• Protoplast fusion (somatic hybridization) requires overcoming the strong negative surface charges of isolated protoplasts to induce plasma membrane coalescence.
• Standard chemical fusogens and conditions include:
• Polyethylene glycol (PEG): Bridges membranes and dehydrates local membrane surfaces.
• High $\text{Ca}^{2+}$ concentration (e.g., $50\text{ mM } \text{CaCl}_2$): Neutralizes negative surface charges and facilitates lipid mixing.
• High pH (pH 9.0–10.5): Alters membrane surface potentials to promote membrane fusion.
• Cesium chloride (CsCl) is used in equilibrium density gradient ultracentrifugation for nucleic acid banding/purification; it is neither a fusogen nor used in protoplast fusion.
• Therefore, $\text{CsCl}_2$ (CsCl) is not used in protoplast fusion (Option 4).

Thomas Hunt Morgan's linkage and crossing over experiments in Drosophila melanogaster:

• Grey body ($b^+$) is dominant over black body ($b$), and normal wings ($vg^+$) are dominant over vestigial wings ($vg$). Both loci reside on chromosome 2.
• Parents: Grey normal ($b^+ vg^+ / b^+ vg^+$) $\times$ Black vestigial ($b\, vg / b\, vg$).
• In a testcross of female $F_1$ dihybrids ($b^+ vg^+ / b\, vg$) with double recessive black vestigial males ($b\, vg / b\, vg$), the non-parental recombinant offspring—grey vestigial ($b^+ vg / b\, vg$) and black normal ($b\, vg^+ / b\, vg$)—arise exclusively due to crossing over and recombination between linked genes.
• Because the genes reside on the same chromosome (linked) and produce non-parental recombinant types via crossing over, statements (A) and (C) only are true.