(a) What is chromosomal aberration? Briefly discuss the changes in chromosomal structure due to aberrations. (20 marks) (b)…

(a) Chromosomal aberrations and structural changes

Chromosomal aberration is any change in the normal structure or number of chromosomes. Numerical changes (aneuploidy and euploidy) are a separate class. Structural aberrations arise when chromosomes break and the broken ends rejoin wrongly. Breaks occur spontaneously or are induced by radiation and chemicals, altering the amount or arrangement of genes.

1. Deletion (deficiency): loss of a segment.

• Types: terminal, when the tip is lost, or interstitial, when an internal segment is lost.
• Cytology: in a heterozygote, the normal homologue buckles into a deletion loop at pachytene.
• Genetic effect: recessive alleles on the normal homologue are expressed, which is called pseudo-dominance. Geneticists have used this to map genes.
• Viability: large deletions are usually lethal in the homozygote, and deficient pollen is often non-functional.
• Example: cri-du-chat syndrome in humans.

2. Duplication: a segment is present more than once.

• Types: tandem, reverse tandem, or displaced.
• Origin: often unequal crossing-over.
• Example: Bar eye in Drosophila is a position and dosage effect of a duplicated segment.
• Significance: extra gene copies can evolve new functions.

3. Inversion: a segment is reversed by 180°.

• Paracentric inversion (centromere outside the segment): a crossover within the inversion loop gives a dicentric bridge and an acentric fragment at anaphase I.
• Pericentric inversion (centromere inside the segment): crossovers give duplication-deficiency gametes.
• Consequences: in both cases the recombinant products are eliminated. Inversions therefore act as crossover suppressors and cause partial sterility.

4. Translocation: a segment moves to a non-homologous chromosome.

• Simple (non-reciprocal): a one-way transfer of a segment.

Reciprocal: segments are exchanged.

• A heterozygote forms a cross-shaped quadrivalent at pachytene and a ring or chain of four at metaphase I.
• Adjacent segregation produces unbalanced gametes, so the plant shows about 50% semi-sterility.
• Robertsonian (centric fusion): two acrocentric chromosomes fuse at their centromeres.
• Use in breeding: the wheat–rye 1BL·1RS translocation brought rust resistance and adaptation into wheat. Sears transferred leaf-rust resistance from Aegilops umbellulata to wheat through an induced translocation.

Significance. They alter gene dosage, linkage and fertility, and are used in mapping, alien gene transfer and evolutionary study.

(b) Importance of conserving and using crop genetic resources

Crop genetic resources (CGR) are the sum of heritable variation available for crop improvement. They include:

• landraces and traditional varieties;
• obsolete and modern cultivars;
• breeding lines and genetic stocks;
• crop wild relatives.

Why conserve them

1. Genetic erosion. High-yielding uniform varieties and monocultures have displaced landraces since the Green Revolution. Their loss is irreversible.
2. Genetic vulnerability. Uniformity invites epidemics. The Irish potato famine of the 1840s and the 1970 Southern corn leaf blight in the USA, which struck maize carrying T-cytoplasm, are classic warnings.
3. Food and nutritional security, and climate resilience. CGR hold genes for tolerance to heat, drought, flooding and salinity, for resistance to new pests, and for nutritional quality. Future varieties must come from this pool.
4. Heritage and rights. CGR are protected under the CBD (1992), ITPGRFA (2001) and India's PPV&FR Act (2001).

How they are conserved

In situ:

• gene sanctuaries, for example for wild Citrus in the Garo Hills;
• biosphere reserves;
• on-farm conservation of landraces by farming communities.

Ex situ:

• seed genebanks, with long-term base collections at about −18 °C;
• field genebanks for vegetatively propagated and perennial crops;
• cryopreservation in liquid nitrogen at −196 °C;
• Institutions: in India the ICAR-NBPGR, New Delhi, runs the National Genebank. Globally, CGIAR genebanks and the Svalbard Global Seed Vault provide back-up.

Utilisation, the real purpose of conservation

Germplasm is first characterised, evaluated and summarised in core and mini-core collections. It then reaches farmers' fields through pre-breeding and genomics-assisted allele mining. Landmark examples include:

• Norin 10 semi-dwarfing genes in wheat, and the **Dee-geo-woo-gen *sd1*** gene in rice, which together made the Green Revolution possible;
• Oryza nivara as the source of grassy stunt virus resistance;
• wild-abortive cytoplasm for hybrid rice;
• the Sub1 gene from the Indian landrace FR13A, which gave Swarna-Sub1 for flood-prone eastern India;
• the Saltol gene from Pokkali for salinity tolerance.

Conclusion. Conservation without use is a museum, and use without conservation is unsustainable. Linking the two through genebanks, pre-breeding and benefit-sharing is essential to sustain yields under climate change.

(c) SPAC and rooting characteristics

The Soil-Plant-Atmosphere Continuum (SPAC), a concept introduced by J. R. Philip in 1966, treats soil, plant and atmosphere as one physically continuous hydraulic system.

• Direction of flow: water moves from higher (less negative) to lower water potential (Ψ): from soil, into the root, up the xylem, into the leaf, and out to the air.
• Driving force: the transpiration pull. Evaporation from leaf mesophyll creates tension that is transmitted down the cohesive water columns (cohesion–tension theory).
• Flux: the flux follows an Ohm's-law analogy. Flow equals the difference in Ψ divided by the resistances in series: soil, root, xylem, leaf and stomata/boundary layer.
• Largest drop: the steepest fall occurs between leaf and air. Air at 50% relative humidity has a Ψ of about −95 MPa, and stomata regulate this step.
• Drying soil: hydraulic conductivity falls steeply and soil resistance limits uptake.

How rooting characteristics shape moisture extraction

• Where water is taken from: water is extracted mainly where root length density is highest. In a uniformly moist profile, crops follow roughly the 40-30-20-10 pattern: 40% of water use comes from the top quarter of the root zone, then 30%, 20% and 10% from successively deeper quarters. As the upper layers dry, extraction shifts downward.

Depth of rooting:

• deep-rooted crops (sorghum, pigeonpea, cotton, sugarcane, lucerne) draw on subsoil moisture and tolerate drought;
• shallow or fibrous-rooted crops (rice, potato, onion, most vegetables) exhaust the surface layers and need frequent, light irrigation.

Other root traits:

• root hairs, a high root-to-shoot ratio under stress, and osmotic adjustment increase uptake;
• narrow xylem vessels ration water so that some remains for grain filling.
• Practical use: irrigation is scheduled at about 50% depletion of available water in the effective root zone.