Genetics, Plant Breeding, Biostatistics & Evolution BBO4T07

Syllabus topics (official RTMNU)

1. Gene Interaction: Allelic gene interaction (Incomplete dominance 1:2:1); non-allelic gene interaction (Complementary genes 9:7, Dominant epistasis 12:3:1)
2. Multiple alleles and Polygenic inheritance
3. Linkage: Definition, theories of linkage (Coupling and Repulsion); Types of linkage (Complete and incomplete); significance.
4. Crossing over: Definition; Breakage and reunion theory; significance.

1. Gene Interaction: Allelic and non-allelic interaction

Gene interaction means that the expression of one gene is changed by another gene. If the two genes are alleles of the same locus it is allelic (intra-allelic) interaction; if they sit at different loci it is non-allelic (inter-allelic) interaction. Such interactions modify the classical Mendelian ratios (3:1 and 9:3:3:1).

(a) Incomplete dominance (1:2:1) – neither allele is completely dominant, so the heterozygote shows an intermediate phenotype. Example: flower colour in four o'clock plant, Mirabilis jalapa (Correns).

• P: Red (RR) × White (rr) → F₁: all Pink (Rr).
• F₁ selfed: Rr × Rr → F₂: 1 RR Red : 2 Rr Pink : 1 rr White.
• Phenotypic ratio = genotypic ratio = 1:2:1. Other example: snapdragon (Antirrhinum majus).

(b) Complementary genes (9:7) – two dominant genes at different loci must both be present to produce a character; either one alone gives no effect. Example: flower colour in sweet pea, Lathyrus odoratus (Bateson and Punnett).

• P: White (CCpp) × White (ccPP) → F₁: CcPp – Purple (both C and P present).
• F₂ (16 combinations): 9 C_P_ Purple : 3 C_pp White : 3 ccP_ White : 1 ccpp White.
• Ratio = 9 Purple : 7 White. Biochemically, C and P control two steps of one pathway making anthocyanin; a block at any step gives white.

(c) Dominant epistasis (12:3:1) – a dominant allele of one gene (epistatic gene) masks the expression of another gene (hypostatic gene). Example: fruit colour in summer squash, Cucurbita pepo. W (white) is epistatic; Y gives yellow, yy gives green.

• P: White (WWYY) × Green (wwyy) → F₁: WwYy – White.
• F₂: 9 W_Y_ White + 3 W_yy White = 12 White : 3 wwY_ Yellow : 1 wwyy Green.
• Ratio = 12:3:1.

2. Multiple alleles and Polygenic inheritance

Multiple alleles: when a gene has more than two alternative forms (alleles) at the same locus in a population, they are called multiple alleles. A diploid individual carries only two of them.

• Examples: self-incompatibility (self-sterility) alleles S₁, S₂, S₃… in Nicotiana (East); ABO blood group in man (I^A, I^B, i); coat colour in rabbit.
• In Nicotiana, pollen carrying an S allele that is also present in the style fails to grow; S₁S₂ × S₁S₂ is sterile, while S₁S₂ ♀ × S₂S₃ ♂ allows only S₃ pollen to function.
• Features: all alleles occupy the same locus; they affect the same character; no crossing over among them; number of genotypes with n alleles = n(n+1)/2.

Polygenic (quantitative) inheritance: a character controlled by several genes, each with a small, equal and additive effect. It shows continuous variation and is influenced by environment. Example: kernel colour in wheat (Nilsson-Ehle, 1909); also height, yield, skin colour in man.

• Two genes: Dark red (R₁R₁R₂R₂) × White (r₁r₁r₂r₂) → F₁ intermediate red.
• F₂ by number of dominant alleles (4,3,2,1,0): 1 : 4 : 6 : 4 : 1 (dark red, medium-dark red, medium red, light red, white).
• With three genes the F₂ ratio is 1:6:15:20:15:6:1 and only 1/64 is white. The frequency curve is bell-shaped (normal distribution).

3. Linkage: Definition, theories, types and significance

Linkage is the tendency of genes located on the same chromosome to stay together and be inherited together. Linked genes do not show independent assortment. First observed by Bateson and Punnett (1906) in sweet pea; explained by T.H. Morgan (1910) in Drosophila.

Coupling and repulsion theory (Bateson and Punnett):

• Coupling: two dominant alleles (or two recessive alleles) come from the same parent and tend to stay together, e.g. AB/ab.
• Repulsion: a dominant and a recessive allele come from each parent and tend to stay apart, e.g. Ab/aB.
• In sweet pea (purple flower, long pollen × red flower, round pollen) the test cross gave far more parental types than the 1:1:1:1 expected.
• Morgan's chromosome theory of linkage showed that coupling and repulsion are two phases of the same phenomenon: genes lie in a linear order on a chromosome and the closer they are, the stronger the linkage.

Types of linkage:

• Complete linkage: linked genes are always inherited together, no recombinants form, e.g. male Drosophila (no crossing over). Test cross gives only parental types (1:1).
• Incomplete linkage: linked genes are occasionally separated by crossing over, giving a few recombinants, e.g. coloured-shrunken (C Sh) genes in maize (Hutchinson); test cross gave about 96.4% parental and 3.6% recombinant types.

Significance: keeps useful gene combinations together; helps in preparing linkage (chromosome) maps; linkage groups = haploid chromosome number (e.g. maize 10, pea 7); plant breeders must break undesirable linkages to combine good characters.

4. Crossing over: Definition, Breakage and reunion theory, significance

Crossing over is the exchange of segments between non-sister chromatids of homologous chromosomes during meiosis, producing new combinations of linked genes (recombinants). It occurs at the pachytene stage of prophase I (four-strand stage); the visible cross-shaped point is the chiasma, seen at diplotene.

Breakage and reunion theory (Darlington) – most accepted classical theory. Steps:

1. Synapsis: homologous chromosomes pair at zygotene to form bivalents.
2. Tetrad formation: each chromosome has two chromatids, so a bivalent has four chromatids.
3. Breakage: two non-sister chromatids break at corresponding points (enzyme endonuclease).
4. Exchange and reunion: broken segments exchange and rejoin crosswise (enzyme ligase), forming the chiasma.
5. Terminalisation: chiasmata move towards chromosome ends during diplotene–diakinesis; homologues separate at anaphase I.

Recombination frequency = (number of recombinants ÷ total progeny) × 100. Example: test cross gives 420 AB, 410 ab, 85 Ab, 85 aB (total 1000). Recombinants = 170, so RF = 17%; the genes are 17 map units (centiMorgans) apart.

Significance: produces new gene combinations and genetic variation; source of material for natural selection and evolution; proves the linear arrangement of genes; used to construct linkage maps; helps breeders combine desirable genes from different parents.

5 likely exam questions

Long answer

1. Explain non-allelic gene interaction with suitable examples of complementary genes (9:7) and dominant epistasis (12:3:1), showing the crosses.
2. Define linkage. Describe the coupling and repulsion theory and the types of linkage with examples. Add a note on its significance.

Short answer / short notes

3. Explain incomplete dominance in Mirabilis jalapa with a cross.
4. Write a short note on polygenic inheritance in wheat kernel colour.
5. Describe the breakage and reunion theory of crossing over.

Syllabus topics (official RTMNU)

1. Variation in chromosome number and its significance: Euploidy (Auto- and Allo-polyploidy); Aneuploidy (Nullisomic, monosomic, trisomic and tetrasomic)
2. Structural changes in chromosomes and its significance: Deficiency, Duplication, Inversion and Translocation
3. Gene mutation: Spontaneous and Induced mutations; Types (Frame shift and substitution), physical and chemical mutagens; Applications of induced mutation in crop improvement.

1. Variation in chromosome number and its significance

Every species has a fixed chromosome number. A change in this number is called heteroploidy. It is of two types: euploidy (change in whole sets) and aneuploidy (change in one or a few chromosomes).

A. Euploidy – the number is an exact multiple of the basic set (x). Monoploid (x), diploid (2x), triploid (3x), tetraploid (4x) etc. Organisms with more than two sets are polyploids.

• Autopolyploidy: multiplication of the same genome (AA → AAA or AAAA). Arises by failure of cell division or by the use of colchicine (from Colchicum autumnale), which stops spindle formation. Examples: triploid banana, seedless watermelon, triploid sugar beet; tetraploid grapes. Effects: larger cells, flowers, fruits ("gigas" effect); triploids are sterile because of unbalanced meiosis.
• Allopolyploidy: combination of two or more different genomes followed by doubling (AA × BB → AB → AABB). Examples: Raphanobrassica (Karpechenko, radish × cabbage, 2n = 36); Triticale (wheat × rye, the first man-made cereal); bread wheat Triticum aestivum (AABBDD, 2n = 6x = 42); cotton Gossypium hirsutum (AADD). Allotetraploids behave like diploids and are fertile (amphidiploids).

B. Aneuploidy – gain or loss of one or a few chromosomes, caused by non-disjunction during meiosis.

Significance: polyploidy is a major way of speciation in plants (about 30–35% of flowering plants are polyploid); produces larger, more vigorous and seedless varieties; allopolyploidy restores fertility in sterile hybrids and creates new crops; aneuploids are used to locate genes on particular chromosomes (nullisomic and monosomic analysis in wheat). In man, aneuploidy causes disorders (Down syndrome = trisomy 21).

2. Structural changes in chromosomes and its significance

Breakage of chromosomes followed by wrong rejoining changes the number or arrangement of genes. These are chromosomal aberrations.

• Deficiency (Deletion): loss of a segment. Terminal (end piece lost, one break) or intercalary (middle piece lost, two breaks). In a heterozygote the normal homologue forms a deletion loop at pachytene. Large deletions are usually lethal; may cause pseudo-dominance (recessive allele expressed).
• Duplication: a segment present twice. Types: tandem (ABC BC DEF), reverse tandem (ABC CB DEF), displaced. Example: Bar eye in Drosophila (duplication of 16A region). Duplications provide extra genes for evolution of new functions.
• Inversion: a segment is reversed by 180°. Paracentric – centromere outside the inverted piece; pericentric – centromere included. In heterozygotes an inversion loop forms at pachytene; crossing over in the loop of a paracentric inversion gives a dicentric bridge and an acentric fragment at anaphase I. Inversions suppress recombination and keep gene blocks together.
• Translocation: transfer of a segment to a non-homologous chromosome. Simple (one-way) or reciprocal (exchange between two non-homologues). A reciprocal translocation heterozygote forms a cross-shaped quadrivalent at pachytene and a ring or figure-of-eight at metaphase I, leading to about 50% pollen sterility (semi-sterility). Example: Oenothera lamarckiana forms a ring of 14 chromosomes.

Significance: source of variation and evolution; duplications create new genes; inversions and translocations help speciation by reducing fertility of hybrids; used in gene mapping and in locating genes; translocations can transfer useful genes (e.g. disease resistance from wild relatives into wheat).

3. Gene mutation: types, mutagens and applications

Mutation is a sudden heritable change in the genetic material. The term was given by Hugo de Vries (1901) from Oenothera lamarckiana. A gene (point) mutation is a change in the nucleotide sequence of a single gene.

Spontaneous vs Induced mutation:

• Spontaneous: occur naturally without known cause, due to errors in DNA replication, tautomeric shifts of bases, natural radiation; very low rate (about 10⁻⁶ per gene per generation).
• Induced: produced artificially by mutagens. H.J. Muller (1927) induced mutations in Drosophila by X-rays; L.J. Stadler (1928) in barley and maize.

Types of gene mutation:

• Substitution (base-pair substitution): one base replaced by another. Transition – purine by purine or pyrimidine by pyrimidine (A↔G, C↔T); transversion – purine by pyrimidine or vice versa. Effect may be silent, missense (changed amino acid, e.g. sickle-cell anaemia, GAG → GTG) or nonsense (stop codon).
• Frame shift: insertion or deletion of one or two bases shifts the reading frame, so all codons after it are misread. Usually harmful. Example: THE CAT ATE THE RAT → deleting one letter gives THE ATA TET HER AT.

Mutagens:

Applications of induced mutation in crop improvement (mutation breeding):

• Quick creation of new variation where natural variation is lacking.
• Sharbati Sonora wheat – amber-grained mutant of Sonora 64 developed by gamma irradiation at IARI.
• Jagannath rice – semi-dwarf gamma-ray mutant (CRRI, Cuttack); Aruna castor – early-maturing mutant (BARC); many TG groundnut varieties from BARC, Trombay.
• Improvement of disease resistance, earliness, dwarfness, yield, oil and protein quality; useful in vegetatively propagated crops (new flower colours in chrysanthemum, rose).

5 likely exam questions

Long answer

1. What is euploidy? Describe autopolyploidy and allopolyploidy with examples. Add a note on aneuploidy and the significance of variation in chromosome number.
2. Describe structural changes in chromosomes (deficiency, duplication, inversion and translocation) with labelled diagrams and their significance.

Short answer / short notes

3. Differentiate between nullisomic, monosomic, trisomic and tetrasomic.
4. Explain frame shift and substitution mutations with examples.
5. Write the applications of induced mutation in crop improvement.

Syllabus topics (official RTMNU)

1. Fine structure of gene: Classical concept – cistron, muton and recon
2. Extra-chromosomal Inheritance: Cytoplasmic inheritance, male sterility in higher plants.
3. Population Genetics: Gene pool, genetic drift, gene frequencies, Hardy-Weinberg Law.
4. Evolutionary genetics: Role of natural selection; Genetic variation and Speciation.

1. Fine structure of gene: Classical concept – cistron, muton and recon

According to the classical concept (Morgan's "beads on a string" view), a gene was a single indivisible unit that acted at the same time as the unit of function, the unit of mutation and the unit of recombination. Crossing over was thought to occur only between genes, never inside a gene.

Seymour Benzer (1955–1961), working on the rII locus of bacteriophage T4 infecting E. coli, showed that a gene can be divided into smaller parts. He proposed three sub-units:

• Cistron – the unit of function; a segment of DNA that codes for one polypeptide. It is identified by the cis-trans complementation test: if two mutations in the trans position (one on each chromosome) give the wild phenotype, they are in different cistrons; if they give the mutant phenotype, they lie in the same cistron. The rII locus has two cistrons, rIIA and rIIB.
• Muton – the smallest unit of DNA that can undergo mutation; may be as small as a single nucleotide pair.
• Recon – the smallest unit of DNA that can take part in recombination (crossing over); one or two nucleotide pairs. Benzer found recombination even between mutations within one cistron.

Thus one gene (cistron) contains many mutons and recons. This is called the fine structure of gene.

2. Extra-chromosomal Inheritance: Cytoplasmic inheritance, male sterility

Extra-chromosomal (cytoplasmic) inheritance is the transmission of characters controlled by genes present outside the nucleus, in plastids (plastome) and mitochondria (chondriome). These genes are called plasmagenes.

Features: reciprocal crosses give different results; progeny resemble the female (mother) parent because the egg contributes almost all the cytoplasm (maternal inheritance); Mendelian ratios are not obtained; characters are not affected by nuclear substitution.

Example – plastid inheritance in Mirabilis jalapa (Correns, 1909): the plant has green, white and variegated branches. Seeds from flowers on a green branch give green plants, on a white branch give white plants (which die), and on a variegated branch give green, white and variegated plants – whatever the pollen source.

Male sterility is the inability of a plant to produce functional pollen while the female part is normal. Types:

• Genetic male sterility (GMS): controlled by recessive nuclear genes (ms ms).
• Cytoplasmic male sterility (CMS): controlled by mitochondrial genes (S cytoplasm), e.g. Texas (T) cytoplasm in maize; all progeny are male sterile.
• Cytoplasmic-genetic male sterility (CGMS): sterile cytoplasm (S) with nuclear restorer gene (Rf). S rfrf = sterile; S Rf_ or N (normal cytoplasm) = fertile.

Use in hybrid seed production (three-line system): A-line (male sterile, S rfrf) × R-line (restorer, RfRf) gives fertile F₁ hybrid seed without hand emasculation; B-line (maintainer, N rfrf) maintains the A-line. Used in maize, sorghum, pearl millet (bajra), rice and sunflower.

3. Population Genetics: Gene pool, genetic drift, gene frequencies, Hardy-Weinberg Law

• Population (Mendelian population): a group of interbreeding individuals of a species in an area.
• Gene pool: the total of all genes and their alleles present in a population at a given time.
• Gene (allele) frequency: the proportion of a particular allele among all alleles of that gene in a population. For alleles A and a, p = frequency of A, q = frequency of a, and p + q = 1.
• Genetic drift (Sewall Wright effect): random change in gene frequency by chance, important in small populations; an allele may be lost or fixed regardless of its usefulness. Special cases: founder effect (a few individuals start a new population) and bottleneck effect (population drastically reduced by disaster).

Hardy-Weinberg Law (1908): In a large, randomly mating population, gene and genotype frequencies remain constant from generation to generation in the absence of mutation, selection, migration and genetic drift.

Equation: (p + q)² = p² + 2pq + q² = 1, where p² = AA, 2pq = Aa, q² = aa.

Conditions: large population; random mating; no mutation; no migration (gene flow); no natural selection; no genetic drift.

Worked example 1: In a population of 1000 plants, 160 have white flowers (recessive, aa). Find the frequencies.

• q² = 160/1000 = 0.16, so q = √0.16 = 0.4; p = 1 − 0.4 = 0.6.
• AA = p² = 0.36 → 360 plants; Aa = 2pq = 2 × 0.6 × 0.4 = 0.48 → 480 plants; aa = 0.16 → 160 plants. Check: 360 + 480 + 160 = 1000.

Worked example 2: AA = 490, Aa = 420, aa = 90 (N = 1000). p = (2×490 + 420) / 2000 = 1400/2000 = 0.7; q = 0.3. Expected: p² = 0.49, 2pq = 0.42, q² = 0.09 – exactly as observed, so the population is in Hardy-Weinberg equilibrium.

4. Evolutionary genetics: Natural selection, genetic variation and speciation

Genetic variation is the raw material of evolution. Sources: gene mutation, chromosomal changes, recombination (crossing over and independent assortment), gene flow (migration) and polyploidy (in plants).

Role of natural selection (Darwin): individuals with favourable variations survive and reproduce more (differential reproduction), so the frequency of favourable alleles increases in the gene pool. Types:

• Stabilising – favours the average form, removes extremes (e.g. human birth weight).
• Directional – favours one extreme, mean shifts (e.g. industrial melanism in peppered moth Biston betularia; insecticide and herbicide resistance).
• Disruptive – favours both extremes, may split a population into two.

Speciation is the formation of new species. It needs reproductive isolation, which may be pre-zygotic (geographical, seasonal, ecological, mechanical) or post-zygotic (hybrid sterility, hybrid inviability).

• Allopatric speciation: a population is separated by a geographical barrier; the parts accumulate different variations and become separate species.
• Sympatric speciation: new species arise in the same area, commonly by polyploidy in plants (e.g. bread wheat, Raphanobrassica).

5 likely exam questions

Long answer

1. What is cytoplasmic inheritance? Explain plastid inheritance in Mirabilis jalapa and describe male sterility and its use in hybrid seed production.
2. State and explain the Hardy-Weinberg law with its conditions. In a population of 1000 plants, 90 show the recessive phenotype; calculate p, q and the expected number of each genotype.

Short answer / short notes

3. Define cistron, muton and recon.
4. Write a note on genetic drift.
5. Explain the role of natural selection in evolution.

Syllabus topics (official RTMNU)

1. Plant Breeding: Definition, objective and domestication of crop plants.
2. Methods of Plant Breeding: Pure line selection; Technique of hybridization; Heterosis
3. Biostatistics: Mean, Mode, Median, Standard deviation, Standard error, and Student's 't' test.
4. Evolution: Origin of life and organic evolution (Miller's theory)

1. Plant Breeding: Definition, objective and domestication of crop plants

Plant breeding is the art and science of improving the genetic make-up of crop plants to produce new varieties that are more useful to man.

Objectives: higher yield; better quality (protein, oil, taste, cooking quality); resistance to diseases and insect pests; tolerance to drought, salinity and cold; early maturity; suitable plant type (dwarf, non-lodging, responsive to fertilizer); wider adaptability; removal of toxic substances.

Domestication is the process of bringing wild plants under human cultivation and selecting them for useful traits. It began about 10,000 years ago. N.I. Vavilov proposed centres of origin of crop plants (8 main centres), e.g. the Hindustan (Indian) centre for rice, sugarcane, mango, pigeon pea and brinjal. Domestication brought changes such as non-shattering seeds, larger fruits and grains, loss of dormancy and uniform maturity (e.g. wheat, maize from teosinte).

2. Methods of Plant Breeding: Pure line selection; Technique of hybridization; Heterosis

A. Pure line selection – a pure line is the progeny of a single homozygous self-pollinated plant (Johannsen, 1903, working on beans, Phaseolus vulgaris). Used in self-pollinated crops (wheat, rice, gram).

1. Year 1: select a large number of superior single plants from a mixed local variety; harvest separately.
2. Year 2: grow progeny rows of each plant; reject inferior or segregating rows.
3. Years 3–7: replicated yield trials of the best lines against standard check varieties.
4. Year 8 onward: the best line is released as a new variety and multiplied.

Merits: uniform variety, simple. Demerit: creates no new genotype, only isolates the best existing one.

B. Technique of hybridization – crossing genetically different parents to combine their desirable characters. Steps:

1. Selection of parents with complementary desirable traits.
2. Emasculation: removal of anthers from the female parent's bisexual flower before they dehisce, by hand with forceps; also by hot water, alcohol, suction or use of male sterility.
3. Bagging: emasculated flower covered with a butter-paper bag to prevent unwanted pollen.
4. Tagging: label with parents' names and date.
5. Pollination: pollen from the male parent dusted on the receptive stigma; flower re-bagged.
6. Harvesting F₁ seed and raising F₁, then selection in segregating generations (pedigree, bulk or backcross method).

C. Heterosis (hybrid vigour) – the superiority of F₁ hybrid over its parents in yield, size, growth rate and vigour. Term coined by G.H. Shull (1914). Heterosis (%) over mid-parent = (F₁ − MP) ÷ MP × 100. Example: parents yield 40 and 60 q/ha (MP = 50), F₁ = 70 q/ha → heterosis = 40%.

• Dominance hypothesis (Davenport; Bruce; Jones): harmful recessive genes of one parent are hidden by dominant genes of the other.
• Over-dominance hypothesis (Shull; East): the heterozygote (Aa) itself is superior to both homozygotes.
• Exploited in hybrid maize, sorghum, bajra, cotton, rice and vegetables. Vigour declines in F₂ (inbreeding depression), so fresh hybrid seed is needed each year.

3. Biostatistics: Mean, Mode, Median, Standard deviation, Standard error, Student's t test

Biostatistics is the application of statistics to biological data. Worked data: heights (cm) of 10 plants of variety A – 12, 15, 11, 15, 17, 14, 13, 15, 16, 12.

• Mean (x̄ = Σx ÷ n): Σx = 140, n = 10, x̄ = 14 cm.
• Median (middle value of arranged data): 11, 12, 12, 13, 14, 15, 15, 15, 16, 17. n is even, so median = (5th + 6th) ÷ 2 = (14 + 15) ÷ 2 = 14.5 cm.
• Mode (most frequent value): 15 occurs three times, mode = 15 cm.
• Standard deviation (SD) – measure of spread of values around the mean: SD = √[Σ(x − x̄)² ÷ (n − 1)]. Deviations: −2, 1, −3, 1, 3, 0, −1, 1, 2, −2; squares: 4, 1, 9, 1, 9, 0, 1, 1, 4, 4; Σ = 34. SD = √(34 ÷ 9) = √3.778 = 1.94 cm. (For large samples n is used in place of n − 1.)
• Standard error (SE) – reliability of the sample mean: SE = SD ÷ √n = 1.944 ÷ 3.162 = 0.61 cm.

Student's t test (W.S. Gosset, pen-name "Student", 1908) tests whether the difference between the means of two small samples (n < 30) is significant. Null hypothesis (H₀): no real difference.

Variety B heights: 16, 18, 15, 17, 19, 16, 17, 18, 16, 18. Σx = 170, x̄₂ = 17; Σ(x − x̄)² = 14.

• Pooled variance S² = (34 + 14) ÷ (10 + 10 − 2) = 48 ÷ 18 = 2.667; S = 1.633.
• t = (x̄₂ − x̄₁) ÷ [S × √(1/n₁ + 1/n₂)] = 3 ÷ (1.633 × 0.4472) = 3 ÷ 0.730 = 4.11.
• Degrees of freedom = n₁ + n₂ − 2 = 18; table t at 5% level = 2.101.
• Calculated t (4.11) > table t (2.101), so H₀ is rejected: the difference in height is significant.

4. Evolution: Origin of life and organic evolution (Miller's theory)

Oparin–Haldane theory (1924–1929): life arose on the primitive earth by chemical evolution from inorganic molecules. The early atmosphere was reducing (no free oxygen) and contained methane (CH₄), ammonia (NH₃), hydrogen (H₂) and water vapour. Energy from lightning, UV rays and volcanic heat formed simple organic molecules, which collected in the sea as a "hot dilute soup".

Miller's experiment (Stanley Miller and Harold Urey, 1953) – experimental proof of chemical evolution:

• A closed glass apparatus contained CH₄, NH₃, H₂ and water vapour (from boiling water in a flask).
• Electric sparks between two tungsten electrodes imitated lightning.
• A condenser cooled the gases; the liquid collected in a U-tube trap.
• After about a week the liquid contained amino acids (glycine, alanine, aspartic acid), simple sugars and other organic compounds.

Steps of organic evolution after chemical evolution:

1. Simple monomers (amino acids, sugars, nucleotides) joined into polymers (proteins, nucleic acids).
2. Polymers aggregated into coacervates (Oparin) or microspheres (Fox) – protobionts with a membrane-like boundary.
3. Self-replicating RNA gave the protobionts heredity; the first cells were anaerobic heterotrophic prokaryotes.
4. Autotrophs (cyanobacteria-like) evolved photosynthesis, releasing oxygen; the atmosphere became oxidising and the ozone layer formed.
5. Aerobic organisms and eukaryotes evolved; variation and natural selection then produced the diversity of life (biological or organic evolution).

5 likely exam questions

Long answer

1. Define plant breeding and state its objectives. Describe the technique of hybridization and explain heterosis with its theories.
2. Describe the origin of life according to the Oparin–Haldane theory and explain Miller's experiment with a labelled diagram.

Short answer / short notes

3. Describe the method of pure line selection.
4. Calculate the mean, median and mode of: 12, 15, 11, 15, 17, 14, 13, 15, 16, 12.
5. What is Student's t test? State its uses.