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It is thought that a combination of several genetic predisposition factors interact with environmental factors to trigger complex diseases, and that it is not a single gene but multiple genes that contribute to such traits, and the identification of each of these genes is correspondingly more difficult.

Complex traits can constitute various other challenges for researchers. Genetic heterogeneity is where alleles at more than one locus trigger the same phenotype, or mutations in the same gene cause different phenotypes. Reduced penetrance is where a predisposing genotype does not necessarily cause the phenotype to manifest itself. Phenocopy is where a trait looks identical but has a different cause than the one being studied.

To address these challenges, scientists use association studies, which are based on the principle that if a particular allele and trait occur simultaneously at a statistically significant frequency, the allele is likely to be involved in the development of the trait. (Linkage studies, by contrast, are based on finding DNA markers and traits that are linked within families.)

Alzheimer's disease is one example of a complex trait. Three genes have been found to contribute to the rare, early forms of the disease. Genetic screens have found that a fourth locus is linked to the common, late-onset form of the disease. Association studies have revealed that one allele of this fourth locus increases a patient's risk of developing Alzheimer's in a dose-dependent fashion, where the risk posed by having two alleles is greater than the risk posed by having just one. see also Alzheimer's Disease; Bioin-formatics; Cloning Genes; Complex Traits; Cystic Fibrosis; DNA Libraries; Human Genome Project; Internet; Linkage and Recombination; Mendelian Genetics; Twins.

Sofia A. Oliveria and Jeffery M. Vance

Bibliography

Lewis, Ricki. Human Genetics: Concepts and Applications, 5th ed. Boston: McGraw-Hill, 2002.

Peltonen, Leena, and Victor A. McKusick. "Dissecting Human Disease in the Postge-nomic Era." Science 291 (2001): 1224-1229.

alleles particular forms of genes

Gene Expression: Overview of Control

The chromosomes of an organism contain genes that encode all of the RNA and protein molecules required to construct that organism. Gene expression is the process through which information in a gene is used to produce the final gene product: an RNA molecule or a protein.

Each cell in a multicellular organism such as a human contains the same genes as every other cell. Nonetheless, there are hundreds of distinct types of cells in the human body, each expressing a unique set of genes. Indeed, it is this unique constellation of expressed genes that makes each cell type distinct.

Cells may also change the genes they express over time, and they are constantly adjusting the amount of protein made in response to changing enome the total conditions. How does a cell express some, but not all, of the genes in its genetic material in a genome? How does it react to environmental changes to adjust the level of cell or organism

Transcription cytoplasm nucleus

Transcription pre-mRNA

cytoplasm pre-mRNA

RNA Transport inactive mRNA

RNA Transport

RNA degradation 5

Translation protein

Protein regulation and degradation inactive protein

Figure 1. The flow of genetic information from DNA to proteins. Control can be exerted at each numbered step. Most control occurs at step one, transcription.

transcription messenger RNA formation from a DNA sequence cytoplasm the material in a cell, excluding the nucleus phosphorylation addition of the phosphate group PO43-

gene expression? These are the problems of control of gene expression. While the genes whose final products are RNA molecules are also regulated, this entry will focus on genes that encode proteins.

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