Abstract
We test the hypothesis that polymorphisms of the brain regulator genes MCPH1 and ASPM contribute to variations in human brain size and its correlates. We measured general mental ability, head circumference and social intelligence in 644 Canadian adults (496 Caucasians, 36 Orientals, 84 Mixed Race/Other and 28 Blacks; 257 men and 387 women). The gene polymorphisms were assessed from buccal DNA; mental ability by Wonderlic Personnel Test and Multidimensional Aptitude Battery; head circumference by stretchless tape; and social intelligence by prosocial attitude questionnaires. Although all measures were construct valid and the allele frequencies showed expected population differences, no relationship was found between the genes and any of the criteria. Among Caucasian 18–25 year olds, for example, the two mental ability tests correlated with each other (r=0.78, N=476, p<0.001), with head circumference (r=0.17, N=182, p<0.05) and with prosocial attitudes (r=0.23, N=182, p<0.001).
Highlights
Two newly discovered genes, Microcephalin (MCPH1) on chromosome 8p23 and abnormal spindle-like microcephaly associated (ASPM) on chromosome 1q31, attracted much attention when reported to be (i) associated with autosomal recessive primary microcephaly, (ii) positively accelerated in molecular evolutionary rate through the simian line leading to Homo sapiens, and (iii) under recent positive selection in modern humans (Evans et al 2005; Mekel-Bobrov et al 2005)
The MCPH1 allele favoured by selection in modern humans, known as the D allele, was estimated to have arisen approximately 37 000 years ago, about the time symbolic behaviour became widespread in Europe, while the favoured D allele for ASPM was estimated to have arisen approximately 5800 years ago, about the time cities developed in the Near East
The D allele of MCPH1 had an overall frequency of 84% and was in Hardy–Weinberg equilibrium among all population groups and among Caucasians was more often in homozygote than heterozygote form
Summary
Microcephalin (MCPH1) on chromosome 8p23 and abnormal spindle-like microcephaly associated (ASPM) on chromosome 1q31, attracted much attention when reported to be (i) associated with autosomal recessive primary microcephaly, (ii) positively accelerated in molecular evolutionary rate through the simian line leading to Homo sapiens, and (iii) under recent positive selection in modern humans (Evans et al 2005; Mekel-Bobrov et al 2005). The D alleles for both MCPH1 and ASPM exist in high frequency Both genes were hypothesized to confer a selective advantage such as increased brain size and general mental ability (GMA). One MRI study of 112 extended twin families found heritabilities of 82% for whole-brain grey matter volume, 87% for whole-brain white matter volume and 86% for general intelligence (Posthuma et al 2002). It reported that the correlation between grey-matter volume and GMA, as with white-matter volume and GMA, was completely due to genetic factors and not due to environmental factors. The purpose of the present study was to investigate the hypothesis of an association between the two microcephaly alleles and GMA, head size and social intelligence
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