|MadSci Network: Genetics|
You are absolutely right that genes and their protein products interact, both with each other and with the environment, to produce a particular phenotype. Plant breeders used to talk about finding a "happy home" for a particular gene or trait in a plant, meaning that it wasn't enough to breed one specific allele connected to a desirable trait into a commercial variety. You then have to breed for a whole genotype in which that allele can be exploited fully. There are, also, many phenotypic traits that are produced by large numbers of genes working together – human intelligence, however you wish to define it, is undoubtedly one such trait, for instance. I doubt seriously that anyone will ever find a "gene for high IQ" for that reason. That said, one of the surprising findings over the past twenty years or so has been that a number of pretty complex phenotypic traits are profoundly influenced by a single gene. Just this month, for instance, a study was published in the journal Science (Sage et al., 2005) in which researchers were able to induce new inner ear "hair cells" to grow by knocking out expression of a single gene. This restored hearing in mice whose hair cells had been destroyed experimentally, and has some promise for the treatment of hearing loss in humans. Now it's true that the mice could not grow the hair cells without the presence of lots of other genes, but the activity of just one gene made the difference between growing them only once, during embryonic development, and being able to re-grow them as adults. So it looks as if we can't assume that complex traits will always be controlled by complex gene interactions. In some important cases (some more examples: cystic fibrosis, hemophilia, sickle-cell anemia, and the inherited immune deficiency that afflicted the "boy in the bubble" are all single-gene defects), altering a single gene can produce major effects. Actually doing the research is probably the only way we'll find out which conditions fall into this category. Cyrille Sage, Mingqian Huang, Kambiz Karimi, Gabriel Gutierrez, Melissa A. Vollrath, Duan-Sun Zhang, Jaime García-Añoveros, Philip W. Hinds, Jeffrey T. Corwin, David P. Corey, Zheng-Yi Chen. 2005. Proliferation of Functional Hair Cells in Vivo in the Absence of the Retinoblastoma Protein. Science 307: 1114-1118.
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