Twin studies, heritability, and intelligence

How culture shaped the human genome: bringing genetics and the human sciences together

Researchers from diverse backgrounds are converging on the view that human evolution has been shaped by gene-culture interactions. Theoretical biologists have used population genetic models to demonstrate that cultural processes can have a profound effect on human evolution, and anthropologists are investigating cultural practices that modify current selection. These findings are supported by recent analyses of human genetic variation, which reveal that hundreds of genes have been subject to recent positive selection, often in response to human activities. Here, we collate these data, highlighting the considerable potential for cross-disciplinary exchange to provide novel insights into how culture has shaped the human genome.

Heritability in the genomics era--concepts and misconceptions

Heritability allows a comparison of the relative importance of genes and environment to the variation of traits within and across populations. The concept of heritability and its definition as an estimable, dimensionless population parameter was introduced by Sewall Wright and Ronald Fisher nearly a century ago. Despite continuous misunderstandings and controversies over its use and application, heritability remains key to the response to selection in evolutionary biology and agriculture, and to the prediction of disease risk in medicine. Recent reports of substantial heritability for gene expression and new estimation methods using marker data highlight the relevance of heritability in the genomics era.

Platelet genomics and the risk of atherothrombosis

Platelets play a pivotal role in atherothrombosis after coronary artery plaque rupture. The extent of response of platelets to such an event varies between individuals. This variation is for a large extent genetically controlled. A comprehensive study of sequence variation that modifies the platelet response to agonists is, however, lacking. We set out to discover the regulatory nodes of platelet function by an integrated systems biology approach. The high density genotyping of 110 genes in a cohort of more than 500 individuals, in whom the platelet response to ADP and collagen-related peptide was determined, allowed the robust definition of the first set of regulatory nodes. Microarray and proteomics studies on platelets from individuals with a so-called 'extreme end' response phenotype provided further insight into key regulators of platelet function. In addition, the completion of the HapMap project allows the comprehensive surveying of the genome for sequence variation by the testing of a limited number of single nucleotide polymorphisms (SNPs). With the advent of high density (i.e. 500,000 SNPs) genotyping arrays large number of case and control samples can be tested at an affordable cost. The recently completed Wellcome Trust Case Control Consortium (WTCCC) study has allowed us to address the question of whether common sequence variation confers risk for seven common diseases, one being myocardial infarction. The results of the WTCCC genome-wide association study and issues of case-control study design, particularly the selection of suitable controls, will be reviewed. In conclusion the integration of the results from the platelet systems biology study with those of the WTCCC project enhances our understanding of the mechanisms underlying common conditions such as atherothrombosis and provides pointers to novel cellular mechanisms and pathways.

Genetics of brain structure and intelligence

Genetic influences on brain morphology and IQ are well studied. A variety of sophisticated brain-mapping approaches relating genetic influences on brain structure and intelligence establishes a regional distribution for this relationship that is consistent with behavioral studies. We highlight those studies that illustrate the complex cortical patterns associated with measures of cognitive ability. A measure of cognitive ability, known as g, has been shown highly heritable across many studies. We argue that these genetic links are partly mediated by brain structure that is likewise under strong genetic control. Other factors, such as the environment, obviously play a role, but the predominant determinant appears to be genetic.

Functional neural anatomy of talent

The terms gifted, talented, and intelligent all have meanings that suggest an individual's highly proficient or exceptional performance in one or more specific areas of strength. Other than Spearman's g, which theorizes about a general elevated level of potential or ability, more contemporary theories of intelligence are based on theoretical models that define ability or intelligence according to a priori categories of specific performance. Recent studies in cognitive neuroscience report on the neural basis of g from various perspectives such as the neural speed theory and the efficiency of prefrontal function. Exceptional talent is the result of interactions between goal-directed behavior and nonvolitional perceptual processes in the brain that have yet to be fully characterized and understood by the fields of psychology and cognitive neuroscience. Some developmental studies report differences in region-specific neural activation, recruitment patterns, and reaction times in subjects who are identified with high IQ scores according to traditional scales of assessment such as the WISC-III or Stanford-Binet. Although as cases of savants and prodigies illustrate, talent is not synonymous with high IQ. This review synthesizes information from the fields of psychometrics and gifted education, with findings from the neurosciences on the neural basis of intelligence, creativity, profiles of expert performers, cognitive function, and plasticity to suggest a paradigm for investigating talent as the maximal and productive use of either or both of one's high level of general intelligence or domain-specific ability. Anat Rec (Part B: New Anat) 277B:21-36, 2004.

Mapping genetic influences on human brain structure

Recent advances in brain imaging and genetics have empowered the mapping of genetic and environmental influences on the human brain. These techniques shed light on the 'nature/nurture' debate, revealing how genes determine individual differences in intelligence quotient (IQ) or risk for disease. They visualize which aspects of brain structure and function are heritable, and to what degree, linking these features with behavioral or cognitive traits or disease phenotypes. In genetically transmitted disorders such as schizophrenia, patterns of brain structure can be associated with increased disease liability, and sites can be mapped where non-genetic triggers may initiate disease. We recently developed a large-scale computational brain atlas, including data components from the Finnish Twin registry, to store information on individual variations in brain structure and their heritability. Algorithms from random field theory, anatomical modeling, and population genetics were combined to detect a genetic continuum in which brain structure is heavily genetically determined in some areas but not others. These algorithmic advances motivate studies of disease in which the normative atlas acts as a quantitative reference for the heritability of structural differences and deficits in patient populations. The resulting genetic brain maps isolate biological markers for inherited traits and disease susceptibility, which may serve as targets for genetic linkage and association studies. Computational methods from brain imaging and genetics can be fruitfully merged, to shed light on the inheritance of personality differences and behavioral traits, and the genetic transmission of diseases that affect the human brain.

Genetic influences on brain structure

Here we report on detailed three-dimensional maps revealing how brain structure is influenced by individual genetic differences. A genetic continuum was detected in which brain structure was increasingly similar in subjects with increasing genetic affinity. Genetic factors significantly influenced cortical structure in Broca's and Wernicke's language areas, as well as frontal brain regions (r2(MZ) > 0.8, p < 0.05). Preliminary correlations were performed suggesting that frontal gray matter differences may be linked to Spearman's g, which measures successful test performance across multiple cognitive domains (p < 0.05). These genetic brain maps reveal how genes determine individual differences, and may shed light on the heritability of cognitive and linguistic skills, as well as genetic liability for diseases that affect the human cortex.

Assessing the heritability of attentional networks

BACKGROUND

Current efforts to study the genetics of higher functions have been lacking appropriate phenotypes to describe cognition. One of the problems is that many cognitive concepts for which there is a single word (e.g. attention) have been shown to be related to several anatomical networks. Recently we have developed an Attention Network Test (ANT) that provides a separate measure for each of three anatomically defined attention networks. In this small scale study, we ran 26 pairs of MZ and DZ twins in an effort to determine if any of these networks show sufficient evidence of heritability to warrant further exploration of their genetic basis.

RESULTS

The efficiency of the executive attention network, that mediates stimulus and response conflict, shows sufficient heritability to warrant further study. Alerting and overall reaction time show some evidence for heritability and in our study the orienting network shows no evidence of heritability.

CONCLUSIONS

These results suggest that genetic variation contributes to normal individual differences in higher order executive attention involving dopamine rich frontal areas including the anterior cingulate. At least the executive portion of the ANT may serve as a valid endophenotype for larger twin studies and subsequent molecular genetic analysis in normal subject populations.

The importance of cultural inheritance

Cultural inheritance refers to the storage and transmission of information by communication, imitation, teaching and learning. It is transmitted by the brain rather than by genes. However, it does have a genetic basis, the genes involved determining the structure of the brain. Cultural inheritance is considered to be the latest stage in the evolution of heredity. It is thought to have evolved by epigenetic mechanisms from genetic inheritance. This article proposes that cultural inheritance underlies normal behaviour and mental disorders.

The phenomenon of claimed memories of previous lives: possible interpretations and importance

Several disorders or abnormalities observed in medicine and psychology are not explicable (or not fully explicable) by genetics and environmental influences, either alone or together. These include phobias and philias observed in early infancy, unusual play in childhood, homosexuality, gender identity disorder, a child's idea of having parents other than its own, differences in temperament manifested soon after birth, unusual birthmarks and their correspondence with wounds on a deceased person, unusual birth defects, and differences (physical and behavioral) between monozygotic twins. The hypothesis of previous lives can contribute to the further understanding of these phenomena.