This review is a little out-of-date due to the rapid pace of research. But it seems well worth the read. See for example Fig. 4, which summarizes the underlying biology, cell types, etc., and Table 3 and the Appendix tables, which summarize what was known then about a long list of putative fly orthologs of human genes linked to mental retardation.
Inlow JK, Restifo LL. Molecular and comparative genetics of mental retardation. Genetics. 2004 Feb;166(2):835-81. PubMed PMID: 15020472; PubMed Central PMCID: PMC1470723.
An advantage of a paper with a little age on it--free access through PubMed Central!