If the HMPK carries the DT gene you'll get 50% DTs and 50% STs that carry the DT gene. If the HMPK doesn't carry the DT gene, you'll get 100% STs but they will all carry the DT gene, so that in F2 you'll get 25% DTs, 25% STs and 50% STs that carry the DT gene.
ETA: If the DT is a PK, all the offspring will be PKs. If the DT is a long-finned betta, then all the offspring will be long-finned as well.