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The genetics of G in human and mouse

Abstract

The g factor refers to the substantial overlap that exists between individual differences in diverse cognitive processes in humans. In this article, I argue that a mouse model of g could provide a powerful analytic tool for exploring cognitive processes that are linked functionally by genes.

Key Points

Summary

  • Cognitive neuroscience focuses on species-universal processes rather than on differences between individuals. Heredity is based on the naturally occurring genetic variation that underlies individual differences. For complex traits, many genes contribute to heritable variation. Common disorders may be the quantitative extreme of the same genetic factors that contribute to variability throughout the distribution of individual differences.

  • One of the most consistent findings from individual-variability research on human cognitive abilities and disabilities is that very diverse processes such as general reasoning, spatial ability and vocabulary highly interrelate. A technique called factor analysis best captures this overlap and yields a g factor that accounts for about 40% of the variance of diverse cognitive tests. The existence of g does not imply that a single physical, physiological or psychological process is responsible for g.

  • There are more studies addressing the genetics of human g than any other trait. These studies consistently converge on the conclusion that genetic factors contribute substantially to g. Multivariate genetic analysis indicates that what is common between cognitive abilities is genetic in origin whereas what is specific to each cognitive ability is largely environmental.

  • An animal model of g is needed to apply sophisticated neurobiological techniques to understand the brain mechanisms that mediate genetic influences on g. Most of the research on individual differences and genetics in learning and memory has used mice than all other non-human species combined, and the general acceptance of g in man rekindled interest in g in rodents. Evidence for g has emerged from several studies of diverse cognitive tasks in mice.

  • More research is needed to prove that g in mice is the same as g in man. Investigating g in mice requires measures that are reliable at the level of the individual mouse, large samples, and a battery of diverse cognitive measures. The strongest genetic evidence for congruence of g in mouse and man will come from identifying genes that are associated with g in both mouse and man. The strongest neurobiological evidence for congruence will come from showing congruence of brain function across species.

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Acknowledgements

M. Galsworthy, J. L. Paya-Cano and S. Monleon contributed to this essay and are collaborators in our ongoing research on g in HS mice funded in part by a grant from the US National Institute of Child Health and Human Development.

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FURTHER INFORMATION

Robert Plomin's lab

Glossary

QUANTITATIVE TRAIT LOCI

(QTLs) Genes (loci) in multiple-gene systems in which each QTL contributes quantitatively to a continuous distribution, thus creating a quantitative trait.

PRINCIPAL COMPONENTS ANALYSIS

A statistical technique that weights tests to create a dimension that best represents the intercorrelations of the items.

MULTIVARIATE GENETIC ANALYSIS

A quantitative genetic technique that analyses the covariance between traits rather than the variance of each trait considered on its own.

GENETIC CORRELATION

A statistic from multivariate genetic analysis that indicates the extent to which genetic effects on one trait are the same as genetic effects on another trait.

HERITABILITY

A statistic that describes the extent to which individual differences in a population can be ascribed to genetic differences between individuals.

INBRED STRAINS

Animals produced by mating brother to sister for at least 20 generations, which results in homozygous animals with two copies of the same allele (form of gene) at all loci.

HEBB-WILLIAMS MAZE

Rectangular field with a start box a a goal box at opposite ends of the apparatus. Different configurations are obtained by placing barriers at different points of the field.

OUTBRED LINES

Animals produced by crossing inbred strains, which increases heterozygous animals with different alleles at all loci.

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Further reading

Figure 1: Strain differences for performance on the Morris water maze.
Figure 2: Successful selective rat breeding on the basis of maze-learning performance.