(a) What are the major requirements for a successful back-crossing programme ? Describe its procedure, advantages and…

(a) Back-crossing: requirements, procedure, advantages and limitations

Requirements: a recurrent parent that is superior but lacks one trait; a donor parent with that trait; a trait that is highly heritable and easily scored, ideally governed by one or few genes; enough back-crosses (usually six or more) with a sufficiently large population in each; and (for a recessive trait) a selfing or progeny-test step.

Procedure: Cross the recurrent parent (A) with the donor (B) to get F₁. Back-cross F₁ to A to produce BC₁, select plants with the donor trait, and back-cross them again to A. Repeat for six or more generations, selecting for the trait each time. Finally self the last BC generation and select homozygous plants. The recurrent-parent genome recovered after n back-crosses is 1 − (½)ⁿ⁺¹ (about 99.2% after BC₆). Marker-assisted back-crossing speeds up recovery.

Advantages: the variety's good traits are kept while one trait is added; results are predictable, need less testing than new varieties; and it does not need large segregating populations. Example: Sub1 submergence tolerance introduced into popular rice cultivars such as Swarna.

Limitations: linkage drag; useful only for traits of simple inheritance; the donor trait must be identifiable in each generation; the variety is improved for only one trait; it takes many generations, and no new gene combinations are produced.

(b) Membrane transport and vesicular transport

Passive transport (down the gradient, no energy): simple diffusion of small non-polar molecules and gases, and facilitated diffusion through channels (e.g. aquaporins, ion channels) or carrier proteins (uniporters), and osmosis.

Active transport (against the gradient, using energy): primary transport uses ATP directly, as by the plasma-membrane H⁺-ATPase in plants and the Na⁺/K⁺-ATPase in animals; secondary transport couples movement to an ion gradient, by symport (proton-sucrose) or antiport (Na⁺/H⁺ exchange).

Vesicular transport: bulk movement in membrane-bound vesicles.

• Endocytosis: phagocytosis (particles), pinocytosis (fluid), and receptor-mediated endocytosis through clathrin-coated pits, pinched off by dynamin. Vesicles then fuse with endosomes and lysosomes or vacuoles.
• Exocytosis: Golgi-derived vesicles fuse with the plasma membrane to secrete proteins or cell-wall material, constitutively or on signal. Rab GTPases direct vesicles and SNARE proteins (v-SNARE and t-SNARE) drive membrane fusion.
• Transcytosis and ER–Golgi traffic use COPII (forward) and COPI (return) coated vesicles.

(c) Theory of natural selection and its significance

Darwin and Wallace (1858), Darwin in On the Origin of Species (1859), explained evolution by natural selection.

Postulates: (1) organisms produce more offspring than can survive, leading to a struggle for existence; (2) individuals of a population vary; (3) some variations are heritable; (4) individuals with variations better suited to the environment survive and reproduce more (survival of the fittest, i.e. differential reproductive success); (5) over generations, favourable variations accumulate and new species can form.

Modes of selection: stabilising (favours the mean, e.g. human birth weight), directional (shifts the mean, e.g. peppered moth melanism, pesticide and antibiotic resistance) and disruptive (favours both extremes).

Significance: it explains adaptation and the apparent design of organisms; it is the main cause of divergence and speciation, as in Darwin's finches; it links with genetics through the modern synthesis, in which mutation and recombination provide variation, and selection, drift, gene flow and isolation change allele frequencies; and it explains resistance in pests and pathogens and underpins crop and animal breeding, which mimics selection artificially. Darwin did not know the source of variation, which Mendelian genetics later supplied.