Practice important
Genetic Dwarfism Reversal
MCQ questions and answers.
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MCQ Questions
1. A genetically dwarf pea mutant becomes nearly normal in height after repeated application of gibberellic acid. Which conclusion is most strongly supported?
A.
The mutation probably prevents gibberellin synthesis but leaves response machinery functional
B.
The mutation probably destroys all gibberellin receptors while increasing auxin synthesis
C.
The dwarf phenotype is caused by permanent absence of cell division in the shoot apex
D.
The applied gibberellin acts only by increasing chlorophyll concentration in mature leaves
Correct Answer:
The mutation probably prevents gibberellin synthesis but leaves response machinery functional
Explanation:
Reversal of genetic dwarfism by externally supplied gibberellin indicates that the plant can respond to the hormone, but may be unable to synthesize sufficient endogenous gibberellin.
2. Two dwarf mutants are treated with increasing concentrations of gibberellic acid. Mutant X shows normal elongation at low concentration, whereas mutant Y remains dwarf even at very high concentration. Which interpretation is most appropriate?
A.
Mutant X is likely gibberellin-deficient, whereas mutant Y may be defective in gibberellin perception or response
B.
Mutant X is gibberellin-insensitive, whereas mutant Y is deficient only in photosynthetic pigments
C.
Both mutants must have identical defects because both initially show dwarfism
D.
Mutant Y must synthesize excessive gibberellin that blocks all cell elongation
Correct Answer:
Mutant X is likely gibberellin-deficient, whereas mutant Y may be defective in gibberellin perception or response
Explanation:
A dose-responsive reversal suggests insufficient hormone production, while failure to respond even at high doses suggests a defect in hormone perception, signaling, or downstream response.
3. A dwarf rice variety is crossed with a normal-height variety, and the dwarf offspring regain normal height only after gibberellin treatment. Which physiological process most directly explains this restoration?
A.
Gibberellin-stimulated elongation of cells in the internodes
B.
Gibberellin-mediated conversion of chloroplasts into mitochondria
C.
Gibberellin-induced destruction of all cells in the shoot apical meristem
D.
Gibberellin-dependent replacement of the plant's genome by a normal genome
Correct Answer:
Gibberellin-stimulated elongation of cells in the internodes
Explanation:
Genetic dwarfism can be reversed when gibberellin restores internode elongation, allowing genetically short plants to approach the height of normal plants.
4. A researcher applies gibberellin to a genetically dwarf plant and observes elongated internodes but no change in the plant's genetic makeup. What does this observation demonstrate?
A.
Gibberellin can correct the physiological expression of dwarfism without correcting the underlying mutation
B.
Gibberellin permanently removes the dwarfing allele from every meristematic cell
C.
Gibberellin changes the chromosome number and thereby creates a normal genotype
D.
Gibberellin reverses dwarfism exclusively by increasing seed size before germination
Correct Answer:
Gibberellin can correct the physiological expression of dwarfism without correcting the underlying mutation
Explanation:
Hormone treatment changes growth behavior rather than DNA sequence; therefore, the phenotype may be rescued even though the genetic defect remains present.
5. Which experimental result would best distinguish a gibberellin-biosynthesis mutant from a gibberellin-response mutant among genetically dwarf plants?
A.
The biosynthesis mutant elongates after exogenous gibberellin, whereas the response mutant shows little or no elongation
B.
The biosynthesis mutant remains unaffected, whereas the response mutant becomes normal after exogenous gibberellin
C.
Both mutants show identical elongation because external gibberellin bypasses every genetic defect
D.
Neither mutant changes height because gibberellin functions only in seed germination
Correct Answer:
The biosynthesis mutant elongates after exogenous gibberellin, whereas the response mutant shows little or no elongation
Explanation:
Supplying the missing hormone can bypass a biosynthetic defect, but it cannot reliably overcome a defect in hormone perception or downstream signaling.
6. A dwarf mutant has normal gibberellin concentration in its tissues but fails to elongate after treatment with additional gibberellic acid. Which defect is most consistent with this result?
A.
A defect in gibberellin perception or signal transduction
B.
A defect limited to gibberellin synthesis in the roots
C.
A deficiency of carbon dioxide caused by excessive stomatal opening
D.
A mutation that increases the number of functional internodes
Correct Answer:
A defect in gibberellin perception or signal transduction
Explanation:
Normal endogenous hormone levels combined with insensitivity to added gibberellin point to a failure in recognizing or transmitting the gibberellin signal.
7. Why is exogenous gibberellin expected to be more effective in reversing a biosynthetic dwarf mutant than a receptor-defective dwarf mutant?
A.
It supplies the hormone that the biosynthetic mutant cannot produce, but cannot restore a defective receptor
B.
It replaces the mutant receptor with a functional receptor synthesized directly by the hormone
C.
It permanently silences the dwarfing gene only in plants lacking gibberellin receptors
D.
It increases chromosome duplication, which specifically repairs hormone-perception genes
Correct Answer:
It supplies the hormone that the biosynthetic mutant cannot produce, but cannot restore a defective receptor
Explanation:
A biosynthetic defect can be bypassed by supplying gibberellin externally, whereas a defective receptor prevents the plant from responding to that hormone.
8. A genetically dwarf plant is treated with gibberellin at three developmental stages. The greatest recovery of height occurs when treatment begins before rapid internode elongation. Which explanation is most defensible?
A.
Gibberellin must act while responsive internode tissues are actively elongating
B.
Gibberellin can reverse the mutation only before DNA replication begins in the seed
C.
Gibberellin affects height only by increasing the number of chromosomes in the embryo
D.
Gibberellin becomes toxic to all mature tissues and therefore acts exclusively on roots
Correct Answer:
Gibberellin must act while responsive internode tissues are actively elongating
Explanation:
Early treatment is more effective because developing internodes retain tissues capable of gibberellin-promoted cell elongation, allowing greater reversal of the dwarf phenotype.
9. A genetic dwarf plant treated with gibberellin develops taller stems, but its offspring remain genetically dwarf when grown without treatment. What does this result indicate?
A.
The treatment produced a temporary phenotypic rescue rather than a heritable genetic correction
B.
The treatment permanently converted the dwarf allele into a dominant normal allele
C.
The offspring are unaffected because gibberellin is inherited through chloroplast DNA only
D.
The original dwarfism was caused entirely by nutrient deficiency rather than by a mutation
Correct Answer:
The treatment produced a temporary phenotypic rescue rather than a heritable genetic correction
Explanation:
Gibberellin application can restore growth in the treated plant without altering the allele responsible for dwarfism, so untreated descendants can remain dwarf.
10. In a screening experiment, four genetically dwarf lines show the following responses to gibberellin: Line P reaches normal height, Line Q shows partial recovery, Line R shows no recovery, and Line S responds only at extremely high concentration. Which line most strongly suggests a severe defect in gibberellin response rather than a simple shortage of hormone?
A.
Line P
B.
Line Q
C.
Line R
D.
Line S
Correct Answer:
Line R
Explanation:
Complete absence of growth recovery despite abundant external gibberellin most strongly indicates a defect in hormone perception, signaling, or downstream responsiveness rather than merely inadequate hormone production.
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