​Shown below is the inversion product of the X chromosome of wild type Drosophila. The w+ gene coding for the red eye colour is near the telomere and another gene roughest (rst+) is close to w+ but towards the centromere. The eyes of roughest mutants have rough appearance. The inversion brings w+ near the vicinity of highly compact centromeric heterochromatin and places rst+ little farther away from the centromeric heterochromatin.

Which of the following phenotypes will NEVER be observed in the Drosophila strain with inversion?

Correct option is D

​The inversion alters gene expression due to position effect variegation:

1. w+ moves near centromeric heterochromatin, causing silencing, which results in white eyes.
2. rst+ moves away from heterochromatin, so it remains active, leading to smooth eyes.

Thus, the following phenotypes are possible:
White and Smooth Eyes → w+ is silenced (White), rst+ is active (Smooth).
White and Rough Eyes → In rare cases, rst+ may also be silenced (White + Rough).
Red and Rough Eyes → Red eyes are impossible because w+ is always silenced.

Therefore, Red and Rough Eyes (Option 4) will NEVER be observed.

Information Booster:

• Position Effect Variegation (PEV): When a gene is moved near heterochromatin, its expression is suppressed due to chromatin inactivation.
• Gene Silencing in Inversions:
• Genes placed near heterochromatin (like w+ here) are turned off.
• Genes moved away from heterochromatin (like rst+) remain active.
• In Drosophila, w+ (red eye gene) is silenced, leading to white eyes in the inversion strain.
• rst+ suppresses rough eyes, meaning its activity leads to smooth eyes, unless silenced.