Well done! Programming is all about making tools that make our lives easier. This takes me right back to the first useful program I wrote; my Mum was doing an Open University degree and was doing a load of chi-squares; each took about half n hour with a calculator and she'd got bored with it having done enough to understand them and still having loads left over; so she showed me how they worked and I knocked up a quick program on my trusty old Commodore 64 which took the numbers in and spat out a chi square almost instantly.

I don't know anything about punnet squares (thought at first it was something to do with strawberries), but the code looks just fine to me and if it works I think you've got something there worth keeping. The only nit I would pick on the above code is that the indentation could be better; use 3 or 4 spaces per indent not just one, and code can sometimes look clearer if it's formatted into a tabular layout rather than following strict code layout rules, e.g.:
Code:
     if (b2=='A' && c2=='A') { ++homdcount;  }
else if (b2=='a' && c2=='a') { ++homrcount;  }
else                         { ++hetrocount; }