- Given below are some statements that are associated with transgenic plants. Each statement has a blank space indicated by ‘_______’.
A. A transgenic plant with two functional copies of a transgene can segregate in a _______ ratio for the transgenic phenotype on self-pollination if the two genes are linked.
B. The _______ system can be used for removal of marker genes from transgenic plants.
C. The endogenous plant gene,_____, can be used to engineer resistance to imidazolinone herbicides.
D. Variations in transgene expression levels between five independent transgenic lines generated using the same T-DNA construct can be due to _______
Which one of the following options has the correct sequence of terms that can be used to fill in the blanks in the above statements (from A to D) such that all statements become true?
(1) A-9:3:3:1; B-Cre/loxP: C-ALS: D-codon usage of the transgene
(2) A-3:1; B-FLP/FRT; C-bar; D-copy number of transgene
(3) A-3:1; B-Cre/loxP: C-ALS; D-position effect
(4) A-1:2:1; B-FLP/FRT; C-EPSPS; D-position effectThe correct option is (3) A‑3:1; B‑Cre/loxP; C‑ALS; D‑position effect.
Filling each blank correctly
A. Segregation ratio with two linked functional copies
Statement: “A transgenic plant with two functional copies of a transgene can segregate in a _______ ratio for the transgenic phenotype on self‑pollination if the two genes are linked.”
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If both functional copies are tightly linked and inherited together as one unit, the plant behaves genetically as if it carries a single dominant locus for the trait.
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Selfing a heterozygote at that locus gives 3 transgenic : 1 non‑transgenic.
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So the blank must be 3:1, not 9:3:3:1 or 1:2:1.
Thus A = 3:1.
B. System used to remove marker genes
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Site‑specific recombination systems for marker excision include Cre/loxP (from phage P1) and FLP/FRT (from yeast).
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Both can be used, but only one appears in the correct option combined with correct answers to other blanks.
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Here, the widely cited system in plant transgenics is Cre/loxP.
So B = Cre/loxP.
C. Endogenous gene for imidazolinone resistance
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Imidazolinone and sulfonylurea herbicides inhibit acetolactate synthase (ALS/AHAS), the key enzyme in branched‑chain amino acid biosynthesis.
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Herbicide‑tolerant crops carry mutant ALS genes that no longer bind these herbicides.
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bar encodes phosphinothricin acetyltransferase (glufosinate/bialaphos resistance), and EPSPS mutations confer glyphosate resistance, not imidazolinone.
So C = ALS.
D. Cause of expression variation among independent lines
Statement: “Variations in transgene expression levels between five independent transgenic lines generated using the same T‑DNA construct can be due to _______.”
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With identical constructs, expression differences usually arise from position effects: different chromosomal insertion sites, local chromatin state, and surrounding regulatory elements.
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Copy number can also contribute, but only one of the options pairs correctly with A–C.
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Codon usage is constant for the same construct, so it cannot explain variation between lines.
So D = position effect.
Checking the options
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A-9:3:3:1; B-Cre/loxP; C-ALS; D-codon usage – wrong A and D.
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A-3:1; B-FLP/FRT; C-bar; D-copy number – wrong C; B not uniquely correct here.
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A-3:1; B-Cre/loxP; C-ALS; D-position effect – all four blanks correctly filled.
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A-1:2:1; B-FLP/FRT; C-EPSPS; D-position effect – wrong A and C.
Therefore, the correct sequence is given in option (3).
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