I wanted a fort that would be based only on the original seven and their offspring. The climate outside was bad and their tribe was dead, so there would only be two reinforcement waves. I sealed the fort, and let the reinforcements freeze to death outside. After many years I had no babies. I checked and discovered I only had one female dwarf and she was the daughter/sister of all but one of the others. I left the game running and many more years later she did eventually have a baby and later a second. One of the babies was female, so maybe she would eventually have children? But before that could happen, the oldest dwarf died of old age. The survivors were pretty upset and one started kicking chairs in the dining room. This escalated into fighting. By the time it was over, there was only one crippled dwarf left. Much fun!