- ATO transgenic plant containing a transgene for herbicide resistance shows two bands on Southern blot analysis using a probe that is internal to the restriction sites used for genomic DNA digestion. However, it segregates in a 3: 1 ratio for herbicide resistance: sensitivity in the T1 progeny obtained by self- pollination. Which one of the following statements is correct?
(1) The T0 plant is a single-copy event
(2) The T0 plant is a double-copy event and the two transgene copies are tightly linked
(3) The T0 plant is a double-copy event and the two transgene copies are integrated in two different chromosomes
(4) The T0 plant contains two unlinked copies of the transgene, both of which are truncated versions of the herbicide resistance gene.The correct statement is (2) The T₀ plant is a double‑copy event and the two transgene copies are tightly linked.
Step-by-step reasoning
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Southern blot with a probe internal to the restriction sites, and digestion with a single enzyme, shows two bands → at least two integrated copies of the transgene.
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In the T₁ progeny (selfed T₀), herbicide resistance segregates 3 resistant : 1 sensitive.
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This is the Mendelian ratio for a single dominant locus, i.e. the T₀ behaves as heterozygous at one functional locus.
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Reconciling both facts:
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Genetically the resistance trait behaves as if controlled by one locus, but physically Southern shows two copies.
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This is best explained if the two copies are very tightly linked, inserted adjacent or in tandem at the same chromosomal locus, so they segregate together as one unit.
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The plant is then effectively (T t) at that “compound” locus, giving a 3:1 phenotype ratio on selfing.
Option-by-option explanation
(1) “The T₀ plant is a single-copy event” – Incorrect
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A single internal-probe band would be expected for a true single-copy event.
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The presence of two bands clearly indicates more than one copy.
(2) “Double-copy event, two copies tightly linked” – Correct
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Two copies (two bands) explain the Southern blot.
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Tight linkage (same locus, no recombination in meiosis) makes them inherit as a single Mendelian unit, giving 3:1 segregation for resistance vs sensitivity in T₁.
(3) “Double-copy event, copies in two different chromosomes” – Incorrect
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If the two functional copies were unlinked, and T₀ were heterozygous at both loci, selfing would give:
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Resistant (at least one copy present) = 15/16, sensitive = 1/16 → 15:1, not 3:1.
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Even if one copy were non-functional, that would not match the stated Southern (both seen by internal probe as intact) and 3:1 ratio simultaneously.
(4) “Two unlinked truncated copies” – Incorrect
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Truncated copies detected by an internal probe but lacking full coding sequence would not confer resistance.
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The plant would then be herbicide sensitive, contradicting the observed 3:1 resistant:sensitive segregation in T₁.
SEO‑oriented introduction (for article use)
When a herbicide‑resistant T₀ transgenic plant shows two internal-probe bands on a Southern blot yet its selfed T₁ progeny segregate 3:1 for resistance, the only consistent explanation is that it carries two tightly linked copies of the transgene at one locus. These copies travel together in meiosis and behave genetically as a single dominant locus, distinguishing this situation from single‑copy events or unlinked double‑copy insertions, which would give different segregation ratios.
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