And this informality is more than skin deep; it extends to the way the book is conceived. Its basic aim is to educate the reader. I don't mean that the tone is patronising, but Rutherford wants to convey certain messages and to counteract some erroneous ideas about genetics that are widespread today.
One of his targets is genetic determinism: the idea that if you have the gene 'for' a particular disease you are fated to suffer from it. In fact, as he explains, this is seldom the case; usually environmental factors are needed for the disease to appear in an individual who carries the gene concerned. In any case, it is now apparent that most diseases, and most traits in general, such as intelligence, height, and eye colour, are influenced by dozens or even hundreds of genes, and we simply don't know how this works. In a nice phrase Rutherford compares genetic determinism to astrological determinism.
Another concept that Rutherford is keen to attack is race. He insists that this has no scientific basis. When humans left Africa they were almost certainly black and the development of pale skin and blond hair in humans living in northern latitudes is quite recent in evolutionary terms. (The Neanderthals may also have been pale-skinned but via a different genetic mechanism.) And, on average, two 'black' people will probably differ from each other genetically more than either will differ from a 'white' person. Criticism of racism is a recurrent theme throughout the book and its lack of scientific credibility is convincingly demonstrated.
Many people these days pay to have their DNA analysed in order to discover where their ancestors came from. Rutherford has done this himself, twice, and concludes that the results are either meaningless or trivial. It is obvious on arithmetical grounds that you are related to everyone who was alive a few thousand years ago, so the fact that your DNA shows a connection with, say, the Middle East if you are a European is not surprising. In any case, you may well have no genetic similarity to an ancestor of yours from only a few generations back, so to pride yourself on your family connections is pretty absurd.
Rutherford is adept at explaining difficult scientific concepts. He has an excellent analogy to illustrate how the genetic code works at the molecular level: he takes a sentence, 'Imagine, if you will, that this very sentence is a gene', and subjects it to interpolation of chains of meaningless letters and other transformations to illustrate what happens to genes in our DNA. He also has a useful explanation of what is meant by epigenetics, which is sometimes wrongly represented as conflicting with natural selection.
I wish there were a lot more of this kind of thing here. But in fact the structure of the book is itself reminiscent of the transformations that it describes for genes, with a tendency to include not very relevant material along with what really matters. There are quite lengthy historical digressions, including a potted history of the Spanish Hapsburgs, for example, and an explanation of why the New Testament statement of Mary's virginity is based on a mistranslation. While all this may be interesting, it expands the book a good deal without adding anything useful. Perhaps it is meant to make the science more approachable for readers for whom most of the ideas are unfamiliar, but others may simply find it distracting.
What this amounts to is that if you like to have your science mixed in with a lot of chat and jokes and treated as informally as possible, this is the book for you. If not, you can still find much to interest you (the book is remarkably up to date in the research it cites) but you will probably find yourself doing a fair amount of skipping.