semantics in schema (xsd)

Greetings,

I'm creating a schema file but by several times I feel the needed of
something to validate my sub-elements in tree (children elements)
subordinated to an attribute value on the parent element.

Is it possible to validate this with the schema semantics?

For instance, a schema may define an element called "human" that would
allow to have a <wife> or <husband> element below it. However, I would
like to apply an extra semantic rule that would only allow the <wife>
element be inside a human of gender type "female", and the same thing
for <husband> with "male". Is it possible?  

<human gender="female">
   <wife />
</human>

<human gender="male">
   <husband />
</human>

thanks,
Pedro Salazar.
-- 
PS
pedro-b-salazar_at_ptinovacao_dot_pt
PGP:0E129E31D803BC61