The dorsal gene in Drosophila melanogaster encodes a transcription factor critical for establishing the dorsoventral axis during early embryogenesis. It functions in a gradient manner, with high nuclear concentrations of Dorsal protein activating ventral-specific gene expression, which is essential for proper tissue differentiation.

Mutation or loss of function in the dorsal gene results in a failure to form ventral cell fates correctly. This leads to the dorsalization of the ventral side, where ventral cell types are lost or transformed into dorsal-like cells. Key ventral markers, such as twist and snail, fail to be expressed, and as a result, the ventral furrow does not form properly during gastrulation.

Due to this altered patterning, the embryo loses its normal ventral structures and instead exhibits dorsal characteristics in the ventral region. This phenotype confirms the pivotal role of dorsal in encoding positional information for tissue specification along the dorsoventral axis.

Therefore, the phenotype observed in a dorsal mutant is:

(1) Dorsalization of ventral side

Answer: (1) Dorsalization of ventral side