- p + q = 1; p² + 2pq + q² = 1
- q² = freq. recessive homozygotes; q = √(disease freq.)
- 5 conditions: large pop, random mating, no mutation/migration/selection
- Drift: bottleneck + founder effect
- Natural selection: directional / stabilising / disruptive
- Gene flow homogenises; mutation = raw material
Speciation & Evidence
- Allopatric: geography separates; then divergence (finches, Himalayan endemics)
- Sympatric: polyploidy (plants) or ecological (cichlids)
- Homologous = same anatomy, different function (divergent)
- Analogous = different anatomy, same function (convergent)
- Lucy (A. afarensis) 3.2 Mya; H. sapiens 300 kya
- Punctuated equilibrium: Eldredge & Gould 1972
- Chapter 11 — Cell Biology & Cell Division: meiosis (basis of segregation and independent assortment), mitosis, cell cycle checkpoints relevant to mutation.
- Chapter 13 — Biotechnology: recombinant DNA, PCR, gene cloning, GMO crops — all apply operon/gene-regulation principles directly.
- Chapter 14 — Ecology & Environment: population size fluctuations create bottlenecks; habitat fragmentation drives founder effects and allopatric speciation in HP wildlife corridors.
- Chapter 1 — Plant Diversity and Taxonomy: angiosperm families evolved by speciation from common ancestors; molecular phylogenetics uses the same DNA-sequence tools as molecular evidence for evolution.
- Chapter 5 — Plant Physiology: photosynthesis genes (chloroplast genome) evolved via endosymbiosis — extreme gene transfer between ancestors.
Practice Questions
1. Mendel's Law of Independent Assortment holds only when genes are located — HPRCA-pat.
- on the same chromosome, far apart
- on different chromosomes, or far apart on the same chromosome
- in the same linkage group
- on the sex chromosomes only
Law 3 applies when genes assort without linkage bias. Genes in different linkage groups (different chromosomes) or those more than ~50 cM apart on the same chromosome satisfy this criterion. Closely linked genes violate Law 3.
2. In a monohybrid cross Tt × Tt, what is the probability of getting a true-breeding tall offspring?
- 1/4
- 1/2
- 3/4
- 1/3
F₂ genotype ratio: 1 TT : 2 Tt : 1 tt. True-breeding tall = TT = 1 out of 4 total. But among tall plants (3/4), TT is 1/3 of tall plants. The question asks for probability among ALL offspring: 1/4. If question implies “among tall offspring”, answer is 1/3. With standard phrasing “probability of true-breeding tall in ALL offspring”: 1/4 (A). The answer D = 1/3 applies if “among tall phenotype.” Verify the phrasing in your exam; 1/4 (A) is the probability from ALL F₂ seeds.
3. In four-o'clock plant (Mirabilis jalapa), red-flowered × white-flowered gives pink F₁. This is an example of — HPRCA-pat.
- Codominance
- Incomplete dominance
- Polygenic inheritance
- Epistasis
Incomplete dominance: heterozygote is intermediate (pink), not identical to either parent. Codominance would require both parental phenotypes expressed simultaneously (like ABO AB blood group). F₂ gives 1 red : 2 pink : 1 white, where phenotypic = genotypic ratio 1:2:1.
4. Which F₂ phenotypic ratio is characteristic of duplicate dominant epistasis? HPRCA-pat.
- 9 : 3 : 4
- 12 : 3 : 1
- 15 : 1
- 9 : 7
Duplicate dominant epistasis (15:1): dominant allele at either locus A OR locus B is sufficient to produce the trait. In the F₂ of AaBb × AaBb, only aabb (1/16) fails to show the trait. Example: leaf shape in Capsella bursa-pastoris. All ratios sum to 16.
5. Assertion (A): In the ABO blood group system, Iₐ and Iₛ alleles are codominant. Reason (R): In genotype IₐIₛ, both A antigen and B antigen are expressed on red blood cells.
- Both A and R are true, and R is the correct explanation of A
- Both A and R are true, but R is not the correct explanation of A
- A is true, but R is false
- A is false, but R is true
Codominance means both alleles are fully expressed in the heterozygote. IₐIₛ individuals produce both A and B antigens — they are blood group AB. R correctly explains A: codominance is defined by both products being present simultaneously, not blended. This is different from incomplete dominance where products average.
6. Linkage was first experimentally demonstrated in which organism? HPRCA-pat.
- Pisum sativum by Mendel
- Lathyrus odoratus by Bateson and Punnett
- Drosophila melanogaster by Morgan
- Neurospora by Beadle and Tatum
Bateson and Punnett (1906) observed that flower colour and pollen shape in sweet pea (Lathyrus odoratus) did not assort independently, thus providing the first evidence of linkage. Morgan (1910) later demonstrated the chromosomal basis of linkage in Drosophila and received the Nobel Prize (1933).
7. A woman carrier of haemophilia A (XᴴXʰ) marries a normal man (XᴴY). The probability that their son will have haemophilia is — HPRCA-pat.
- 0%
- 25%
- 50%
- 100%
Sons receive Y from father and either XH or Xh from mother. Probability of receiving Xh = 50%, so 50% of sons are haemophilic. 50% of daughters are carriers; no daughters will be haemophilic (they all receive XH from father).
8. Match the sex-determination system with the correct example:
(P) XX-XO (Q) ZW-ZZ (R) Haplo-diploidy (S) XX-XY mammals
| (i) Honey bee | (ii) Snake | (iii) Grasshopper | (iv) Human |
- P-iii, Q-ii, R-i, S-iv
- P-iv, Q-iii, R-i, S-ii
- P-i, Q-iv, R-iii, S-ii
- P-ii, Q-i, R-iv, S-iii
P (XX-XO) = grasshopper (Melanoplus), (iii). Q (ZW-ZZ) = snakes and birds, (ii). R (haplo-diploidy) = honey bee (Apis), (i). S (XX-XY mammals) = humans, (iv).
9. Which of the following statements about DNA B-form is INCORRECT?
- It is a right-handed double helix
- It has approximately 10 base pairs per turn
- The diameter is approximately 2 nm
- G≡C bonds have two hydrogen bonds while A=T have three
It is the reverse: A=T forms 2 hydrogen bonds; G≡C forms 3 hydrogen bonds. The extra hydrogen bond in G≡C makes GC-rich DNA more stable and requires higher temperature to denature. All other options (right-handed, 10 bp/turn, 2 nm diameter) are correct for B-DNA.
10. Meselson and Stahl proved semiconservative replication by growing E. coli in ⁻N medium, then transferring to ⁺N medium. After TWO generations, the expected result in CsCl density gradient would be — HPRCA-pat.
- All DNA at heavy (¹⁵N) density
- All DNA at intermediate (hybrid) density
- Equal amounts of intermediate and light (¹⁴N) density DNA
- Equal amounts of heavy and light density DNA
After one generation: all hybrid (¹⁵N/¹⁴N). After two generations: half hybrid + half light (¹⁴N/¹⁴N). This matches only the semiconservative model (each parental strand templates one new strand). Conservative model would give heavy + light after one generation; dispersive model would give all intermediate after two generations but lighter than first-generation hybrid.
11. Assertion (A): The leading strand is synthesised continuously during DNA replication. Reason (R): DNA polymerase can synthesise DNA only in the 5'→3' direction.
- Both A and R are true, and R is the correct explanation of A
- Both A and R are true, but R is not the correct explanation of A
- A is true, but R is false
- A is false, but R is true
Both true, and R explains A. Since DNA polymerase can only add nucleotides to the 3'-OH end (synthesis 5'→3'), the leading strand (running 3'→5' as template toward the fork) is synthesised continuously. The lagging strand template runs 5'→3' away from the fork, requiring discontinuous Okazaki fragments synthesised in the opposite direction.
12. The enzyme that removes RNA primers and fills gaps during DNA replication in prokaryotes is —
- DNA Polymerase I
- DNA Polymerase III
- Primase
- DNA Ligase
DNA Pol I has 5'→3' exonuclease activity (removes the RNA primer) and 5'→3' polymerase activity (fills the gap with DNA). DNA Pol III is the main replicative polymerase. Primase synthesises RNA primers. Ligase seals the final nick between the filled gap and the adjacent Okazaki fragment.
13. Which of the following is the CORRECT property of the genetic code? HPRCA-pat.
- It is overlapping and universal
- It is non-overlapping, degenerate, and nearly universal
- It is ambiguous (some codons specify more than one amino acid)
- UGG codes for two amino acids due to degeneracy
The genetic code is non-overlapping (each nucleotide belongs to one codon), degenerate (multiple codons can specify one amino acid), nearly universal (minor exceptions in mitochondria and some ciliates), unambiguous (each codon specifies only one amino acid), and comma-less. UGG is the only codon for tryptophan — no degeneracy for Trp. The code is not ambiguous — option C is incorrect.
14. The peptidyl transferase activity in ribosomes resides in —
- Ribosomal proteins of the large subunit
- 23S rRNA (prokaryotes) / 28S rRNA (eukaryotes)
- tRNA at the A-site
- EF-Tu elongation factor
Peptidyl transferase is a ribozyme — the catalytic activity is an intrinsic property of the large subunit rRNA (23S rRNA in prokaryotes, 28S rRNA in eukaryotes). This was a landmark discovery confirming RNA's catalytic role and supporting the RNA World hypothesis. Crystallographic studies (Nobel Chemistry 2009) confirmed that ribosomal proteins do not contact the peptidyl transferase centre directly.
15. In the lac operon, allolactose acts as — HPRCA-pat.
- A co-repressor that activates the repressor
- An inducer that inactivates the repressor
- A positive activator that binds RNA polymerase directly
- A structural gene product that degrades lactose
Allolactose (an isomer of lactose produced by residual β-galactosidase) is the true inducer. It binds the Lac repressor, causing a conformational change that reduces repressor affinity for the operator by 1000-fold. The operon is then transcribed. CAP-cAMP (not allolactose) acts as a positive activator binding upstream of the promoter. Trp is a co-repressor (activates the trp repressor), not lactose.
16. Assertion (A): The trp operon is an example of negative repressible control. Reason (R): When tryptophan levels are high, it acts as a co-repressor and activates the trp repressor, switching the operon OFF.
- Both A and R are true, and R is the correct explanation of A
- Both A and R are true, but R is not the correct explanation of A
- A is true, but R is false
- A is false, but R is true
Both correct. Negative = repressor-based regulation (turns transcription OFF). Repressible = product of the pathway (Trp) co-represses its own synthesis. Trp + aporepressor → active repressor → binds operator → OFF. Contrast with lac operon: negative inducible (inducer inactivates repressor).
17. Which of the following statements about Down syndrome are CORRECT? (Select ALL that apply)
- It results from trisomy of chromosome 21
- Affected individuals have 47 chromosomes
- It is more common in offspring of older mothers (risk increases with maternal age)
- Affected males are almost always fertile
Down syndrome (trisomy 21): 47 chromosomes including three copies of chr. 21. Risk rises with maternal age (non-disjunction during meiosis I more common with older oocytes; e.g., ~1/1500 at age 20 vs ~1/25 at age 45). Males with Down syndrome are almost always infertile due to defective spermatogenesis, so (D) is incorrect.
18. Turner syndrome is characterised by karyotype — HPRCA-pat.
- 47 XXY
- 47 XYY
- 45 X (XO)
- 48 XXXY
Turner syndrome (45 X or 45 XO): monosomy of the X chromosome. Clinical features: short stature, webbed neck, shield chest, gonadal dysgenesis (streak ovaries), infertility in females, normal or near-normal intelligence. Klinefelter = 47 XXY (male, infertile). 47 XYY = Jacob's syndrome (tall male, normal fertility usually).
19. The molecular basis of sickle-cell anaemia is — HP-spec.
- Deletion of three nucleotides in β-globin gene
- Point mutation: GAG → GTG (Glu → Val at position 6 of β-globin)
- Frameshift mutation in the α-globin gene
- Trinucleotide repeat expansion in HBB gene
Sickle-cell anaemia is caused by a single nucleotide substitution A→T in codon 6 of the β-globin gene: GAG (Glu) → GUG/GTG (Val). This missense mutation makes HbS, which polymerises under low O₂ → sickle shape. Autosomal recessive, pleiotropic. Heterozygotes (HbA/HbS, sickle-cell trait) have partial protection against falciparum malaria — a classic balanced polymorphism maintained by natural selection.
20. In a population, the frequency of a recessive lethal allele a is 0.04. The frequency of carriers (Aa) in the population is approximately —
- 0.0016
- 0.04
- 0.077
- 0.16
q = 0.04; p = 1 − 0.04 = 0.96. Carrier frequency = 2pq = 2 × 0.96 × 0.04 = 0.0768 ≈ 0.077. Note: q² = 0.0016 (homozygous recessive frequency, not carrier). This illustrates that carriers are always more common than affected homozygotes when q is small.
21. Which of the following is NOT a condition for Hardy-Weinberg equilibrium? HPRCA-pat.
- Random mating
- No mutation
- Polygenic inheritance
- No migration
The five H-W conditions are: large population, random mating, no mutation, no migration, no selection. Polygenic inheritance (multigenic control of a trait) is a type of inheritance pattern, not an H-W condition. Hardy-Weinberg can be applied to any single locus regardless of whether the overall phenotype is polygenic.
22. Arrange the following in chronological order of discovery/publication: HPRCA-pat.
(P) Hershey-Chase blender experiment (Q) Watson-Crick double helix (R) Griffith's transformation (S) Avery-MacLeod-McCarty DNA = transforming principle
- R → S → P → Q
- P → Q → R → S
- R → P → S → Q
- S → R → P → Q
Griffith 1928 (R) → Avery 1944 (S) → Hershey-Chase 1952 (P) → Watson-Crick 1953 (Q). Each built on the previous: Griffith showed transformation occurred; Avery proved DNA was the agent; Hershey-Chase confirmed DNA carried genetic information; Watson-Crick explained its structure.
23. Which scientist coined the term “gene” and proposed the concept of unit factors? HPRCA-pat.
- Gregor Mendel
- Hugo de Vries
- Wilhelm Johannsen
- Thomas Hunt Morgan
Wilhelm Johannsen (1909) coined the terms gene, genotype, and phenotype. He also coined pure lines and demonstrated that variation within pure lines is non-heritable (environmental). Mendel used the term “Merkmal” (factor); De Vries proposed “pangene”; Morgan established gene-chromosome linkage.
24. Identify the ODD ONE OUT in the context of evidence for evolution:
- Homologous organs (forelimb of human and whale)
- Vestigial organs (human coccyx)
- Analogous organs (wings of bird and insect)
- Comparative embryology (gill slits in vertebrate embryos)
Analogous organs are evidence of convergent evolution (similar function, different structural origin), not direct evidence of common ancestry or descent. Homologous organs, vestigial organs, and comparative embryology all provide direct evidence for common ancestry and evolutionary descent. Analogous structures show adaptation to similar selection pressures but cannot be used to reconstruct phylogenies.
25. Sympatric speciation by allopolyploidy is most common in — HP-spec.
- Animals, especially vertebrates
- Plants, because polyploidy is tolerated and produces fertile hybrids
- Bacteria, via horizontal gene transfer
- Viruses, via reassortment
Allopolyploidy (hybridisation between two species + genome doubling) creates instant reproductive isolation because the new allopolyploid cannot backcross productively with either parent species (different chromosome numbers → non-homologous pairing). It is far more common in plants than animals because plants can self-fertilise and tolerate polyploidy. Example: Brassica napus (allotetraploid), wheat (Triticum aestivum, allohexaploid).
26. Assertion (A): A bottleneck event can cause random changes in allele frequencies regardless of whether those alleles are advantageous. Reason (R): Genetic drift acts independently of natural selection and is most pronounced in small populations.
- Both A and R are true, and R is the correct explanation of A
- Both A and R are true, but R is not the correct explanation of A
- A is true, but R is false
- A is false, but R is true
Both correct, and R explains A. A bottleneck reduces population size to very few individuals; random sampling error (genetic drift) determines which alleles survive — not their adaptive value. Cheetahs and northern elephant seals show extreme loss of genetic diversity from bottleneck events. R is the correct mechanistic explanation.
27. Match the genetic disorder with its inheritance pattern:
| (P) Huntington disease | (Q) Cystic fibrosis | (R) Red-green colour blindness | (S) Hairy ear pinna |
| (i) X-linked recessive | (ii) Autosomal dominant | (iii) Autosomal recessive | (iv) Y-linked (holandric) |
- P-ii, Q-iii, R-i, S-iv
- P-i, Q-ii, R-iii, S-iv
- P-iii, Q-ii, R-iv, S-i
- P-ii, Q-i, R-iii, S-iv
Huntington = autosomal dominant (CAG repeat expansion; heterozygote affected). Cystic fibrosis = autosomal recessive (ΔF508 CFTR). Red-green colour blindness = X-linked recessive (OPN1LW/MW on Xq28). Hairy ear pinna = Y-linked holandric (passed from father to all sons).
28. Which of the following best describes the “founder effect”? HPRCA-pat.
- Loss of genetic diversity when a large population crashes to a small one
- Allele frequency changes when a small group colonises a new area, carrying only a subset of the source population's alleles
- Directional selection favouring a new advantageous mutation
- Random mating causing all allele frequencies to reach H-W equilibrium
The founder effect is a type of genetic drift where a new colony is established by a small number of individuals (founders), carrying only a random sample of the source population's alleles. Classic examples: Amish community (high frequency of Ellis-Van Creveld syndrome), Pingelap islanders (high frequency of complete achromatopsia). Bottleneck effect (option A) is a separate but related phenomenon within the same existing population.
29. Which of the following are differences between the lac operon and the trp operon?
- Lac is catabolic; trp is anabolic
- Lac is induced by allolactose; trp is repressed by tryptophan
- Both operons are positively regulated only by CAP-cAMP
- Lac operon is OFF by default; trp operon is ON by default
A: Correct — lac breaks down lactose (catabolism); trp synthesises tryptophan (anabolism). B: Correct — allolactose (inducer) inactivates lac repressor; Trp (co-repressor) activates trp repressor. C: Incorrect — only the lac operon has CAP-cAMP positive regulation; the trp operon does not. D: Correct — lac operon is normally OFF (repressor bound); trp operon is normally ON (aporepressor cannot bind operator without Trp).
30. The phenomenon where a single gene produces multiple, apparently unrelated phenotypic effects is called — HPRCA-pat.
- Epistasis
- Polygenic inheritance
- Pleiotropy
- Codominance
Pleiotropy: one gene → many phenotypic effects. Classic examples: sickle-cell gene (anaemia, pain, splenomegaly, eye and CNS effects — all from HbS polymerisation); Marfan syndrome (fibrillin-1, affects heart, lens, skeleton); PKU (one enzyme deficiency affects brain, skin, urine odour). Polygenic = many genes, one trait (opposite direction). Epistasis = interaction between two genes. Codominance = both alleles expressed in heterozygote.
End of Chapter 12 · Genetics and Evolution. HPRCA-pat. indicates HPRCA / state-TGT pattern questions; literal past-paper items will be flagged with year when official papers are sourced.
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Sections — Ch. 12
- 01 Overview
- 02 12.1 Mendel and the Laws of Inheritance
- 03 12.2 Beyond Mendel — Incomplete Dominance, Codominance, Lethality, Multiple Alleles, Pleiotropy, Polygenic Inheritance
- 04 12.3 Linkage, Recombination & Genetic Mapping
- 05 12.4 Sex Determination & Sex-Linked Inheritance
- 06 12.5 Mutations & Pedigree Analysis
- 07 12.6 The Molecular Basis of Inheritance — DNA Structure & Replication
- 08 12.7 Transcription, Translation & the Genetic Code
- 09 12.8 Gene Expression & Regulation — Lac Operon, Trp Operon, Eukaryotic Regulation
- 10 12.9 Theories of Evolution — Lamarck, Darwin, Modern Synthesis
- 11 12.10 Hardy-Weinberg Equilibrium & Population Genetics
- 12 12.11 Evidence for Evolution & Speciation
- 13 12.12 Quick-Reference Tables
- 14 Recap & Cheatsheet
- 15 Practice Questions
Other chapters
- Ch. 1 Plant Diversity and Taxonomy
- Ch. 2 Economic Botany
- Ch. 3 Plant Anatomy
- Ch. 4 Plant Physiology
- Ch. 5 Animal Diversity
- Ch. 6 Comparative Anatomy & Developmental Biology
- Ch. 7 Animal Physiology & Immunology
- Ch. 8 Reproductive Biology
- Ch. 9 Applied Zoology
- Ch. 10 Medical Diagnostics
- Ch. 11 Cell Biology
- Ch. 13 Biotechnology
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- Ch. 15 Ecology
- Ch. 16 Teaching of Life Science
- Ch. 17 Himachal Pradesh — General Knowledge
- Ch. 18 General Knowledge & Current Affairs
- Ch. 19 Everyday Science, Reasoning & Social Science
- Ch. 20 General English & General Hindi
- Ch. M1 Mock Test 1
- Ch. M2 Mock Test 2
- Ch. M3 Mock Test 3