How to setup a form to allow input of multi-level values?

My form has a radio box group "genus" (a lookup from the genus table/form) but also needs another - "species" - that shows only the species values that are in the selected genus - i.e. that have a matching genus field in my species table/form.

I guess I need an Update action script for this, using input.genus, but I cannot find out how to

a) lookup the all matching species in the special table/form
b) put these into a radio box group.

Any help greatly appreciated. Thanks.