↓

F 1: bb (gray body)

↓ (free mating)

F2: bb (gray body) × F2:bb (gray body) BB (black body)

Except for B. melanogaster bb, others are free to mate.

F2: (1) 1/3bb (gray body) × 1/3BB (gray body)

↓

F3: 1/9bb (gray body)

F2: 21/3bb (gray body) ×2/3Bb (gray body)

↓

F3: 1/9BB (gray body) 1/9BB (gray body)

F2: (3) 2/3bb (gray body) × 1/3BB (gray body)

↓

F3: 1/9BB (gray body) 1/9BB (gray body)

F2: (4) 2/3bb (gray body) ×2/3Bb (gray body)

↓

F3: 1/9bb (gray body) 2/9Bb (gray body) bb (black body)

So:1/9+1/9+1/9+1/9+19 = 8/9.

→ gray body: black body = 8: 1