There is no simple one-to-one mapping, then, between genes and bits of body, any more than there is mapping between words of recipe and crumbs of cake. The genes, taken together, can be seen as a set of instructions for carrying out a process, just as the words of a recipe, taken together, are a set of instructions for carrying out a process. The reader may be left asking how, in that case, it is possible for geneticists to make a living. How is it possible ever to speak of, let alone do research on, a gene 'for' blue eyes, or a gene 'for' colour blindness? Doesn't the very fact that geneticists can study such single-gene effects suggest that there really is some sort of one-gene/one-bit-of-body mapping? Doesn't it disprove everything I have been saying about the set of genes being a recipe for developing a body? Well no, it certainly doesn't, and it is important to understand why.
Perhaps the best way to see this is to go back to the recipe analogy. It will be agreed that you can't divide a cake up into its component crumbs and say 'This crumb corresponds to the first word in the recipe, this crumb corresponds to the second word in the recipe', etc. In this sense it will be agreed that the whole recipe maps onto the whole cake. But now suppose we change one word in the recipe; for instance, suppose 'baking-powder' is deleted or is changed to 'yeast'. We bake 100 cakes according to the new version of the recipe, and 100 cakes according to the old version of the recipe. There is a key difference between the two sets of 100 cakes, and this difference is due to a one-word difference in the recipes. Although there is no one-to-one mapping from word to crumb of cake, there is one-to-one mapping from word difference to whole-cake difference. 'Baking-powder' does not correspond to any particular part of the cake: its influence affects the rising, and hence the final shape, of the whole cake. If 'baking-powder' is deleted, or replaced by 'flour', the cake will not rise. If it is replaced by 'yeast', the cake will rise but it will taste more like bread. There will be a reliable, identifiable difference between cakes baked according to the original version and the 'mutated' versions of the recipe, even though there is no particular 'bit' of any cake that corresponds to the words in question. This is a good analogy for what happens when a gene mutates.
An even better analogy, because genes exert quantitative effects and mutations change the quantitative magnitude of those effects, would be a change from '350 degrees' to '450 degrees'. Cakes baked according to the 'mutated', higher-temperature version of the recipe will come out different, not just in one part but throughout their substance, from cakes baked according to the original lower-temperature version. But the analogy is still too simple. To simulate the 'baking' of a baby, we should imagine not a single process in a single oven, but a tangle of conveyor belts, passing different parts of the dish through 10 million different miniaturized ovens, in series and in parallel, each oven bringing out a different combination of flavours from 10,000 basic ingredients. The point of the cooking analogy, that the genes are not a blueprint but a recipe for a process, comes over from the complex version of the analogy even more strongly than from the simple one.
