Genes for Intelligence

The search for specific genes associated with IQ is proceeding at a rapid pace with the completion of the Human Genome Project. While defects in single genes, such as the fragile X gene, can cause mental retardation, the heritability of general cognitive ability is most likely due to multiple genes of small effect (called quantitative trait loci, or QTLs) rather than a single gene of large effect. QTLs contribute additively and interchangeably to intelligence.

Genetic studies have identified QTLs associated with "g" on chromosomes 4 and 6. These studies involved both children with high "g" and children with average "g." QTLs on chromosome 6 have been identified and shown to be active in the regions of the brain involved in learning and memory. The gene identified is for insulin-like growth factor 2 receptor, or IGF2R, the exact function of which is still unknown. One allele (alternative form) of IGR2R was found to be present 30 percent of the time in two groups of children with high "g." This was twice the frequency of its occurrence in two groups of children with average "g," and these findings have been successfully replicated in other studies. QTLs associated with "g" have also been identified on chromosome 4. Future identification of QTLs will allow geneticists to begin to answer questions about IQ and development and gene-environment interaction directly, rather than relying on less specific family, adoption, and twin studies.

In summary, intelligence measurements ranging from specific cognitive abilities to "g" have a complex relationship. Genetic contributions are large, and heritability increases with age. Heritability remains high for verbal abilities during adulthood. Finally, the identification of QTLs associated with "g" and with specific cognitive abilities is just beginning. see also Behavior; Complex Traits; Eugenics; Fragile X syndrome; Genetic Discrimination; Quantitative Traits; Twins.

Harry Wright and Ruth Abramson


Casse, D. "IQ since 'The Bell Curve.'" Commentary Magazine 106, no. 2 (1998): 33-41.

Chiacchia, K. B. "Race and Intelligence." In Encyclopedia of Psychology, 2nd ed., Bonnie Strickland, ed. Farmington Hills, MI: Gale Group, 2001.

Deary, I. J. "Differences in Mental Ability." British Medical Journal 317 (1998): 1701-1703.

Fuller, J. L., and W. R. Thompson. "Cognitive and Intellectual Abilities." In Foundations of Behavior Genetics. St. Louis, MO: C.V. Mosby Co., 1978.

Plomin, R. "Genetics of Childhood Disorders, III: Genetics and Intelligence." Journal of the American Academy of Childhood and Adolescent Psychiatry 38 (1999): 786-788.

Sternberg, R. J., and J. C. Kaufman. "Human Abilities." Annual Review Psychology 49: 479-502.

Sternberg, R. J., and E. L. Grigorenko. "Genetics of Childhood Disorders, I: Genetics and Intelligence." Journal of the American Academy of Childhood and Adolescent Psychiatry 38 (1999): 486-488.


Biologists often use two terms to describe alternative approaches for conducting experiments. "In vitro" (Latin for "in glass") refers to experiments or organisms typically carried out in test tubes with purified biochemicals. "In vivo" ("in life") experiments are performed directly on living organisms. In recent years, the indispensable use of computers and the Internet for genetic and molecular biology research has introduced a new term into the language: "in silico" ("in silicon"), referring to the silicon used to manufacture computer chips. In silico genetics experiments are those that are performed with a computer, often involving analysis of DNA or protein sequences over the Internet.

Geneticists and molecular biologists use the Internet much the same way most people do, communicating data and results through e-mail and discussion groups and sharing information on Web sites, for instance. They also make wide use of powerful Internet-based databases and analytical tools. genomes the total Researchers are determining the DNA sequences of entire genomes at an genetic materia| in ce||s ever accelerating pace, and are devising methods for cataloging entire sets of proteins (termed "proteomes") expressed in organisms. The databases to store all this information are growing at an equal pace, and the computer tools to sort through all the data are becoming increasingly sophisticated.

One of the most important Web sites for biological computer analysis bioinformatics use of (sometimes called bioinformatics) is that of the National Center for inf°rmation techno|ogy Biotechnology Information (NCBI), a part of the National Library of Med-datan^^ biological icine, which, in turn, is part of the National Institutes of Health. The NCBI

?Web site hosts DNA and protein sequence databases, protein three-dimensional structure databases, scientific literature databases, and search engines for retrieving files of interest. All of these resources are freely accessible to anyone on the Internet.

Of all the powerful analytical tools available at NCBI, probably the most important and heavily used is a set of computer programs called BLAST, for Basic Local Alignment Search Tool. BLAST can rapidly search many sequence databases to see whether any DNA or protein sequence (a "query sequence," supplied by the user) is similar to other sequences. Since sequence similarity usually suggests that two proteins or DNA molecules are homologous (i.e., that they are evolutionarily related and therefore may have—or encode proteins—with similar functions), discovering a blast match between an unknown protein or nucleic acid sequence and a well-characterized sequence provides an immediate clue about the function of the unknown sequence. An important scientific discovery that, in the past, may have taken many years of in vitro and in vivo analysis to arrive at is now made in a few seconds, with this simple in silico experiment. see also Bioinformatics; Genome; Genomics; Homology; Proteomics; Sequencing DNA.

Paul J. Muhlrad

Bibliography internet Resources

Basic Local Alignment Search Tool. National Center for Biotechnology Information. <>.

Baxevanis, Andreas D. "The Molecular Biology Database Collection: 2002 Update." Nucleic Acids Research. Oxford University Press. < database/>.

ExPASy Molecular Biology Server. Swiss Institute of Bioinformatics. <http://ca.expasy .org/>.


Virtual Library of Genetics. U.S. Department of Energy. < TechResources/Human_Genome/genetics.html>.

Wellcome Trust Sanger Institute. <>.

WWW Virtual Library: Model Organisms. George Manning. < VL/mo/>.

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