From The Guardian:
What is a gene? Scientists eager to uncover genes for heart disease, autism, schizophrenia, homosexuality, criminality or even genius are finding that their quarry is far more nebulous than they imagined. Uncovering the true nature of genes has turned biology on its head and is in danger of undermining the whole gene-hunting enterprise.
The first clues turned up in study of the cell’s metabolic pathways. These pathways are like Britain’s road networks that bring in raw materials (food) and transport them to factories (enzymes) where the useful components (molecules) are assembled into shiny new products (more cells). A key concept was the “rate-limiting step”, a metabolic road under strict traffic control that was thought to orchestrate the dynamics of the entire network.
Biotechnologists try to engineer cells to make products but their efforts are often hindered, apparently by the tendency of the key genes controlling the rate-limiting steps to reassert their own agenda. Scientists fought back by genetically engineering these genes to prevent them taking control. When they inserted the engineered genes back into the cells they expected to see an increase in yields of their products. But they were disappointed. The metabolic pathways slipped back into making more cells, rather than more products.
More here.