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 :

To understand why a mutated gene causing cancer does not appear on a photographic film during autoradiography, let us break down the technique of hybridization and detection step-by-step:

1. What is a Radioactive Probe?
A probe is a single-stranded DNA or RNA fragment tagged with a radioactive molecule. It is designed to have a complementary base sequence to the specific gene of interest (the normal, non-mutated gene).

2. The Principle of Hybridization:
Double-stranded DNA from the clone of cells is denatured (separated into single strands) and allowed to hybridize (bind) with the radioactive probe. Hybridization only occurs if there is perfect or near-perfect complementarity between the probe and the target DNA sequence.

3. Effect of Mutation:
If a gene has undergone mutation, its nucleotide sequence is altered. Because of this sequence change, the radioactive probe (which is complementary to the normal gene) can no longer hybridize with the mutated gene due to the lack of complementarity.

4. Detection via Autoradiography:
During autoradiography, a photographic film is exposed to the hybridised clone of cells. The radioactive probe emits radiation that exposes the film, making the hybridized DNA fragments visible as dark spots or bands.
Since the probe does not hybridize with the mutated gene, there is no radioactivity attached to the mutated gene's location. As a result, the mutated gene does not appear on the photographic film.

Therefore, the mutated gene does not appear on the photographic film because the radioactive probe has no complementarity with the mutated sequence.