Thanks!

In general, crosses between genera fail unless you use genetic engineering techniques. Inter-generic hybrids are very ambitious. Right offhand, the only successful instances I can think of are with orchids.

In my opinion, you have picked a very difficult goal for a science fair project. My advice would be to scale the goal back to interspecific and intraspecific hybrids. Zinnias might be a suitable plant for that, because they germinate rapidly and develop to a blooming stage in about six weeks.

The Sakata Seed Company has been successful with commercial interspecific zinnia hybrids, and intraspecific hybrids within Zinnia violacea (formerly Zinnia elegans) offer a wide variety of zinnia strains to cross.

It would be dramatic to cross one of the 3-foot tall zinnia varieties with a very short variety like Thumbelina, Zinnita, or Short Stuff. Some of the short varieties can start to bloom when they are only 3 inches tall.

Perhaps a botanist can suggest something even more appropriate for your science fair project. If you could contact someone at the Sakata Seed Company, they might be able to help with your project.

MM

One thing to remember is that systematists in the past almost never considered "cross-ability" in classifying plants. Some are still oblivious.
Successful hybrids of Sinningia and Reichsteineria resulted in the latter genera being abolished.

On the other hand, when Burbank crossed among three species to produce his 'Shasta Daisies', all were in the same genera. Since then, one of the parents has been separated as a distinct genera - making Shasta Daisies intergeneric.

Pardancanda Norissii was an intergeneric hybrid of Pardanthopsis dichotoma and Belamcanda chinensis, until both parents were thrown into Iris.

There are numerous intergeneric hybrids among the pomes: malus, pyrus, sorbus, aronia, etc.

Chitalpa taskentensis is an intergeneric hybrid of the Catalpa and Chilopsis linearis (Desert Willow). It is an attractive tree and more useful than either parent.

Berberis can cross with Mahonia. The Osage Orange (Maclura pomifera) was crossed with the Chinese Mulberry (Cudrania tricuspidata).

I could to on.

Distinctions among genera are often technical, and have no bearing on cross-ability.

In addition to obvious hybrids that might be produced, there are some dubious crosses that have resulted from cross-polliantion. I call these "exceptional" crosses, because they usually appear to be entirely maternal... except for an odd trait that might be paternal.
http://www.bulbnrose.org/Heredity/King/ExceptionalCrosses.html

Karl