Cannabis seeds, like a lot of other seeds and human embroyos, have all the "basic plumbing" built in. That is to say that they can go either way - boy or girl - once a "sexual choice" has been determined. A good analogy would be a human embroyo (please forgive me if my terminology is a bit faulty, it's been many moons since I was in school!). Anyway, the point is that in the early stages of developement a human embroyo has a simple, basic, asexual organ (structure) called a gonad , or gonads. At some point these gonads reach a "turning point" in their developement - some will turn into testis and be born as boys, while some will turn into overies and be born as girls. Cannabis works the same way!
Pot seeds are "Pre-disposed" to be either male or female at about 50% vs. 50% - what this means is that about half will "probably" be male, while the other half will "probably" be female (that is, under "normal" growing conditions!).
DON'T GET PRE-DISPOSED CONFUSED WITH "CHISLED IN STONE".
A "Pre-disposed" male seed will "probably" turn into a male plant, under normal conditions - the operative words here are: probably and normal conditions. To make a long story short - all you have to do is manipulate the growing conditions (a little bit) and you can effect the theorhetical 50-50 ratio of males to females.
The following is something I Cut & Pasted from Meangreen @ hipforums (my old stomping grounds).
INCREASING YOUR CHANCES OF FEMALES
A well maintained grow room with plants growing in optimal conditions will naturally produce more females than males. There are some other things you can do to increase your odds of improving your male to female ratio. These are as follows:
High nitrogen levels in vegetative growth have shown to produce more females than males. This might be a good to reason to use feeds with a higher l\l to P and K ratio rather than foods with an equal N to P and K ratio. Remember though that plant burn will only lead to stress and this will produce more males and hermaphrodites than females.
[FONT=Times New Roman, Times, serif] High potassium levels in vegetative growth and flowering tends to produce more males than females. Keeping your potassium levels down is another good reason to choose a food with a higher N than P and K ratio where the P and K ratios are even, or where the K ratio is kept lower than P. Remember though that lack of K can cause plant stress due to this nutrient deficiency and this can cause more males and hermaphrodites to appear than females.[/FONT]
[FONT=Times New Roman, Times, serif] Cannabis plants grow best under conditions of between 40 and 80 percent relative humidity (rH). In the higher rH range of between 70 and 80 your female to male ratio may increase.[/FONT]
[FONT=Times New Roman, Times, serif] Cannabis plants grow best at 75 degrees Fahrenheit. If you do not allow the temperatures to increase beyond this you will improve your chances at getting more females than males. If the environment is supplemented with C02 the temperature may be allowed to increase as far as 95 degrees Fahrenheit.[/FONT]
[FONT=Times New Roman, Times, serif] Using MH lights in vegetative growth will improve your female to male ratio[/FONT]
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Some of this stuff may seem a little out of date, but I standby the notion that environmental factors can, and do, influence the ultimate sexual expression of a seed - at least as much as "Pre-disposition"!
Some of you may have other opinions (and you are welcome to them). But opinions without proper research is just a "popularity contest". I don't know about you, but I've known some really popular people that were "dumb as a bag of hammers"!