Unlike most plants, the ginkgo trees have separate male and female trees. The fruit of the female ginkgo tree smells horrible, something like a mix of vomit and fecal matter. (the nut inside the fruit is edible). For this reason, grafted clones of male ginkgo trees are almost always planted. However, because UC Davis is, well, UC Davis, they thought it would be a great idea to plant male and female ginkgo trees on campus. You can find a pantsload more information on other species on the town flora page.
We had female ginkgos on the street I grew up on. According to my father (a gardener, so let's trust him) you can't always properly sex a ginkgo tree at the age they're planted, so the unwanted females tend to end up planted by accident. -ImR