If a taxon is monotypic (having only one member, such as a genus that has only 1 species), there is no Freebase topic for the monotypic taxon. Instead, the monotypic taxon is listed as an alias of the next lower taxon that has >1 member, or if there is only one member of the group, that member.
For example, the genus Gavia (Loons) is the only member of the family Gaviidae, which is the only family in the order Gaviformes. There is only topic for Gavia, and Gaviidae and Gaviformes are listed as aliases.
My instinct was to add topics for the family and order, so that we could programatically answer questions like "what order is the Red-throated Loon in?" -- it seems wrong to have structured data in the alias. But it looks like this was a deliberate decision that was made when the topics were loaded, so I thought I'd better throw it out there.
(I have a list of birds from the americas that I wanted to add data for -- there were a couple of hundred genera for which there is no Freebase topic, and a few random searches revealed that many of these genera had only 1 species.)
