Objective: About 50% of severe to profound intellectual disabilities (ID) are caused by genetic factors. In this study we decided to investigate the genetic causes of ID in 69 Bushehrian families to provide information for genetic counseling, carrier detection, and prenatal diagnosis.

Materials & Methods: In this study we excluded known chromosomal abnormalities. The majority of families had more than two affected individuals. Karyotyping for each proband with physical malformations was performed. One affected member from each family was tested for FMR1 mutation and metabolic screening. Families with ID and primary microcephaly were checked for 7 known MCPH genes by linkage analysis.

Results: Chromosomal abnormality was not found in any of the families. One family had full mutation of CGG repeat of Fragile-X syndrome. Six out of 18 families with MCPH showed linkage to one of the MCPH loci. One family had a syndrome associated with microcephaly. Two families with microcephaly and one family with a non-syndromic form of mental retardation without microcephaly showed an autosomal dominant mode of inheritance.

Conclusion: According to our results genetic causes of ID are very heterogeneous and autosomal recessive primary microcephaly has an extremely high prevalence (26.09%) in Bushehr province of Iran.