A study of 44,788 pairs of twins in Swedish, Danish and Finnish twin registries (NEJM July 2000) estimated that heritable factors accounted for 42% of risk for prostate cancer, 35% for colorectal cancer and 27% for breast cancer. At present, current genes identified account for much lower proportions of risk in these diseases. So there remain major gaps in knowledge of the genetics of cancer, but still the majority of risk variation is accountable by other, non-heritable factors. Few diseases are caused purely by genes. Genes are only information carriers. External factors can and do disrupt every level from the genes themselves to their final phenotype. At the other end of the spectrum, injuries sustained in a road traffic accident (RTA) have a genetic component, but you wouldn’t want to look for a gene predisposing to RTAs. To do so would be considered silly.

Lifestyle is the term used to describe the range of exposures a person receives over their life. These exposures may be in the form of toxins, they may be in the form of microbiological agents, they may be in the form of a pattern of behaviour, dictated by a culture or family which modulates exposure to other toxins, such as tobacco or hazards such as work. Lifestyle is the interaction between heritable and non-heritable influences on a person. Genes exist because they serve some purpose. Polymorphisms are conserved because they provide some advantages under certain environmental conditions; for example it may be a “tolerable” side effect of some other advantageous feature, such as HLA typology, and important immune feature, or may confer some advantage in times of dietary stress. An example is that thalassemias and sickle-cell disease provide some benefit in areas of high malarial prevalence, though are disadvantageous at other times and places. Type 2 Diabetes “genes” are likely to be advantageous when nourishment is scarce, not when plentiful.