Reading a genome sequence is like consulting an oracle in many ways—the messages must be interpreted carefully and often in the absence of a clear, or even complete, message. What actions should we take based on the word of the genomic oracle? How sure can we be? How much information can we currently glean? In the short term: perhaps not too much.
Some genes speak loud and clear, most don’t—certainly not yet given our limited knowledge of the language of DNA. The classic textbook example of a very clear message is encoded in the gene that causes the disease sickle cell anaemia. Sickle cell anaemia is caused by a single amino acid mutation in the haemoglobin gene found on chromosome 1. This causes a normally hydrophilic amino acid, glutamic acid, to be replaced with the hydrophobic amino acid, valine. The result is a sickle-shaped cell with a rigid membrane. The sickle cells are not elastic enough to allow the oxygen carrying red blood cells to flow easily through the smallest blood vessels, and this causes blockages.
Some genes only suggest the probability of certain outcomes. Take, for example, the ApoE gene, the strongest genetic determinant of Alzheimer’s disease and the gene that James Watson and Steven Pinker hid from themselves. Most genetic messages are strongly context-dependent. Predictions must be viewed through a maze of complex interactions of genes, environmental factors, random events, and historical contingencies. Many genes interact so heavily with external factors we can’t yet understand the consequences of possessing them.
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