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Question 1Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

In wheat, the gene responsible for the semi-dwarf, gibberellin-insensitive phenotype exploited during the Green Revolution is a mutant allele of which locus?

  1. Lr34 (rust resistance gene)
  2. Ppd-1 (photoperiod gene)
  3. Vrn-1 (vernalization gene)
  4. Rht-B1/Rht-D1 (DELLA repressor) Correct answer.

Explanation

The reduced-height alleles Rht-B1b and Rht-D1b encode mutant DELLA proteins that resist gibberellin-induced degradation, keeping growth repressed and shortening the stem. Vrn-1 and Ppd-1 control flowering time, and Lr34 confers adult-plant rust resistance, so none of them produces the semi-dwarf, GA-insensitive habit.

Question 2Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

A plant breeder wants to estimate the narrow-sense heritability of grain yield from a half-sib progeny trial. Which variance component is the numerator of the estimate?

  1. Dominance variance
  2. Genotype × environment variance
  3. Total phenotypic variance
  4. Additive genetic variance Correct answer.

Explanation

Narrow-sense heritability is the ratio of additive genetic variance to total phenotypic variance, because additive effects are the component transmitted to offspring and thus determine response to selection. Dominance and epistatic variances are non-additive, and genotype × environment variance is part of the environmental portion of the phenotype.

Question 3Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

In a hybrid maize seed production block, detasselling of the female rows is performed to prevent which event?

  1. Cross-pollination of the male rows
  2. Precocious germination of the seed
  3. Wind damage to the tassels
  4. Self-pollination of the female inbred Correct answer.

Explanation

Removing the tassels from the female parent eliminates its own pollen source, so the ears are fertilised only by pollen from the designated male rows. This yields hybrid rather than selfed seed. Cross-pollination of the male rows is the desired outcome, and detasselling does not affect wind damage or vivipary.

Question 4Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

A soil-borne pathogen produces pear-shaped sporangia that release biflagellate zoospores, and it causes a rot of potato tubers in waterlogged fields. Which organism fits this description?

  1. Pythium aphanidermatum Correct answer.
  2. Phytophthora infestans
  3. Fusarium oxysporum
  4. Rhizoctonia solani

Explanation

Pythium aphanidermatum is an oomycete whose lemon- or pear-shaped sporangia produce biflagellate zoospores that swim to roots and tubers in saturated soil, causing watery rots. Rhizoctonia and Fusarium are true fungi without motile zoospores, and Phytophthora infestans causes late blight of foliage rather than a waterlogging-associated tuber rot.

Question 5Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

The gene-for-gene model of plant disease resistance predicts that a plant resistance (R) gene product recognises which pathogen component?

  1. A ribosomal RNA sequence
  2. A corresponding avirulence (Avr) effector Correct answer.
  3. A host susceptibility factor
  4. A conserved chitin synthase

Explanation

In the gene-for-gene model, a plant R gene confers resistance only when the pathogen carries the matching avirulence gene, whose effector protein is recognised directly or indirectly. Recognition triggers defence responses such as the hypersensitive reaction. Ribosomal RNA and chitin synthase are not the specific matching determinants, and a host susceptibility factor is a plant, not pathogen, component.

Question 6Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

An insect has piercing-sucking mouthparts, a pair of cornicles on the abdomen, and reproduces parthenogenetically in summer. To which group does it belong?

  1. Aphididae Correct answer.
  2. Pentatomidae
  3. Thripidae
  4. Cicadellidae

Explanation

Aphids (Aphididae) are characterised by piercing-sucking stylets, abdominal cornicles (siphunculi), and cyclical parthenogenesis with live-bearing summer females. Cicadellids lack cornicles, pentatomids have true piercing-sucking but no cornicles and typically lay eggs, and thrips have rasping-sucking mouthparts with fringed wings.

Question 7Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

Which pest management principle is illustrated by releasing Trichogramma chilonis against the sugarcane shoot borer?

  1. Cultural control by crop rotation
  2. Mechanical control by light traps
  3. Chemical control with a systemic insecticide
  4. Biological control by an egg parasitoid Correct answer.

Explanation

Trichogramma chilonis is a minute hymenopteran that parasitises the eggs of lepidopteran borers, so its release is a classical augmentative biological control tactic. It is neither a chemical, a rotation practice, nor a mechanical trap, and it acts specifically on the egg stage of the pest.

Question 8Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

In mango, the technique of grafting a scion onto a rootstock is used primarily because it preserves which feature of the scion?

  1. Its clonal genotype Correct answer.
  2. Its root system
  3. Its seed dormancy
  4. Its juvenile phase

Explanation

Because mango is highly heterozygous, seed propagation would give variable offspring; grafting propagates the scion vegetatively, so the new plant is a clone retaining the cultivar's genotype and fruit quality. The rootstock supplies the root system, the scion is already past juvenility, and seed dormancy is irrelevant to a grafted plant.

Question 9Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

A tomato crop shows blossom-end rot on the fruit while leaf analysis shows adequate calcium. Which explanation is most consistent with this observation?

  1. Excess boron in the soil
  2. A viral infection of the fruit
  3. A systemic nitrogen deficiency
  4. Localised calcium deficiency in the fruit caused by irregular water supply Correct answer.

Explanation

Calcium moves in the xylem with the transpiration stream and is largely phloem-immobile, so rapidly growing fruit tips can become locally deficient when water supply fluctuates, even if leaves test adequate. The result is blossom-end rot. Nitrogen deficiency, viruses, and boron toxicity produce different symptoms.

Question 10Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

The C4 photosynthetic pathway concentrates CO2 in bundle-sheath cells using which primary carboxylating enzyme in the mesophyll?

  1. Carbonic anhydrase
  2. Phosphoenolpyruvate carboxylase Correct answer.
  3. Rubisco
  4. Pyruvate phosphate dikinase

Explanation

In C4 plants, PEP carboxylase fixes bicarbonate in the mesophyll to form oxaloacetate or malate, which is decarboxylated in bundle-sheath cells to feed Rubisco with concentrated CO2. Carbonic anhydrase supplies bicarbonate but does not fix carbon, and pyruvate phosphate dikinase regenerates PEP. Rubisco performs the Calvin cycle in the bundle sheath.

Question 11Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

A breeder crosses two pure lines and finds that the F1 is superior to both parents for yield. This phenomenon is termed:

  1. Pleiotropy
  2. Heterosis Correct answer.
  3. Inbreeding depression
  4. Transgressive segregation

Explanation

Heterosis, or hybrid vigour, is the superiority of the F1 hybrid over its parents, commonly exploited in cross-pollinated crops such as maize. Transgressive segregation describes F2 or later progeny exceeding parental ranges, inbreeding depression is the decline on selfing, and pleiotropy is one gene affecting several traits.

Question 12Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

Which cultural practice most directly reduces the carry-over of Xanthomonas oryzae pv. oryzae between rice crops?

  1. Flooding fields continuously during the fallow period
  2. Raising nursery beds close to the previous season's crop
  3. Applying nitrogen in a single heavy dose
  4. Removing infected stubble and avoiding clipping of seedling tips at transplanting Correct answer.

Explanation

Bacterial blight survives mainly in infected straw and stubble and enters plants through wounds, so destroying residue and avoiding clipping of seedling tips at transplanting remove the inoculum and the wound entry points. Continuous flooding does not eliminate the bacterium, heavy nitrogen increases susceptibility, and proximity to old crop residue raises inoculum pressure.

Question 13Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

The sterile insect technique suppresses pest populations by releasing insects that have been:

  1. Selected for insecticide resistance
  2. Infected with a cytoplasmic incompatibility symbiont
  3. Sterilised by gamma irradiation Correct answer.
  4. Treated with a juvenile hormone analogue

Explanation

The sterile insect technique mass-releases males sterilised by ionising radiation; their matings with wild females produce no viable offspring, progressively reducing the population. Juvenile hormone analogues disrupt development rather than sterility, and neither symbiont-based incompatibility nor resistance selection is the defining mechanism of SIT.

Question 14Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

A plant physiologist measures a leaf water potential of −1.2 MPa at predawn and −2.0 MPa at midday. Which interpretation is most defensible?

  1. The plant is accumulating solutes only at night
  2. Predawn values indicate severe water stress
  3. The leaf is fully turgid at midday
  4. The midday drop reflects transpirational water loss exceeding uptake Correct answer.

Explanation

Predawn water potential approximates soil water status because transpiration is minimal, while the more negative midday value arises when transpiration outpaces root uptake and creates tension in the xylem. A leaf at −2.0 MPa is under considerable tension, not turgid, and the predawn value here indicates relatively moist soil.

Question 15Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

In a self-pollinated crop, which breeding method is most appropriate for improving a well-adapted cultivar by transferring a single simply inherited disease resistance gene from a donor line?

  1. Synthetic variety development
  2. Recurrent selection
  3. Topcross testing
  4. Backcross breeding Correct answer.

Explanation

Backcross breeding repeatedly crosses the donor to the recurrent parent while selecting for the resistance allele, recovering the recurrent parent's genome except for the introgressed locus. Recurrent selection and synthetic varieties are used in cross-pollinated crops, and topcross testing evaluates combining ability rather than transferring a specific gene.

Question 16Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

The hypersensitive response at the site of pathogen attack is typically associated with which early cellular event?

  1. Closure of plasmodesmata by callose only
  2. Loss of membrane integrity before any signalling
  3. A rise in cytosolic pH to alkaline values
  4. A burst of reactive oxygen species Correct answer.

Explanation

Recognition of an avirulence effector triggers a rapid oxidative burst, mainly from membrane-bound NADPH oxidases, which contributes to the hypersensitive response and localised cell death. Pathogen attack usually causes cytosolic acidification, callose deposition is one of several later responses, and membrane integrity is lost after, not before, signalling.

Question 17Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

Which statement correctly distinguishes a parasitoid from a predator in insect biological control?

  1. A parasitoid develops on or in a single host individual, killing it Correct answer.
  2. A parasitoid kills several prey per day as an adult
  3. A predator is always host-specific
  4. A predator completes development inside one host

Explanation

A parasitoid spends its immature stages on or within one host and typically kills that host, whereas a predator consumes many prey during its life. Predators are not necessarily host-specific, and the description of killing many prey per day applies to predators, not parasitoids.

Question 18Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

In banana, the disease that threatens Cavendish plantations worldwide and is caused by Fusarium oxysporum f. sp. cubense tropical race 4 is known as:

  1. Moko disease
  2. Panama wilt Correct answer.
  3. Bunchy top
  4. Sigatoka leaf spot

Explanation

Fusarium oxysporum f. sp. cubense causes Fusarium wilt, commonly called Panama disease, and tropical race 4 is virulent on Cavendish cultivars. Moko disease is bacterial, bunchy top is viral, and Sigatoka is a leaf spot disease.

Question 19Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

The physiological basis of the juvenile phase in woody perennials is best described as:

  1. A deficiency of carbohydrates in young shoots
  2. A permanent block to flowering throughout the plant's life
  3. A lack of vascular connection to the roots
  4. An inability to flower that is gradually lost with age and node number Correct answer.

Explanation

Juvenile wood cannot flower even under inductive conditions; the phase is measured by node number and time, and the shoot gradually acquires the competence to flower as it matures. It is not permanent, is not caused by carbohydrate shortage, and young shoots are fully connected to the roots.

Question 20Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

Which mechanism best explains why Bt cotton remains resistant to Helicoverpa armigera while a non-Bt refuge is maintained alongside it?

  1. Refuge sterilises resistant individuals
  2. Refuge increases the dose of toxin ingested by larvae
  3. Refuge allows survival of susceptible moths that dilute resistance alleles Correct answer.
  4. Refuge prevents the toxin from entering the plant

Explanation

A nearby non-Bt refuge supports susceptible moths that mate with any rare resistant survivors, so resistance alleles remain dilute and the trait is inherited recessively. The refuge does not raise toxin dose, block toxin expression, or sterilise insects; it works through population genetic dilution.

Question 21Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

A rice variety flowers at the same date whether sown early or late in the season. This behaviour indicates:

  1. Photoperiod insensitivity Correct answer.
  2. Dominance of the juvenile phase
  3. Strong vernalization requirement
  4. Thermosensitivity to low temperature

Explanation

A genotype whose flowering date shifts with sowing date only in proportion to accumulated temperature, and not with day length, is photoperiod-insensitive. Vernalization and thermosensitivity would make flowering depend on cold exposure, and a fixed juvenile phase would still shift the date with sowing.

Question 22Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

Seed dormancy in many temperate cereals is broken by a period of moist cold treatment. This treatment is called:

  1. Stratification Correct answer.
  2. Vernalization
  3. Scarification
  4. After-ripening

Explanation

Stratification is the moist-cold treatment that breaks seed dormancy by allowing the embryo to overcome inhibitors and complete physiological preparation for germination. Scarification breaks hard seed coats, vernalization induces flowering in the growing plant, and after-ripening is dry storage that relieves dormancy in some species.

Question 23Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

A breeder observes that a trait has high broad-sense heritability but low narrow-sense heritability. What does this imply for a recurrent selection programme?

  1. Response will be high because most variance is genetic
  2. Response will be high because dominance is heritable
  3. Response cannot be predicted from heritability
  4. Response will be limited because additive variance is small Correct answer.

Explanation

Broad-sense heritability includes dominance and epistatic variance, which are not transmitted predictably to progeny, whereas narrow-sense heritability measures the additive portion that drives response to selection. A large gap between the two means most genetic variance is non-additive, so genetic gain will be modest.

Question 24Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

In the context of integrated pest management, economic threshold level refers to:

  1. The maximum pest population the crop can support without any yield reduction
  2. The pest density at which control measures should be initiated to prevent economic loss Correct answer.
  3. The total number of insects caught in a pheromone trap per season
  4. The cost of the pesticide divided by the market price of the crop

Explanation

The economic threshold is the pest density at which intervention is justified to stop the population from reaching the economic injury level, where loss equals control cost. It is not the zero-damage maximum, not a simple cost ratio, and not a seasonal trap catch.

Question 25Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

A crop shows interveinal chlorosis on older leaves, and tissue analysis confirms the deficiency. Which nutrient is most likely involved?

  1. Calcium
  2. Iron
  3. Boron
  4. Magnesium Correct answer.

Explanation

Magnesium is a mobile nutrient and a central atom of chlorophyll, so its deficiency appears as interveinal chlorosis on older leaves as the plant remobilises it to young tissue. Iron, calcium, and boron are phloem-immobile, so their deficiencies appear first on young leaves.

Question 26Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

In a quantitative genetics experiment, the total phenotypic variance (VP) of a trait is 50, and the environmental variance (VE) is 20. What is the broad-sense heritability (H²) of the trait?

  1. 0.70
  2. 0.40
  3. 0.60 Correct answer.
  4. 0.30

Explanation

Broad-sense heritability is the proportion of total phenotypic variance due to genetic variance: H² = VG/VP. Since VG = VP − VE = 50 − 20 = 30, H² = 30/50 = 0.60. This measures the overall genetic contribution to phenotypic variation.

Question 27Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

Which type of gene action is indicated when the heterozygote expresses a phenotype that is distinct from and often more extreme than either homozygote?

  1. Additive gene action
  2. Partial dominance
  3. Overdominance Correct answer.
  4. Complete dominance

Explanation

Overdominance occurs when the heterozygous genotype has a phenotype outside the range of both homozygotes, often leading to heterosis. This is distinct from complete or partial dominance, where the heterozygote resembles one parent or is intermediate.

Question 28Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

A plant pathologist observes water-soaked lesions on rice leaves that turn yellowish-white with wavy margins, and bacterial ooze streams from cut ends when placed in water. Which disease is most likely?

  1. Rice blast
  2. Bacterial leaf blight Correct answer.
  3. Brown spot
  4. Sheath blight

Explanation

Bacterial leaf blight of rice, caused by Xanthomonas oryzae pv. oryzae, produces water-soaked lesions that become yellowish-white with wavy margins, and bacterial ooze is a diagnostic sign. Blast and brown spot are fungal diseases, and sheath blight affects the sheath with different symptoms.

Question 29Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

Which insect order is characterized by having one pair of wings (the forewings) and a pair of halteres, and includes important pests such as fruit flies and mosquitoes?

  1. Lepidoptera
  2. Coleoptera
  3. Diptera Correct answer.
  4. Hymenoptera

Explanation

Diptera (true flies) have only one pair of functional wings; the hindwings are reduced to halteres used for balance. This order includes fruit flies, mosquitoes, and many other pests. The other orders have two pairs of wings or different wing modifications.

Question 30Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

In a horticultural nursery, a crop is grown under a photoperiod of 14 hours, and flowering is promoted. If the same crop is grown under a 10-hour photoperiod, flowering is delayed. What is the term for this response?

  1. Short-day plant
  2. Vernalization requirement
  3. Day-neutral plant
  4. Long-day plant Correct answer.

Explanation

Long-day plants flower earlier when the day length exceeds a critical photoperiod (typically more than about 12 hours). In this case, 14 hours promotes flowering, while 10 hours delays it, indicating a long-day response. Short-day plants would flower under short days, and day-neutral plants are insensitive to photoperiod.

Question 31Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

A farmer wants to control weeds in a field of a broadleaf crop without harming the crop. Which herbicide selectivity principle is most appropriate?

  1. Using a non-selective herbicide at a high rate
  2. Applying a contact herbicide that burns all foliage
  3. Applying a selective herbicide that targets broadleaf weeds Correct answer.
  4. Using a pre-emergence herbicide before crop planting

Explanation

Selective herbicides kill specific weed species while leaving the crop unharmed, often due to differences in uptake, metabolism, or target-site sensitivity. This is the basis for safe weed control in broadleaf crops. Non-selective or contact herbicides would damage the crop.

Question 32Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

In a plant breeding program, a breeder wants to develop a pure line from a segregating population of a self-pollinated crop. Which method involves continuous selfing and selection of individual plants until homozygosity is achieved?

  1. Pedigree method Correct answer.
  2. Backcross method
  3. Mass selection
  4. Recurrent selection

Explanation

The pedigree method involves selecting individual plants in segregating generations and maintaining records of their progeny, with continuous selfing to achieve homozygosity. It is commonly used in self-pollinated crops to develop pure lines. Mass selection is less precise, backcrossing introgresses specific traits, and recurrent selection improves populations.

Question 33Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

A plant physiologist measures the osmotic potential of a leaf cell as −1.5 MPa and the turgor pressure as 0.8 MPa. What is the water potential of the cell?

  1. 0.7 MPa
  2. −2.3 MPa
  3. −0.7 MPa Correct answer.
  4. 2.3 MPa

Explanation

Water potential (Ψw) is the sum of osmotic potential (Ψs) and pressure potential (Ψp): Ψw = Ψs + Ψp. Here, −1.5 MPa + 0.8 MPa = −0.7 MPa. This indicates the cell has a negative water potential, allowing water uptake.

Question 34Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

In an integrated pest management program, which practice best conserves natural enemies of insect pests in a crop field?

  1. Removing all non-crop vegetation around the field
  2. Using soil fumigation before planting
  3. Planting flowering strips that provide nectar and pollen Correct answer.
  4. Applying broad-spectrum insecticides on a calendar schedule

Explanation

Flowering strips provide alternative food sources (nectar and pollen) and shelter for natural enemies such as parasitoids and predators, enhancing their survival and effectiveness. This is a key conservation biological control tactic. Broad-spectrum insecticides and removal of vegetation reduce natural enemy populations.

Question 35Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

Which soil condition most directly favors the development of cavity spot in carrot, caused by Pythium species?

  1. Well-drained sandy soil
  2. Low soil temperature
  3. High soil pH
  4. Waterlogged soil Correct answer.

Explanation

Pythium species are oomycetes that thrive in wet conditions; waterlogged soil promotes zoospore movement and infection, leading to cavity spot. Well-drained soil reduces disease. Soil pH and temperature can influence but are less directly linked than moisture.

Question 36Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

In a population of an insect pest, resistance to an insecticide develops because:

  1. The insecticide induces mutations that confer resistance
  2. Insects learn to avoid treated surfaces
  3. The insecticide is degraded by soil microbes
  4. Pre-existing resistant genotypes survive and reproduce Correct answer.

Explanation

Resistance evolves through natural selection: resistant individuals already present in the population survive insecticide application and pass on resistance alleles. Insecticides do not induce resistance mutations; they select for them. This is a fundamental concept in pest management.

Question 37Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

A plant breeder evaluates a new genotype across multiple environments and finds that genotype × environment interaction is significant. What does this imply for selection?

  1. Environmental variance is negligible
  2. The genotype performs consistently across environments
  3. The genotype is universally superior
  4. Selection in one environment may not predict performance in another Correct answer.

Explanation

A significant genotype × environment interaction means that the relative performance of genotypes changes across environments, so selection in one environment may not be reliable in another. This necessitates multi-environment testing and possibly stability analysis. Consistent performance would imply no interaction.

Question 38Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

In a study of a quantitative trait, the additive genetic variance is 15 and the dominance variance is 5. The total genetic variance is 25. What is the narrow-sense heritability if the phenotypic variance is 50?

  1. 0.40
  2. 0.30 Correct answer.
  3. 0.60
  4. 0.50

Explanation

Narrow-sense heritability (h²) is the ratio of additive genetic variance (VA) to phenotypic variance (VP): h² = VA/VP = 15/50 = 0.30. Dominance variance is not included in narrow-sense heritability. This measures the proportion of phenotypic variance due to additive gene effects.

Question 39Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

Which physiological disorder is characterized by the accumulation of calcium oxalate crystals in the cells of a fruit, often leading to a bitter taste and a gritty texture, and is common in pear?

  1. Stone cells Correct answer.
  2. Water core
  3. Bitter pit
  4. Sunburn

Explanation

Stone cells (sclereids) are dead cells with thick lignified walls and often contain calcium oxalate crystals, causing a gritty texture in pear fruit. This is a structural disorder, not a nutritional deficiency. Bitter pit is a calcium deficiency disorder with dark spots, water core is a physiological disorder with water-soaked tissue, and sunburn is heat damage.

Question 40Plant Sciences (agronomy, genetics and plant breeding, plant pathology, entomology, horticulture, plant physiology)

In a crop rotation scheme, a legume is included to improve soil fertility. Which mechanism primarily explains the benefit?

  1. Legumes fix atmospheric nitrogen through symbiotic rhizobia Correct answer.
  2. Legumes enhance phosphorus solubilization by root exudates
  3. Legumes increase soil pH by releasing organic acids
  4. Legumes release allelopathic compounds that suppress weeds

Explanation

Legumes form symbiotic associations with rhizobia, which fix atmospheric nitrogen into forms usable by plants, thereby enriching soil nitrogen. This is the primary reason for including legumes in rotations. Allelopathy, pH changes, and phosphorus solubilization are not the main benefits.

Question 41Animal and Veterinary Sciences (animal breeding and genetics, animal nutrition, veterinary pathology, veterinary microbiology)

In a herd improvement programme, which statistic directly measures the proportion of total phenotypic variance in a trait that is attributable to additive genetic effects?

  1. Phenotypic correlation
  2. Repeatability
  3. Breeding value
  4. Heritability Correct answer.

Explanation

Heritability in the narrow sense is the ratio of additive genetic variance to total phenotypic variance, so it quantifies the fraction of phenotypic variation transmitted additively to offspring. Repeatability instead expresses consistency of repeated records, and breeding value is an individual's genetic merit rather than a variance ratio.

Question 42Animal and Veterinary Sciences (animal breeding and genetics, animal nutrition, veterinary pathology, veterinary microbiology)

A closed herd of 200 breeding females has an effective population size of about 25. Which consequence is expected from such a small effective size?

  1. Complete absence of inbreeding depression
  2. Rapid loss of heterozygosity and increased inbreeding per generation Correct answer.
  3. Elimination of genetic drift from the population
  4. Immediate fixation of all favourable alleles

Explanation

Effective population size determines the rate of genetic drift and inbreeding; with Ne around 25, inbreeding rises by roughly 1/(2Ne), about 2 percent per generation, and heterozygosity declines accordingly. Small Ne accelerates drift-driven allele frequency changes rather than removing them, and it raises rather than eliminates inbreeding depression risk.

Question 43Animal and Veterinary Sciences (animal breeding and genetics, animal nutrition, veterinary pathology, veterinary microbiology)

Which mating system is deliberately used to increase the frequency of homozygotes and expose recessive alleles in a breeding programme?

  1. Grading up
  2. Topcrossing
  3. Close inbreeding Correct answer.
  4. Outcrossing

Explanation

Close inbreeding, such as full-sib or parent-offspring mating, raises homozygosity each generation and thereby reveals deleterious recessive alleles through inbreeding depression. Outcrossing and topcrossing do the opposite by increasing heterozygosity, and grading up merely increases the proportion of a desired breed over generations.

Question 44Animal and Veterinary Sciences (animal breeding and genetics, animal nutrition, veterinary pathology, veterinary microbiology)

In a selection index, why are economic weights assigned to each trait in the aggregate genotype?

  1. To standardise traits to a common phenotypic scale
  2. To reflect the relative profit contributed by a unit genetic gain in each trait Correct answer.
  3. To remove the influence of genetic correlations between traits
  4. To estimate the heritability of each trait

Explanation

Economic weights convert genetic change in each trait into monetary value so the index maximises total economic merit rather than any single trait. They do not alter heritabilities or remove genetic correlations; those enter the index through the variance-covariance structure.

Question 45Animal and Veterinary Sciences (animal breeding and genetics, animal nutrition, veterinary pathology, veterinary microbiology)

Which statement correctly describes the relationship between accuracy of selection and generation interval in breeding scheme design?

  1. Accuracy and generation interval are always positively correlated
  2. Increasing accuracy usually requires progeny testing, which lengthens the generation interval Correct answer.
  3. Accuracy is fixed by heritability alone and cannot be improved
  4. Generation interval has no effect on annual genetic response

Explanation

Annual genetic response equals accuracy times selection intensity times genetic standard deviation divided by generation interval. Progeny testing raises accuracy for lowly heritable traits but delays use of sires, lengthening the generation interval and offsetting part of the gain.

Question 46Animal and Veterinary Sciences (animal breeding and genetics, animal nutrition, veterinary pathology, veterinary microbiology)

Which crossbreeding effect describes the superior performance of crossbred animals over the average of their parental breeds?

  1. Additive gene action
  2. Heterosis Correct answer.
  3. Epistatic fixation
  4. Inbreeding depression

Explanation

Heterosis, or hybrid vigour, is the advantage of crossbreds over the mean of the parental breeds and is largest for lowly heritable fitness traits. It arises mainly from dominance effects masking recessive deleterious alleles, not from additive gene action.

Question 47Animal and Veterinary Sciences (animal breeding and genetics, animal nutrition, veterinary pathology, veterinary microbiology)

A cow's diet is abruptly changed from high-forage to high-concentrate. Which rumen disturbance is most likely to follow?

  1. Rumen alkalosis with a rise in pH above 7
  2. Enhanced fibre digestion by cellulolytic bacteria
  3. Increased rumination and saliva flow
  4. Subacute ruminal acidosis with a fall in rumen pH Correct answer.

Explanation

Rapid introduction of readily fermentable carbohydrate increases volatile fatty acid production faster than it is absorbed, lowering rumen pH and causing subacute ruminal acidosis. Fibre digestion and rumination decline under acidic conditions, and rumen pH falls rather than rises.

Question 48Animal and Veterinary Sciences (animal breeding and genetics, animal nutrition, veterinary pathology, veterinary microbiology)

Which measure best expresses the energy value of a feed for ruminants by accounting for energy lost in methane and urine?

  1. Total digestible nutrients
  2. Gross energy
  3. Metabolisable energy Correct answer.
  4. Digestible energy

Explanation

Metabolisable energy is digestible energy minus energy lost in urine and fermentation gases such as methane, so it represents energy available for metabolism. Gross energy is total heat of combustion and digestible energy ignores the gaseous and urinary losses.

Question 49Animal and Veterinary Sciences (animal breeding and genetics, animal nutrition, veterinary pathology, veterinary microbiology)

A poultry ration is formulated with a large excess of dietary calcium. Which adverse effect is most likely?

  1. Increased eggshell thickness without limit
  2. Reduced availability of zinc and interference with phosphorus utilisation Correct answer.
  3. Enhanced protein digestibility
  4. Improved absorption of zinc and phosphorus

Explanation

Excess calcium forms insoluble complexes with zinc and competes with phosphorus absorption, reducing availability of both minerals and causing deficiencies. It does not enhance protein digestibility and eggshell quality does not improve indefinitely with rising calcium.

Question 50Animal and Veterinary Sciences (animal breeding and genetics, animal nutrition, veterinary pathology, veterinary microbiology)

Which nitrogen fraction in a feed is used by rumen microbes without prior degradation of true protein?

  1. Neutral detergent insoluble nitrogen
  2. By-pass protein
  3. Acid detergent fibre nitrogen
  4. Non-protein nitrogen Correct answer.

Explanation

Non-protein nitrogen such as urea is rapidly hydrolysed to ammonia in the rumen and captured by microbes for protein synthesis. By-pass protein escapes rumen degradation, and the fibre-bound nitrogen fractions are largely unavailable.