Objective: About 70 percent of mental retardation can be attributed to genetic causes. Whereas the recognization of genetic causes of mental retardation causes better programming and more precision council for prevention and control of mental retardation, so the aim of this study is to investigate the genetic causes of mental retardation in Golestan province.

Materials & Methods: In this descriptive research which is a cross-sectional and applied study, fifty families with two or more affected children from different parts of Golestan province were collected with cooperation of Golestan state welfare organization and by simple sampling method. Mental retardation of affected people had been previously proved by physicians. Blood samples of patients and controls in the family were prepared. Patients were investigated for dysmorphism and microcephaly. Cytogenetic study, metabolic test, fragile X test, and linkage analysis for seven gene loci known for autosomal recessive primary microcephaly (MCPH) was carried out.

Results: Chromosomal abnormalities were not observed in any family. One out of fifty families revealed fragile X syndrome and ten were affected to MCPH. Five out of ten microcephaly families were linked to MCPH (autosomal recessive primary microcephaly) loci.

Conclusion: Autosomal recessive primary microcephaly (MCPH)make up about 20% 0f all mental retardation in Golestan province. This prevalence is very remarkable.