Sex of the plant is determined at the RNA level of the plant not the DNA.

Image reaper is correct, the environment is key to growing female plants over male plants. But you can tilt the chance if your favor with proper control and care.

Somewhere between the 3rd and 4th week of vegetation is when the plant determines it's sex if you are interested.

I don't mean to be cheesy but in my show week #6 I cover this subject in detail if you are interested DL the mp3 and listen. Follow the link in my sig line.