The correct statement is (4) Qualitative traits are typically characterized by discontinuous phenotypic variation while quantitative traits often generate continuous phenotypic variation.

Option-wise explanation

1. “To achieve high expression level of a heterologous gene in a transgenic plant a promoter of bacterial origin is often used.” – Incorrect

• High expression in plants usually uses strong plant or plant-virus promoters (e.g., CaMV 35S), not typical bacterial promoters.

• Bacterial promoters are generally not efficiently recognized by the plant transcription machinery.

2. “F1 progeny derived by crosses between inbred lines with low genetic diversity is more likely to show heterosis.” – Incorrect

• Heterosis (hybrid vigour) is strongest when parents are genetically diverse; crossing very similar (low-diversity) inbreds gives little heterosis.

3. “In Agrobacterium mediated plant transformation, always a single copy of the transgene is inserted in the host genome.” – Incorrect

• Agrobacterium often inserts one or a few copies, and multiple-copy insertions are common.

• It is not “always” a single copy.

4. “Qualitative traits are typically characterized by discontinuous phenotypic variation while quantitative traits often generate continuous phenotypic variation.” – Correct

• Qualitative traits (e.g., flower color, seed shape) are usually controlled by one or few genes and show distinct classes (tall vs dwarf, red vs white).

• Quantitative traits (e.g., height, yield) are polygenic and environmentally influenced, producing continuous variation across a range.

Thus, only option (4) correctly describes a fundamental concept in plant breeding and genetics.