Читать книгу Your Body - The Fish That Evolved - Dr. Keith Harrison - Страница 11

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’.