Final answer:
To identify a nontaster in the PTC tasting lab, one must examine the banding pattern on an agarose gel following digestion with the Haelll restriction enzyme. The individual without a change or with the least change in band size compared to the undigested sample is likely the nontaster.
Step-by-step explanation:
In the context of a PTC tasting lab exercise, where restriction enzyme Haelll recognizes the sequence 'GGCC' and cuts between G and C, an agarose gel electrophoresis is used to visualize the results after digestion.
The banding pattern on the gel will differ depending on whether the genomic DNA from the individuals contains the restriction site or not.
For a nontaster, the DNA sequence at the relevant location would either not be cut (due to a mutation which removes the 'GGCC' site) or show a different pattern indicating a variant form that is not recognized by Haelll.
Assuming that this variant form remains uncut, the nontaster's digested sample would show a band similar in size to the undigested sample, while tasters would show smaller, differently sized fragments due to the successful cleavage by Haelll.
The success of the restriction endonuclease cleavage is evaluated by the presence of distinct bands on the agarose gel, which are the result of DNA fragments of different lengths being separated by electric current.
Whichever individual's digested sample shows no difference or less difference from their undigested sample may carry the non-tasting variant of the PTC gene, indicating they are likely a nontaster.