To take a specific, evolutionary perspective, genes encode information to make proteins. That’s all. Nothing more. Now, a specific protein may provide some advantage to an organism such as a cell, in terms of efficiency of metabolism, or absorption or excretion, or it may not. It might provide a disadvantage to that cell. Whether a protein is advantageous, neutral or disadvantageous is solely dependent on the interaction the cell has with its immediate surroundings, other proteins, and so forth. In environments that confer advantage, the cell is more likely to survive and reproduce, and hence the numbers of cells carrying that particular gene will also increase - the frequency of the gene will increase in the population. If the environmental conditions change, there may be a selective disadvantage against cells carrying that gene, so the gene frequency will decline with each cell that fails to reproduce. Useful genes are usually “conserved”, that is, they may not be expressed, or only be partly expressed under conditions where there is no advantage. Disadvantageous genes under one type of environment may prove to advantageous in a different environment. A gene that alters the structure of the red blood cell membrane so that malarial parasites are less likely to successfully develop will do two things: it makes it more difficult for the cells to function normally, but it doesn’t kill you. Malaria, on the other hand, can kill you, so it is to the organism’s advantage to be a little weaker, but alive to reproduce, than to be strong, perfect and dead before reaching puberty.

So it is the interaction of a specific phenotype with the range of selection pressures presented by environment and lifestyle that leads to the development of specific advantageous and disadvantageous outcomes. The disadvantageous ones we now call disease, but we have simply tended to ignore the advantageous ones, so far.

A key point here to remember is that, from a population perspective, gene frequencies only change over generational time, taking hundreds, if not thousands of years to increase or decline.