Bluefunk, here is your answer:
Plants do not work the same way mammals do. Once a female realizes that it has no chance in being pollinated, it can, in certain situations (i.e. careful stress), produce seeds that are basically clones. These women are called XX females. Every seed will be a female. This is a very rare thing, but can be done. I'm not sure if there is a post about how to make XX females, but I've read a pretty good amount of material on the subject, and it is, as the hairry gorilla said, caused by stress. There's a largely more detailed way to create them, but, seeing as how that was not your question, I will not waste anyone's time.
Here is the scientific theory behind XX females:
When stress is administered to the marijuana plant, it will, in an inherent attempt to keep the species alive, become a hermaphrodite. Hermaphrodites are self-pollinating plants, meaning that there is no other genetic code besides that found in the hermaphrodie that leads to its growth. However, a very small number of plants are not able to become hermaphrodites. As much as they try, they cannot expel pollen, and are only capable of producing seeds. Those that grow these seeds say that the two plants are complete sisters, cloned from seed rather than stem. It is much easier to produce these special females with indicas, though it can be done with sativas.
I hope this information can be helpful to you, bluefunk. If there is anything else you need, don't even hesitate to ask.
If any of the information herein is wrong, and anyone is able to prove thusly, please post the correct information, as we all must learn.