Our results show that the molecular basis of microcephaly is heterogeneous; thus, the Iranian population may provide a unique source for the identification of further genes underlying this disorder.

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IntroductionMental retardation (MR) has heterogeneous aetiology mostly with genetic causes. Chromosomal aberrations are one of the most common causes of MR. Reports on chromosome abnormality rate among consanguineous families are sparse. In order to identify the chromosome abnormality rate in idiopathic mental retardation from consanguineous marriages, a total of 322 Iranian families with positive family history for MR were investigated in the Genetics Research Center.Material and methodsIn the majority of families (92%) at least two sibs were affected with MR and none had specific chromosomal syndromes such as Down syndrome. Standard cytogenetic techniques using high resolution GTG banding were carried out on all the patients.ResultsThe overall chromosome abnormality rate contributing to mental retardation was 1.24% (4 cases), which comprised 46,XY,der(18)t(4;18)(q31.1;q23)mat; 45,XY,-21,-22,+der(22)t(21;22)(q21.1;q13.33)mat; 46,XY,rec(2)dup(2p)inv(2)(p25.1q37.3)pat, and 46,XY,der(11)t(10;11)(q25.2;q25)pat.ConclusionsAlthough the most likely genetic cause of mental retardation in patients with consanguineous parents is autosomal recessive, the fact that 1.24% of our patients had chromosomal abnormalities emphasizes the importance of cytogenetic investigation as the first laboratory genetic tests for all MR patients. To our knowledge, this is the first report on the rate of chromosome abnormality among patients with idiopathic mental retardation from consanguineous marriages.