Final answer:
Without X-gal, researchers would be unable to visually distinguish between bacterial colonies with an intact lacZ gene and those with disrupted lacZ due to the insertion of foreign DNA, as there would be no blue color development.
Step-by-step explanation:
If researchers neglected to add X-gal to the medium when using lacZ as a reporter gene, the consequence would be that the medium would not turn blue, which means option 3 (The medium would turn yellow) is incorrect, option 4 (the lacZ gene would be overexpressed) is also incorrect as X-gal is not involved in the expression levels of lacZ, and finally, option 1 (The lacZ gene would not be expressed) is also incorrect because the presence of X-gal does not affect the expression of the lacZ gene itself. Instead, X-gal is a substrate for the β-galactosidase enzyme expressed by lacZ that, when cleaved, releases a blue dye, indicating the presence of lacZ activity. Hence, without X-gal, there would be no color change regardless of lacZ gene expression, making option 2 (The medium would turn blue) incorrect and mean that researchers would lack the ability to distinguish between colonies with intact lacZ gene and colonies where lacZ has been disrupted due to insertion of foreign DNA.