Your Body - The Fish That Evolved. Dr. Keith Harrison
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mutants are invariably bad things. In the real world, ‘mutation’ just means ‘change’. Genes are copied during growth and as they pass from generation to generation, and whenever anything is copied accidental changes can creep in. An analogy can be made with an old story about the First World War. In this tall tale, the officers in the front line send a message back to headquarters saying: ‘Send reinforcements, we are going to advance.’ The message is not written down but is passed by word of mouth from one trench to the next until it reaches its destination. Unfortunately, by the time it arrives it has mutated into: ‘Send three and four pence, we are going to a dance.’ What is noticeable about this is that the message did not deteriorate into complete gibberish. It remains a perfectly logical message: ‘Send three shillings and four pence’, but now it has no relevance to the situation at hand and no relation to the original message sent. The problem arose because the message, like genes, was repeatedly copied.
Mutation of a gene may stop it working (in an egg, sperm or embryo, if the gene is important enough, this may kill the embryo) or it may not have a serious effect, or it may even make the gene more effective. As the change is a random accident, the results can be very variable. Like the message in the trenches, the gene may not be destroyed or garbled completely but it may cease to do the original job. Whether it survives in the population in its changed form will depend on whether this new form is harmful to its owner. A mutated gene that kills its owner while he or she is still in the womb will die with them. It will never be inherited by anyone else and will disappear from the world immediately. On the other hand, some mutations may be beneficial and will spread quickly in the population.
Here is an another analogy. Genes are instructions. They are like a cake recipe where the body is the cake. Imagine a rather uninspiring cake recipe that includes the instruction to add 100 grams of coconut. Not everyone likes coconut. Some people make this cake and some eat it but very few ask for a copy of the recipe. One day when someone does ask and is transcribing it into their notebook, they have difficulty reading the original handwriting. Instead of ‘coconut’, they think it says ‘chocolate’. They make the cake and instead of coconut add 100 grams of chocolate. The cake is delicious. Everyone who tastes it asks for the recipe. Suddenly instead of there being five copies of the recipe in the world, there are 500, then 5,000 and soon there are 5 million. The copying error, the mutation, has been very successful for this recipe and the cakes are now everywhere. This can happen to genes too. This can even happen to genes without the change being obviously beneficial. A mutation has occurred in blood which does not appear to have any benefit at all, yet like the cake recipe it has been very successful; for most of us it determines which blood group we are.
Blood groups
On the surface of each red blood cell are molecules called antigens. These are of two types: Type A and Type B. The type we have in our blood is inherited from our parents and this gives us our blood group.
Everyone talks about blood groups using three letters A, B and O. This is the convention, but in reality it is wrong. The ‘O’ here is not the letter O, it is the number zero, used to indicate that the A and B antigens are absent.
In the early history of our species, each blood group gene produced either A or B. If a child inherited an ‘A’ gene from their mother and an ‘A’ gene from their father, they would be blood group A. If they inherited two ‘B’ genes, they would be group B. If they inherited one of each, they would be group AB. However, somewhere in the past, one of these genes mutated in such a way that it failed to produce either antigen. Although this would not appear to have conferred any advantage to its owner, this mutation has now spread throughout the human species until today it is the most common form of the gene. The range of blood groups is now therefore far greater than it used to be.
Today, if we get a gene from each of our parents that creates antigen A, we will still have two A genes (‘AA’) and will be blood group A. On the other hand, if we get a gene from one parent that produces ‘A’ and a gene from the other parent that produces nothing, we will have the genes ‘A0’ (A + zero) but when our blood is tested we will still show a positive test for antigen A and will still be blood group A. There are therefore now two ways of being blood group A (three if you count as different options getting the single A from the mother or getting the single A from the father – ‘AA’, ‘A0’ or ‘0A’). The same applies for blood group B. It is still possible to get an A from one parent and a B from the other, when we will be group AB (now quite rare), but if we get the gene for nothing from one parent and the gene for nothing from the other parent (‘00’) we will produce neither antigen and will be blood group O (which despite its meaning is still pronounced as the letter O, ‘oh’). There is therefore only one way of being group O (by being ‘00’) but this is the most common blood group. This is because most parents who are group A are ‘A0 or 0A’ not ‘AA’ and most who are group B are ‘B0 or 0B’ not ‘BB’, so adding these to the parents who are ‘00’ means genes producing neither antigen are the most common.
The complication caused by this mutated gene is that it is now possible for children to have blood groups quite different from either of their parents. For example, both parents can be group A and the child can be group O,
or one parent can be A and the other B, yet the child can be AB or O.
The child in the last example could also be group A (‘A0’) or group B (‘0B’) but they could never be ‘AA’ or ‘BB’. Which of the possibilities becomes reality is pure chance. It depends which of the mother’s genes occurs in the egg that is fertilised, her A gene or her 0 gene, and which gene occurs in the father’s successful sperm, his B gene or his 0 gene (only one chromosome of each pair goes into an egg or sperm). If the parents above had more than one child, the children could also be different from each other.
The percentages of the different blood groups in today’s British population is [with the percentages for the United States in brackets] – Group O: 45% [45%]; Group A: 43 [40]; Group B: 9 [11]; Group AB: 3 [4]. However, both the UK and USA have populations composed mainly of immigrants. (Those people in the UK who complain about immigration undermining traditional Anglo-Saxon culture tend to forget that the Angles and the Saxons were immigrants. Without immigration, there would be no Anglo-Saxon culture. England takes its very name, not from its native peoples but from its immigrants: Angle-land.) In the immigrant populations of the UK and USA, the genes for blood groups will therefore be quite mixed and will not reflect the distribution of blood groups in the original Celtic inhabitants of the British Isles or in the native North Americans.
Native North Americans had virtually no blood group B in their original populations and this has led to suggestions that they are all descended from one small group of individuals who migrated to North America from Asia across the Bering Strait at the time of the last Ice Age – a small group who just happened to have no blood group B among their members. This creates a situation like a survivor population (as noted for Northern Elephant Seals earlier) but as in this case the rest of humanity had not become extinct; scientists call this type of bottleneck, where one group descends from a small number of breakaway founders, a ‘founder population’.
Teams of genes
The situation with blood groups is complicated enough, but for most of our genes it gets worse. It may be that few features are controlled by just one gene (or correctly one gene pair) which we can conveniently label as an ‘eye-colour gene’ or a ‘height gene’. For most characteristics, it seems many genes contribute to the final result, and they do this by interacting with each other. Eye colour is now known to be influenced by at least three different gene pairs, although there is a suspicion many more are involved. Skin colour is also controlled by more than one gene and this will probably be true for most aspects of our body.
Nor does the complexity stop there. It would be wrong to see the body simply as a machine, programmed by genes and built piece by piece like bolting prefabricated