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phenylalanine and so on. This is the genetic code which is used by all genes in the cell nuclei of all species of plants and animals.
    The cell makes a temporary copy of this code, as if it were photocopying a few pages of a book, then dispatches it to the protein-making machinery in another part of the cell. When it arrives here, the production plant swings into action. It reads the first triplet and decodes it as meaning the amino-acid methionine. It takes a molecule of methionine off the shelf. It reads the second triplet for the amino-acid threonine, takes a molecule of threonine down and joins it to the methionine. The third triplet means serine, so a molecule of serine gets tacked on to the threonine. The fourth triplet is for phenylalanine, so one of these is joined to the serine. Now we have the four amino-acids specified by the DNA sequence of the keratin gene assembled in the correct order: methionine–threonine–serine–phenylalanine. The next triplet is read, and the fifth amino-acid is added, and so on. This process of reading, decoding and adding amino-acids in the right order continues until the whole instructions have been read through to the end. The new keratin molecule is now complete. It is cut loose and goes to join hundreds of millions of others to form part of one of the hairs that are growing out of your scalp. Well, it would if you had not pulled it out.

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FROM BLOOD GROUPS TO GENES
    There are few things more distinctive about a person than their hair. It is one of the very first features we ask for in any description of a new baby, a stranger or a wanted criminal. Dark or blonde, wavy or straight, thick or balding: all these different possibilities add immediately to the picture we build up in our minds of someone we have never met. We certainly know how to manipulate the way our own hair appears. Salons are full as we pay to have our hair cut and shaped. Pharmacy shelves are lined with products to lighten, darken, straighten and curl. We are all working to make the best of the hair we were born with; but it is our genes which deal out the basic raw material. The difference between a natural redhead and a blonde lies in a difference in their DNA. Within the genes for keratin and the many others involved in the process of growing hair are small differences in the DNA sequence. These are responsible for giving the hair different characteristics of colour and texture. Most of these genes have yet to be identified, but they are certain to be inherited from both parents, although not necessarily in a straightforward way – which is why it is a fairly frequent occurrence that a new baby does not have the hair colour of either of its parents.
    Hair type is a highly visible distinguishing feature by which we tell individuals apart, but by far the greatest inherited differences between us are invisible and remain hidden unless something brings them to our attention. The first of these inherited differences to be revealed were the blood groups. You cannot tell just by looking at someone which blood group he or she belongs to. You can’t even tell by simply looking at a drop of their blood. All blood looks pretty much the same. It is only when you begin to mix blood from two people that the differences begin to make themselves apparent; and, since no-one had any reason to mix one person’s blood with another until blood transfusions were invented, our blood groups stayed hidden.
    The first blood transfusions were recorded in Italy in 1628, but so many people died from the severe reactions that the practice was banned there, as well as in France and England. Though there were some experimental transfusions using lamb’s blood, notably by the English physician Richard Lower in the 1660s, the results were no better and the idea was given up for a couple of centuries. Transfusions with human blood started up again in the middle of the nineteenth century, to combat the frequently fatal