Now a days it is possible to detect the mutated gene causing cancer by allowing radioactive probe to hybridise its complimentary DNA in a clone of cells, followed by its detection using autoradiography because :

The correct option is: mutated gene does not appear on a photographic film as the probe has no complimentarity with it.

Detailed Explanation:

1. Radioactive Probes and Hybridisation:
A probe is a single-stranded DNA or RNA fragment tagged with a radioactive molecule. In genomic testing, a probe is designed to be complementary to a specific target DNA sequence (usually the normal or wild-type gene of interest). During hybridisation, the probe binds to its complementary single-stranded DNA sequence in the host cells.

2. Impact of Mutation:
When a gene undergoes mutation, its nucleotide sequence is altered. Because of this structural change in the DNA sequence, the probe designed for the normal gene no longer finds a matching complementary sequence in the mutated gene. Thus, the probe cannot hybridise with the mutated gene.

3. Detection via Autoradiography:
Autoradiography uses photographic film to locate radioactive substances. If hybridisation is successful, the radioactive probe binds to the target DNA and exposes the photographic film, leaving a visible mark. Since the probe lacks complementarity to the mutated gene, it does not bind, and no radioactivity is present at the location of the mutated gene. As a result, the mutated gene does not appear on the photographic film, allowing researchers to easily identify the mutation by its absence on the developed film.