Well, I have had 6 (i think) groups of dratini hatched. 
First group:
1 boy, 2 girls
second group:
1 boy, 3 girls
third group:
1 boy, 2 girls
fourth group:
3 (i think) girls, 2 boys
fifth group:
this one i'm not sure about.
sixth group:
3 girls? 
that's it. as you can see, it's weird.