Final answer:
A drug that prevents spindle formation in a zygote disrupts proper chromosome segregation, potentially leading to aneuploidy, where cells have an abnormal number of chromosomes.
Step-by-step explanation:
If a drug that prevents spindle formation is added to a zygote for a single round of the cell cycle, the effect on chromosome number would be detrimental. The spindle apparatus, which is essential for proper chromosome segregation during cell division, would not function as intended. As a result, the chromosomes would not be distributed evenly into the daughter cells. Typically, such drugs bind tubulin, which is a building block of microtubules that make up the spindle fibers, thereby inhibiting proper spindle formation and attachment to chromosomes during cell division. Without functional spindles, chromosomes could be incorrectly distributed, leading to cells with abnormal chromosome numbers. Depending on the stage of the cell cycle in which the spindle is inhibited, this could result in gametes or somatic cells with too few or too many chromosomes, potentially leading to conditions such as aneuploidy.