From Mark Peplow, Science Media Centre, The Royal Institution

Your report about genetic differences between people in different parts of the world was interesting (10 August, p 17). Apparently “modern inhabitants of Paris… share an average of 50 per cent of their genes with people from Baghdad”.

Compare this with the frequently cited statistic: “Humans share 50 per cent of their genes with a banana.” Does this mean that the residents of Baghdad are bananas?

This is a common source of confusion, and no wonder. There are many different ways of comparing genetic similarity. The often quoted fact that people are genetically nearly identical to chimps, for example, comes from the analysis of amino acids coded for by just a couple of genes that we share with chimps. 98 per cent of those amino acids are the same.

The 50 per cent figure for people and bananas roughly means that half of our genes have counterparts in bananas. For example, both of us have some kind of gene that codes for cell growth, though these aren’t necessarily made up of the same DNA sequences.

The figures quoted for human populations are much more specific and are about gene variants. In this study, what is meant is that 50 per cent of the genes of all the people in Europe, on average, are specific types that have come directly from Middle Eastern populations. So no, the people of Baghdad are not bananas, and neither are we – Ed.

London