The Ultrabithorax (Ubx) gene is a key member of the Hox gene family responsible for specifying segment identity along the anterior-posterior body axis in Drosophila. It plays a critical role in distinguishing the identity of the third thoracic segment (T3), where the halteres—small balancing organs—develop instead of wings.

When the Ultrabithorax gene is deleted or mutated, this regulatory function is lost, and the developmental program of the third thoracic segment switches to become like the second thoracic segment (T2), which normally develops two wings. As a result of this homeotic transformation, the fly develops an additional pair of wings in place of the halteres, leading to a four-winged phenotype.

This phenotype was one of the classic discoveries that established the role of Hox genes in segmental patterning and is a remarkable example of how gene regulation can generate morphological diversity. It also highlights the precision of developmental gene networks and how single gene changes can cause major morphological changes.

Therefore, the phenotype obtained upon deletion of the Ultrabithorax gene in Drosophila is:

(1) Fly with four wings

Answer: (1) Fly with four wings