Develop a musical piece incorporating the designated alphabet-based pattern and the supplied melodic theme.
X:1
L:1/8
Q:1/4=120
M:4/4
K:G
['B2 GB dgdB', 'B,/A,/G, DG,']  
X:1
L:1/8
Q:1/4=120
M:4/4
K:G
"^Allegro" d | B2 GB dgdB | gg a/g/f/g/ aAAd | B2 GB dgdB | cA d/c/B/A/ BGG :: D | 
 B,/A,/G, DG, EG,DG, | B,/A,/G, DG, EA,A,D | B,/A,/G, DB, EG,DG, | EGFA GG,G, :|  